Wednesday, October 8, 2014

Late Autumn Insects

It's a bit counter-intuitive perhaps, but my favorite time to look for insects is in late fall.  I'm still very much new to field entomology, so my experiences are largely limited to easier-to-identify groups like Odonata (dragonflies and damselflies) and Lepidoptera (butterflies right now -- I'm scared of learning moths).

I haven't quite put my finger on exactly why I find late season insects so fascinating, but I suspect it's largely to do with the seeming absurdity that some of these things can survive nighttime frosts and even daytime snow.  For the record, it's snowed at least twice here in Laramie so far this fall, nightly temperatures have been hovering in the low 40s lately, and we've had a handful of nights dip below freezing.  I also find it interesting to contemplate the possibility that, within my lifetime, I could easily witness changes in the phenology of these insects as climate change progresses.  I think as humans who are accustomed to thinking on very immediate time scales, there's a tendency for us to develop a static view of the world.  Indeed, I think this is likely to be at least partially responsible for the disturbingly low acceptance of the evidence for phenomena like evolution and climate change.  I increasingly find myself seeking out and cherishing experiences that remind me how dynamic the world really is, and insects in this context seem so well-suited for such a shaking up of perspective.   But anyway, here are a few species I've had fun with recently.

Variegated Meadowhawk (Sympetrum corruptum) on 9/13 from Cheyenne.  The most recent individual I've seen was on 10/4.
Common Checkered Skipper (Pygrus communis) on a chilly morning (9/23).  I've seen quite a few since, including one today!

Today was a surprisingly warm and sunny day and, as such, butterflies were out in decent numbers.  I managed to find 6 species throughout the day, including three Painted Ladies (Vanessa cardui), one Clouded Sulphur (Colias philodice), one Mourning Cloak (Nymphalis antiopa), one Comma species (not sure which exactly), one Milbert's Tortoiseshell (Aglais milberti), and one Common Checkered Skipper.

One of three Painted Ladies from today.  Interestingly, these and a Clouded Sulphur were getting viciously attacked and even dislodged by a large group of bees.

Monday, September 29, 2014

Migratory Speciation (?)

Since arriving in Laramie a few weeks ago, I've been spending a fair amount of time birding a few local spots, trying to soak in as much migration as possible before the gauntlet of grant proposals, research obligations, coursework, and -40F temperatures precludes me from getting out much.  And while I seriously enjoy all groups of birds, migration for me is really about one group: warblers.

I'm not alone in this regard.  It seems that virtually all birders I know, across all skill sets and interest levels, have an inordinate fondness for warblers.  I know several individuals who can tell me the single bird that got them hooked and the vast majority of the time that bird is a species of eastern wood warbler.  Indeed, a trip to Magee Marsh in the spring of 2009 was the single experience that pushed me from a casual bystander of ornithology to the point where I've seriously had to consider the non-zero probability that I will end up dying a hermit in an isolated shack (with good windows, of course) in the north woods because of birds.  For the record, it was a particularly striking male Cape May Warbler (Setophaga tigrina) on that trip which I suspect put me on this precarious path.

Warbler mania seems to be a sort of cliche in the birding world and, like most cliches, has its roots in some universal truth.  That truth being, of course, that eastern wood warblers are the single greatest group of birds ever (p<0.05).  I mean, look (all warbler pictures courtesy of fellow warbler fanatic, Mr. Skye Haas):

Mourning Warbler (Geothlypis philadelphia)

Cape May Warbler (Setophaga tigrina)

Black-and-white Warbler (Mniotilta varia)
American Redstart (Setophaga ruticilla)
Kirtland's Warbler (Setophaga kirtlandii)
Even in the very small sample of species here, all from Michigan, there is an astonishing amount of behavioral, morphological, and ecological diversity.  Mourning Warblers are relatively big, long-legged warblers that spend most of their time on the ground in dense vegetation.  Black-and-white Warblers have converged on a body plan and ecological niche with distantly-related nuthatches, spending most of their time climbing tree trunks in search of bark insects.  American Redstarts flash their bold, flaming tails at insects and tumble madly through vegetation in pursuit of their next meal.  The association between Kirtland's Warblers (also known as American Warblemasters) and Jack Pine (Pinus banksiana) approaches a degree of specialization that exceeds that in any host specialist insect that I can think of.

Growing up in the upper midwest, one is truly spoiled by the ease with which this diversity can be accessed.  In Michigan alone, ~34 species of warblers can be found during the breeding season, and this number increases slightly during spring and fall migration.  Unfortunately, the situation is not so idyllic out west, where warbler diversity is much lower.  Thus far, I've seen a mere 10 warbler species in and around Laramie.  To help put things into perspective, it is not terribly difficult to see more than 15 species of warblers in a single morning this time of year in Michigan.  While I do dearly miss these warblers, I'm increasingly appreciative of -- and sometimes even prefer -- being in areas with fewer species.  As great as it is to see 100+ species in a day, I find that it's very easy to overlook subtle details of single species -- details that, paradoxically, can lend insight into the dynamics governing the very diversity that so often obscures them.

Where the Laramie Valley is lacking in warbler diversity, it makes up for, almost entirely, in numbers of Wilson's Warblers (Cardellina pusilla) throughout much of August and SeptemberWhether I'm walking to campus, from campus, drinking coffee at a cafe, reading in my office, birding anywhere, struggling to wake up in the morning or going to sleep at night, my daily life is currently immersed in Wilson's Warblers.  Back home, I'm lucky if I see more than a handful of these throughout the fall, making this a great chance to look a bit closer at a species I don't know much about.   

One of the first things I noticed about Wilson's Warblers here is their voice -- specifically, the chip notes that warblers give during social contexts, flight, and even when foraging alone.  Despite my comparatively limited time with western Wilson's, it was immediately apparent to me that these birds sounded different than eastern individuals.  Have a listen!
Spectogram of typical Western Wilson's Warbler chip note

Spectogram of Eastern Wilson's Warbler chip note

The differences in amplitude (loudness, or in the case of spectograms, darkness) between the two recordings makes it a touch difficult to compare them visually, but the Eastern Wilson's call is noticeably shorter in duration and has a higher lower frequency bound than Western Wilson's.  This seems to be reflected in the Eastern call sounding sharper than the Western, though I think the amplitude differences could confound this, too.  At any rate, I feel pretty confident that there are real differences in the calls here, even if they are a touch subtle.  I'd like to examine this in more recordings, but there are surprisingly few recordings of eastern Wilson's Warblers calling (not singing).

Of course, this isn't too surprising.  There are loads of species that have distinct western and eastern populations that differ in coloration, morphology, ecology, vocalizations, etc.  However, experiencing these differences in Wilson's Warblers in the field prompted an evening of reading about this species and the discovery of some rather unique features about these populations.  But first, a bit more detail on the biogeography and natural history of Wilson's Warblers is in order.

Wilson's Warblers breed throughout most of Canada, in a large portion of the inter-mountain west, and along most of the pacific coast.  Based on very (and I do mean very) subtle differences in coloration between these populations, 3 subspecies have been described that correspond to these biogeographic and morphological differences.

Range map of Wilson's Warbler subspecies (from Irwin et al., 2011)

These populations are genetically distinguishable, but little was (and kind of still is) known about if and how breeding range and winter range differences were connected -- are different populations overwintering in different regions?  Darren Irwin's group from the University of British Columbia looked at this by sampling birds on the breeding and wintering grounds, trying to find the connections.  And while they didn't find any wintering eastern birds in the area they sampled (points on the map above), they did find something rather remarkable: eastern and western (lumping interior west and pacific populations) birds are as genetically divergent as many well-recognized species.  So divergent, that they are thought to have diverged some 2.3 million years ago, which is more than enough time for speciation in birds.  They didn't even find any birds that were genetically intermediate between the two populations, strongly suggesting that there is substantial reproductive isolation between these populations.

What is most impressive about this to me is that there is so little morphological differentiation between these two populations.  The biggest difference that I know of concerns plumage coloration, with pacific birds being a brighter, more saturated yellow than eastern birds.  Of course, there are also differences in song, which could help maintain reproductive isolation in an area where the two populations might meet and thus potentially hybridize (such an area of contact has not yet been found to my knowledge, though Irwin has looked).  But the fact that they are so similar in appearance after all this time is so fascinating.  Take a look for yourself!

From Sibley

Furthermore, the distinct breeding ranges suggest that differences in migratory behavior have played and are currently playing an important role in maintaining the distinctiveness of these two populations.  Additional study of winter ranges will be especially interesting to this end: are the two populations migrating to the same wintering areas, taking different routes in spring, or are they even overwintering in different areas?  What is the genetic basis of migratory differences -- do hybrid birds have an intermediate migratory route?  If so, are there any costs associated with this intermediate route, as has been implicated in Blackcaps?  If they are overwintering in the same region, why didn't Irwin's group find any eastern birds?  Are they using a different microhabitat? 

This is a system that I will be keeping a close eye on in the coming years, in hopes that Irwin manages to find an area of contact.  It will be particularly interesting to know to what extent and how these two populations are reproductively isolated.  Who knows -- we could have another species of warbler on our checklists in a few years!


Irwin, D.E., J.H. Irwin, and T.B. Smith. 2011. Genetic variation and seasonal migratory connectivity in Wilson’s Warblers (Wilsonia pusilla): species-level differences in nuclear DNA between western and eastern populations. Molecular Ecology 20: 3102-3115.

Tuesday, September 9, 2014

Making the Case for Crossbills

When I tell fellow birders that I study Red Crossbill (Loxia curvirostra) call types, the vast majority of responses range from "what's a call type?" to "wait -- people actually think call types are real?".  I'd increasingly like to devote more time to crossbills on this blog, so I think an overview of the natural history of crossbills is probably in order right about now.  In particular, I'd like to target people on each end of the response spectrum above and hopefully convince anyone unfortunate enough to be reading this that crossbill call types are real and worthy of your attention in the field and the attention they get from evolutionary biologists.  I won't be delving too much into how to identify each call type, as Matt Young from Cornell  has already put together a fantastic piece on this here that you absolutely should read.  Instead, I'd like to focus on the evidence that North American Red Crossbills are composed of distinct evolutionary lineages and some of the evolutionary mechanisms that may account for this pattern. 

Adult male type 9 Red Crossbill
Crossbills are cardueline finches, a group of predominately herbivorous finches that are mostly specialized for foraging on seeds.  There is considerable variation in the degree of specialization on particular types of seeds in this group, with crossbills representing the extreme end of high specialization.  The vast majority of crossbill diet is composed of nothing more than seeds from inside the cones of coniferous trees.  Crossbills are able to extract seeds from closed cones of any stage (fresh green cones or old weathered cones) due to their unique bill morphology.  Specifically, the vertical curvature of both the upper and lower mandible allow them to exert strong biting forces between the scales of cones to open them, which would not be easy if both mandibles were straight.  Additionally, the lower mandible is curved to either the right or the left of the upper mandible, which then allows the bird to laterally abduct the lower mandible to pry open the scales on a cone, giving it access to the seeds inside.  Once a seed is extracted, it is pushed into a groove on one side of the bill.  The lower mandible then puts pressure on the seed and removes the husk with the aid of the tongue.

From Newton (1973)
A South Hills Crossbill (type 9) foraging on Lodgepole Pine cones.  Note the cones are already opened a bit, but the foraging method is illustrative of that on closed cones.

Like all specialists, crossbills face unique challenges that revolve around their food source.  The most obvious challenge being the need to consume A LOT of seeds to survive, especially during colder times of year and when feeding young (unlike many seed-eating birds, crossbills feed their young primarily seeds, not insects, by grinding seeds up into a paste-like consistency).  For example, White-winged Crossbills (Loxia leucoptera), a closely related species, can eat up to 3,000 seeds in a day.  Assuming ~12 hours of foraging time in a day, that's 250 seeds an hour, a little over 4 seeds per minute.  Obviously, this number will be higher during winter when day lengths are shorter, further increasing the need for efficient foraging.  I don't have numbers readily accessible, but I would be willing to bet this number is higher for Red Crossbills, given that they are larger than White-wingeds and thus likely have higher energetic demands.

Red Crossbills have my mind captivated, but White-wingeds will forever hold my heart (Photo credit: Skye Haas)
Another non-independent problem associated with specialization on conifers is the year-to-year variability in cone production in most conifer species.  It is not uncommon for conifers across a large geographic area to produce almost no cones in any given year.  Obviously, this is a huge problem for crossbills and there appears to be two partial solutions to this problem.  One solution is to specialize on conifers that produce more regular cone crops, especially those conifers that also retain seeds throughout winter and early spring instead of dispersing them all (so-called 'key conifers').  However, even still, cone crops will inevitably fail at some point.  When faced with such a situation, crossbills will leave an area en masse and travel widely across the country in search of a sufficient food supply.  In the fall of 2012, the midwest saw a massive influx of type 3 crossbills from the pacific northwest, likely in response to such a cone crop failure.  Another possible stimulus for such irruptions is that crossbills could produce an excess of young in a given year due to favorable cone crops, resulting in strong competition for food and an effective food shortage per individual.  If sufficient food sources are found during these irruptions, crossbills will sometimes even stick around in an area and breed, no matter the time of year (this is responsible for odd breeding records from states such as Kansas).

eBird map of type 3 sightings from 2010-2011 (left) and 2012-2013 (right)
Of course, different conifer species -- even different key conifers -- produce cones that are very different in size, shape, and, importantly for crossbills, the thickness of the scales that protect the seeds.  Key conifer cones for Red Crossbills range in size from the diminutive cones of Western Hemlock (Tsuga heterophylla) to large, hard-scaled pines such as Ponderosa Pine (Pinus ponderosa).

The adaptive landscape for Red Crossbills

Taking into consideration the intense pressure on crossbills to forage efficiently, the potential benefits of specializing on a key conifer species, the diversity of key conifer cones, and the method that crossbills use to extract seeds from these cones, it is perhaps not surprising that crossbills have diversified into several forms (call types), many of which appear to be more or less specialized for foraging on a single or a few key conifers (Benkman, 1993; 2003; Irwin, 2010).  Much like the more famous case of Darwin's Finches to which they are often compared, the primary axis on which this divergence has occurred is bill morphology -- birds specializing on small cones have small bills whereas birds specializing on large cones have large bills.  While the association between bill depth (how 'tall' a bill is) and cone size is pretty striking, there's more evidence inside each bill that some degree of specialization is occurring.  Specifically, there is a near perfect relationship between the width of the husking grooves in a bill and the size of the seeds that many of the different call types specialize on (Benkman, 1993; 2003; Irwin, 2010).  It's worth noting that, based on the presence of stable cone crops and the retention of seeds throughout winter, Craig Benkman actually predicted the existence of a call type that should specialize on Sitka Spruce (Picea sitchensis) in the coastal pacific northwest, even down to the bill depth and width of the husking groove.  In 2010, Ken Irwin described such a bird and there is excellent agreement between Ken's measurements and Craig's predictions. 

7 of 10 call types in North America showing some of the diversity in body size, bill morphology, and flight calls 

In North America, Red Crossbills are currently divided into 10 populations or call types, a few of which have only been described very recently.  In most birds, such populations are categorized as subspecies, which can be thought of as geographical replacements of each other that may differ in any number of traits (e.g. vocalizations, habitat use, coloration, etc.) but are still considered the same species.  Although there is a rough pattern of geographical separation among crossbill call types, there are several places where more than one call type regularly occurs.  The general term used to describe such organisms that overlap extensively yet show differences in ecology and various traits is "ecotypes".  The call type system in crossbills is nothing but a spin on this -- calls are emphasized because different populations are most reliably distinguished in the field by the flight calls they give (see Matt Young's article).

And where more than one call type occurs, all available evidence suggests that hybridization is surprisingly rare (Groth, 1988; Smith and Benkman, 2007).  In other words, crossbill call types behave like different species.  The fact that crossbills have maintained significant genetic (Groth 1993; Parchman et al., 2006, unpublished data), morphological (Benkman, 1993), and vocal differences despite co-occurring together is made more impressive by their frequent nomadic movements across the country.  This should increase the opportunity for gene flow between populations, eroding any differences that had previously accumulated.  Furthermore, it is currently thought that most of the diversity we currently see in crossbills has arisen within the last ~12,000 years, coincident with the northward expansion of different conifers following the retreat of the last glaciers.  This represents one of our biggest gaps in understanding the story of crossbill evolution and is in need of further study.  However, if this scenario is accurate, the diversification of crossbills becomes even more remarkable, as 12,000 years is a mere blink of an eye in evolutionary terms.

This is not a crossbill.  I don't have any applicable pictures to place here, so enjoy this Ring-necked Snake (Diadophis punctatus) that I found in the Upper Peninsula.
The big question, then, is how are these differences maintained -- what barriers to hybridization exist in crossbills?  So far, only one study (Smith and Benkman, 2007) has looked at this in any detail and only in a single group of call types, but it seems likely that the results are somewhat generalizable to other crossbill pairs.  I'm also going to go out on a limb and suggest some other barriers to hybridization based on my own experiences with crossbills, fully acknowledging that they are in need of further study.

1) Habitat isolation: Given that different call types forage most efficiently on different conifers, we might expect to find them in different habitats, which should reduce the probability that they encounter (and thus breed with) each other.  Many conifer species are found in large homogeneous tracts of forest, making it less likely that different call types will encounter each other.  That being said, crossbills regularly fly long distances, so this alone will not be enough to prevent hybridization.  Additionally, there are quite a few conifer species that seemingly all crossbills can utilize well, so the importance of habitat isolation can potentially vary.

2) Immigrant inviability: Say a type 3 crossbill (a western hemlock bird) were to wander into a patch of lodgepole pine where a bunch of type 5 were hanging out.  Given the very small bill of this type 3, it is going to have a very hard time foraging on the big cones of lodgepole pine.  As such, if it were to stay there, the odds that it would survive are not good, given the insane amount of seeds that a crossbill needs to consume.  Even if it were to survive, it may not be able to acquire enough resources to get itself in breeding condition or feed young ('immigrant infecundity').  There is somewhat suggestive evidence that something like this may be happening with type 5 crossbills in the South Hills of Idaho (Smith and Benkman, 2007).

Fast forward to 15:04 for an illustration of how immigrant inviability could work.  Otherwise, do check this video out if you have time!

3) Assortative flocking: This is one of the coolest parts of the crossbill story.  Using playback experiments to types 3 and 4 in an area where they were co-occurring, Smith et al. (2012) found that wild crossbills flying over showed a strong tendency to approach speakers playing their own call type, but not of the different, co-occurring call type.  This is likely to be very important for crossbills, given that they spend much of their lives in flocks.  Furthermore, they seem to choose their mates from within flocks -- if crossbills are, on average, staying with their own call type, this will automatically reduce the probability of hybridization.  Based on my own experiences in the South Hills (with types 2, 5, and 9) and here in Laramie (with types 2 and 5), this tendency for assortative flocking seems to be present in other crossbill groups.  Others have noticed this behavior, too!

4) Conspecifics are sexy: Snowberg and Benkman (2007) gave captive female crossbills a choice of associating with either a male of her own call type or of a different call type (type 2 and 9 were used in this study).  Males were matched for several traits such that the biggest differences between them were their calls.  Despite this, females showed a clear preference for males of their own call type.  Many people I've talked to find this and the assortative flocking results problematic, given that crossbills learn their flight calls from their parents (Groth, 1993) and there has been a documented case of a hybrid pair where the female later changed her call type to match her mate's (Keenan and Benkman, 2008).  As such, if a breeding pair of crossbills was composed of two different call types, the hybrid offspring would inevitably learn one call or the other and then be able to hybridize with members of that call type.  And if crossbills are changing their calls to match their mates, how do we know that a bird giving a type 2 call is actually a 'genetically' type 2 bird?

The first thing to say here is that this call type switching is exceedingly rare.  In the 2008 study, only 1 out of 79 birds (1.27%) changed their call types.  Since 2008, thousands of additional birds from this same area have been banded and recorded and we have not yet found another bird that has changed its call type.  Furthermore, crossbills appear to learn flight calls very early on, after which there is virtually no modification of their distinctive call type (Sewall, 2009; 2011).  So, given that crossbills overwhelmingly prefer their own call type as mates (Snowberg and Benkman, 2007; Groth 1988; Smith and Benkman, 2007) and that we can expect that at least ~99% of these pairs to actually be the 'right' call type, the opportunity for mismatches between flight calls and genetic identity is exceedingly minimal.  There also seems to be a weird notion that learned traits of organisms don't constitute a viable means of inheritance, which is a misguided view (Pfennig and Servedio, 2013).  As a side note, a huge number of birds that we generally consider different species learn their songs just like crossbills learn calls, yet we're perfectly comfortable using songs to delineate species, even when there are well-documented cases of birds giving the 'wrong' song (see link below).

A neat story on a White-throated Sparrow that learned the song of a Black-throated Green Warbler

5) are good foragers: Snowberg and Benkman (2009) also found that females prefer males who are efficient foragers.  By preferring efficient foragers, female mate choice will reinforce natural selection for foraging efficiency.  This will also accelerate divergence between call types and further prevent hybridization.  For example, if a male type 3 joined a flock of large-billed type 2 (or vice versa) foraging on Ponderosa Pine, not only would he struggle to access food, but he would also struggle to attract a mate!

Wow.  You're still reading?  Well, here's a dog (Henslow) in a silly hat.

6) Singing different songs: Surprisingly, differences in the songs that males use to attract females and deter other males have been very under-studied in crossbills.  I've heard rumors that a paper will be published soon on differences in songs among call types, but until then (or at least until I can figure out how to embed my recordings), you'll just have to take my word: there are drastic differences in song!  The differences are so that I'm cautiously suspicious that we will begin using songs more to identify call types in the field.  You'll also have to take my word that these differences in the songs that males produce seem to matter to females.  I've done a few informal playback experiments in the South Hills and have seen striking differences in how females react to songs of their own call type versus those of other call types.  There's also the intriguing possibility that bill size could influence song production, which Julie Smith and I are currently investigating.  Depending on what we find, this could be another example where mate choice reinforces divergent natural selection.

The yooper type 10: one of many crossbill mysteries (Photo credit: Skye Haas)
Really, there's quite a lot more that could be added to this, but I think this post is already much too long.  For example, what happens to hybrid crossbills in the rare event that they're formed?  Do they have bill morphologies that are intermediate between their parents', leaving them poorly adapted to either conifer that their parents specialize on?  A lot of research on crossbills still needs to be done, which is very exciting to me as a birder, crossbill fanatic, and evolutionary biologist.

One point that I'd like to stress is that crossbills are a perfect example of the potential of citizen scientists to make serious contributions to our understanding of the natural world.  Perhaps one of the biggest unresolved areas in crossbill research concerns the distribution and irruptive tendencies of different call types.  While we have a decent idea of where different call types regularly breed, it seems to me that there are several important details that need to be worked out.  Exactly where are different call types co-occuring in the northwest?  Do different suites of call types co-occur in different forest types -- perhaps forests with conifers that all call types can easily exploit?  Are these fine-scale distributions stable over time?  During irruptions, what call types are moving where?  Are they sticking around to breed?

Answers to these questions will greatly enhance our understanding of the ecological and evolutionary dynamics that are writing the crossbill story, even as we speak.  The single best way you can contribute to this is to record crossbills that you encounter in the field so that they can be identified to call type.  Don't have a recorder?  The vast majority of recording/voice memo devices on cell phones are completely capable of capturing sufficiently high quality recordings to identify crossbills (I made the xeno canto recording in this post with my iPhone).  Once you have a recording, you have two options.  You can identify the birds to call type on your own by making a spectogram of the flight calls and visualizing them, which is, in many cases, the only way to confirm call type identification.  For this, you can download a free spectogram-producing software here and then compare the images to the ones in Matt Young's article (see link at the beginning of this post).  Alternatively, you can contact Matt Young (his email is in that article) or myself at empidonaxdvg "at" gmail "dot" com for help.  Finally, once you have your crossbills identified, please submit your observations to eBird, an online database that has bird sightings from all around the world, all contributed by volunteers.


Benkman, C. W. 1993. Adaptation to single resources and the evolution of crossbill (Loxia) diversity. Ecological Monographs 63:305-325.
Benkman, C. W. 2003. Divergent selection drives the adaptive radiation of crossbills. Evolution 57:1176-1181.

Groth, J.G. 1988. Resolution of cryptic species in Appalachian Red Crossbills. Condor 90:745-760.

Groth, J.G. 1993. Evolutionary differentiation in morphology, vocalizations and allozymes among nomadic sibling species in the North American Red Crossbill (Loxia curvirostra) complex. University of California Publications in Zoology 127: 1-143.

Irwin, K. 2010. A new and cryptic call type of the Red Crossbill. Western Birds 41:10-25.

Keenan, P. C., and C. W. Benkman. 2008. Call imitation and call modification in Red Crossbills. Condor 110:93-101.

Parchman, T. L., C. W. Benkman, and S. C. Britch. 2006. Patterns of genetic variation in the adaptive radiation of New World crossbills (Aves: Loxia). Molecular Ecology 15:1873-1887.

Pfennig, D. W. and Servedio, M. R. 2013. The role of transgenerational epigenetic inheritance in diversification and speciation. Non-genetic Inheritance 1: 17-26.

Sewall, K.B. 2009. Limited adult vocal learning maintains call dialects but permits pair distinctive calls in red crossbills. Animal Behavior 77:1303-1311.

Sewall, K.B. 2011. Early learning of discrete call variants in red crossbills: implications for reliable signaling. Behavioral Ecology and Sociobiology 65:157-166.

Smith, J. W., and C. W. Benkman 2007. A coevolutionary arms race causes ecological speciation in crossbills. American Naturalist 169:455-465.

Smith, J. W., S. M. Sjoberg, M. C. Mueller, and C. W. Benkman. 2012. Assortative flocking in crossbills and implications for ecological speciation. Proceedings of the Royal Society of London Series B 279:4223-4229.

Snowberg, L. K., and C. W. Benkman. 2007. The role of marker traits in the assortative mating within red crossbills, Loxia curvirostra complex. Journal of Evolutionary Biology 20:1924-1932.

Snowberg, L. K., and C. W. Benkman 2009. Mate choice based on a key ecological performance trait. Journal of Evolutionary Biology 22:762-769.

Wednesday, August 27, 2014

August in Michigan or Biodiversity in my Backyard

After a two year hiatus, I've somewhat apprehensively decided to have another go at maintaining this blog.  I find myself at a very convenient point in time for doing so -- I've been in Michigan visiting family before starting graduate school and have had an insanely great time naturalizing around the state.  Given the content of my last post, it seems only appropriate to start off with an exciting find in the flooded hardwoods of my parents' neighborhood.

Foraging on buckthorn

I promise it's in there...

I can still remember searching this forest constantly for Prothonotary Warblers (Protonotaria citrea) during my first summer as a birder.  Based on everything I was reading, the habitat here seemed perfect for them, but I never even turned up a migrant bird in four years of searching.  So, when my youngest brother Dawson and I heard a sharp chip note coming from a forested edge of the Grand River, I got really, really excited.  Soon we located a family of four birds (two adults and two juveniles) foraging intensely on buckthorn berries, which are really abundant here right now.  I actually hadn't heard of them doing this but, like many warblers, berries apparently become a more common dietary item for prothontaries in fall (Petit, 1999).  At any rate, I'm very encouraged by finding them here where they likely bred and really hope this continues in subsequent years.  This is a species of special concern in Michigan and is found regularly in only a few spots in Jackson County.

Another encouraging bit of news from this property is the recent discovery of an apparently healthy population of Blue-spotted Salamanders (Ambystoma laterale).  This salamander is the subject of many fond childhood memories and (I suspect) played a major role in getting me interested in biology at an early age.  I distinctly recall flipping a specific log at my grandmother's farm in the northwestern corner of the county during summer visits and seldom finding myself disappointed by the lack of one of these beauties.  How could an eight-year old child not be captivated by this animal?

A summary of my fifth through tenth year on this planet

Blue-spotted Salamanders are pretty common in the state and are capable of handling an impressive diversity of environments -- I recently found one under an old tire near a building in a heavily disturbed, dry field.  Subsequent, post-rain searches have come up empty -- did s/he move in the moist night? Interestingly, many populations in southern Michigan are all-female hybrids involving the closely-related Jefferson's Salamander (Ambystoma jeffersonianum).  They reproduce through kleptogenesis, wherein a female uses sperm from a heterospecific male to stimulate egg division, though the sperm does not actually fertilize the egg (Bonen et al., 2007).  This form of reproduction arose in this group some 5 million years ago (Bi and Bogart, 2010) and results in a high proportion of polyploid individuals with varying numbers of chromosome copies, though most are triploids.  I grew up referring to these salamanders as Blue Jefferson's Salamanders, a la grandma, until I was corrected by a friend some years later.  Who knew grandma Jackie was so well-versed in salamander hybridization?

We found many young individuals this year!
I suspect that this population is likely composed of hybrids, but I'm still not confident in my ability to determine this.  Hybrids tend to have less blue-spotting than pure individuals and also tend to have morphological phenotypes that are intermediate between blue-spotteds and various heterospecifics.  A potentially easier way of evaluating this population is to sex all individuals that I find in the field, but I don't know how to reliably sex salamanders.

Other herps have been doing very well here, such as this large Eastern Milksnake (Lampropeltis triangulum  triangulum) that my brothers found in early May.  Much of my time back home has been devoted to finding this species on the property, as king/milk snakes are my absolute favorite snakes.

Future Dr. Porter, world authority on the utility of denim jackets for herpetological research
The chorus of Wood Frogs (Lithobates sylvaticus) here is absolutely deafening in the spring
Northern Water Snakes (Nerodia sipedon) are really common here.  They are not water moccasins and will not kill you.  Please stop.
A growing interest of mine has been insects, especially Odonata (dragonflies and damselflies).  I'm increasingly struck by the subtle yet staggering diversity of morphology, behavior, and ecology in this group, most of which has been poorly studied compared to other taxa.  As such, I think a better understanding of Odonata natural history will make this an excellent group for addressing many interesting questions in my own field of evolutionary ecology.  Luckily for me, I have a canoe and Odonataphiles as my mentors of natural history, both of which have been invaluable for getting to know these things a bit better.  Here is but a small sampling of the local diversity:

Black-shouldered Spinyleg (Dromogomphus spinosus)
Dragonhunter (Hagenius brevistylus) (Photo credit: Don Henise)
Prince Baskettail (Epitheca princeps) (Photo credit: Don Henise)
Blue Dasher (Pachydiplax longipennis)

Ebony Jewelwings (Calopteryx maculata) in copula (Photo credit: Don Henise)
Violet Dancer (Argia fumipennis violacea) (Photo credit: Don Henise)
It's hard to know where to stop since there's so much more I'd like to cover here, and we still haven't left my back yard (this post was going to be a brief summary of my Michigan travels).  However, it would be downright irresponsible for me to not introduce one of the true outliers of the ode world.

During evening fishing trips to Williams Lake with Carson and Dawson, I began to notice an abnormally abundant damselfly with a light orange thorax and a blue-tipped abdomen, especially where extensive mats of aquatic vegetation were present.  I initially wrote them off as some weird immature or polymorphic female bluet which I'd have no chance of identifying.  However, one curious feature of these things was their crepuscular and even nocturnal activity.  By the time we'd pulled up the anchor and slowly started drifting home for the day, they were still madly fluttering over lily pads and even landing on our hats, fishing poles, and the canoe.  Most Odonata are most active around early to mid afternoon and vanish in evening hours, yet here these odd little creatures were dancing in the soft golden rays of dusk and even well into the night.

They seem to swarm the black lining of the canoe during colder weather
I consulted Don and Robyn Henise about the possibility that I might be observing Vesper Bluets (Enallagma vesperum).  I think a combination of disbelief and excitement was on their faces that evening, as this was a species they had been unsuccessfully searching for for some time.  This instantly lowered my confidence in the true ID of the damselflies I had been seeing -- how could two ode experts living in the same county as me have missed them?  After all, they are exceedingly abundant on Williams Lake, which I don't often regard as being an exceptional ecosystem in many ways.  What was going on?

My understanding is that vespers are often highly localized within a broad region, making them somewhat difficult to find.  Given their affinity for areas with a lot of lily pads (where males defend territories), I suspect that the combination of copious vegetation and high water quality (important for the larval stage) in this area is not often found coexisting in nature, especially in an age of excess fertilizers, herbicides, pesticides, etc.  Though the factors governing their local distributions is still unclear to me, their presence here has afforded me an attempt at better understanding their bizarre phenology and the evolutionary consequences this might have for other traits.  Rather than continue to on too far with this idea, maybe it is appropriate instead to write down a short list of questions I have come up with while observing these lemon-yellow beauties.

1) Why did the nocturnal behavior evolve at all?  Reduced predation/competition with other odonata?  Exploitation of a novel niche?

2) Is the orange coloration of the thorax (an unusual trait within Enallagma) an adaptation to a crepuscular lifestyle?  Interestingly, only other crepuscular species in this genus have such coloration.  Furthermore, light becomes more red-shifted later in the day, meaning an orange-yellow thorax might reflect a lot of light relative to a blue or green thorax, which is preferable if you need to attract the attention of a potential mate.

3) Has the crepuscular lifestyle facilitated the evolution of territoriality in E. vesperum or vice versa?  During the day, the habitat that vespers utilize in late evening has a few individuals of a pretty good diversity of species such as Stream Bluets, Violet Dancers, Widow Skimmers, Slaty Skimmers, Common Whitetail, Prince Baskettails, Royal River Cruisers, Dragonhunters, American Rubyspots, Black-shouldered Spinyleg, etc. -- many potential competitors and predators, especially given the small size of vespers.  However, at dusk, the same habitat is literally covered with vesper bluets (and nothing else) at seemingly every lily pad.  And if you were to visit the same area ~2 hours before vespers come out, you'd find the area similarly dense with Orange Bluets (Enallagma signatum), which are similar to vespers in coloration and territoriality (based on my observations -- this has not been published in the literature to my knowledge).  These observations have had me wondering for some time if vespers and oranges, by virtue of utilizing a clustered resource at a time when no other odes can, regularly reach much higher densities than other bluets ever experience.  It seems to me that this is a perfect recipe for the evolution of territorial behavior.

4) Have the targets of sexual selection changed in this novel (temporal) environment?  Admittedly, I don't know much about sexual selection in dragonflies, but it seems that most visual components of courtship are likely to be difficult to detect after sunlight, when copulation is reported to peak (Moody, 2009).

5) Surely, temperatures at and after dusk are very thermally stressful for an ectotherm -- how do they manage to engage in high performance territorial defense and copulatory flights in these conditions?

6) What are the primary predators of vespers, given that most insectivorous birds and dragonflies are not regularly active during a large chunk of their flight time?  If the predators are different, how is selection for anti-predator mophology, behavior, etc. different than for other bluets?

It's rare to see two males on the same lily pad... (Photo credit: Don Henise)
Most of these questions could completely or partially answered by virtually anyone with a bit of a natural history background and, more importantly, a curiosity about the natural world.  It seems to me that the line between "professional biologist" and "amateur naturalist" is increasingly being drawn, which is a shame, given that a plethora of interesting research could be done on organisms like these bluets by people without a degree.

A supplementary post highlighting other Michigan naturalizing may follow, but I am leaving for Laramie in two days, making this somewhat unlikely.  Besides, this is my first post in years -- I'm taking it easy. However, a post on crossbills in the Upper Peninsula will definitely eek its way into existence over the next week.


  • Bi, Ke; Bogart, James P (2010). "Time and time again: Unisexual salamanders (genus Ambystoma) are the oldest unisexual vertebrates". BMC Evolutionary Biology 10: 238.
  • Bonen, L.; Bogart, James P.; Bi, Ke; Fu, Jinzong; Noble, Daniel W.A.; Niedzwiecki, John (2007). "Unisexual salamanders (genus Ambystoma) present a new reproductive mode for eukaryotes". Genome 50 (2): 119–36
  •  Moody, DL (2009). Mating Behavior and Male Territoriality in Enallagma vesperum (Odonata: Coenagrionidae) on Ponds in Ohio and Northern Michigan. Ohio Journal of Science. 109 (3): 67-70.  
  •  Petit, Lisa J. 1999. Prothonotary Warbler (Protonotaria citrea), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:

Friday, March 9, 2012

A Quest for the Golden Swamp Warbler

Ever since I really got into birding (almost four years ago now) and started exploring the woods around my house through the eyes of a birder, one question has been burning in my mind that remains unresolved to this day: where are the prothonotaries?

Unlike many similar questions I've had concerning the absence of species despite apparently suitable habitat, which are often settled after either finding the species or discovering some critical piece of missing microhabitat, the more I learn about Prothonotary Warbler habitat, the more perplexed I am at their truancy from this area.

This is a map of the area in question, with apparently suitable habitat for prothonotaries highlighted in yellow, both bordered by bodies of water (Grand River to the north and Williams and Browns Lake to the south).  Specifically, these areas are what I would generally call flooded hardwoods, dominated by Red Maples, oak spp., elm spp., and birch spp. (my guess is River Birch, but I have little experience in tree identification).  As you might guess from the nature of this post, this is exactly what prothonotaries prefer to nest in (at least when they're this far north).  Here a a few shots of the habitat from yesterday:

...including a couple close-ups of key trees...

Birch spp.

I think this is a Red Maple...

A bit of searching around on the web has revealed that the following habitat features are important in habitat selection for Prothonotaries:

1) Prefer to nest over large bodies of standing/slow-moving water -- this area borders a slow-moving portion of the Grand River, two lakes, and usually remains flooded well into June/early July.

2) Low elevation and flat terrain are preferred -- low elevation is definitely 'achieved' here and the flooded woods themselves are flat, though the deciduous forest between them is fairly hilly.

3) Canopy height of 12-40 m with 50-75% cover -- yes on both accounts.  There's also a fair bit of diversity in both tree age and canopy cover, both apparently due to fallen trees, which is an indicator of a healthy, mature forest.

4) Forest ≥100 ha -- a quick and dirty estimate using Google Earth puts the northern portion at 13 ha and the southern portion at about 17 ha.  This might be the most obvious factor preventing prothonotary presence, though I'm not sure if this is limited to flooded areas where the birds would directly nest or the forest as a whole.  At any rate, the entire area only comes to about 33 ha.  This does not include adjacent areas that are, for the most part, fragmented only minimally by a few roads and houses.

5) Low exposure to sun -- plenty of this.

Despite the potentially limiting factor of acreage, I plan on installing a few nest boxes in the weeks to come in hopes of luring some warblers into the area this spring.  As many as eight warblers have been reported singing from the Grand River just north of town (in the Berry Rd. area), so I'm somewhat confident that prothonotaries might at least travel through this area (indeed, I got my life Prothonotary during fall migration of 2008 at Dahlem).

A few additional shots from yesterday's trek:

Filamentous green algae(?)

Bark beetle work...

...and a close-up

I'm not sure what's going on in this last photo, but this really stood out against the rest of the dark leaf litter below the water.  Whatever this is, it appeared to be pretty superficial, being restricted to a small layer on top of the leaves.

This is an attempt at cellestial photography that I made at something like 1:00 am yesterday while waiting in vain on the aurora borealis.  I think my telephoto lens has interesting potential for this type of stuff, but I'll definitely need to get my hands on a tripod -- this photo only managed to turn out semi-decent due to the strategic use of my roof as a steady base.
And finally, even though the majority of this post is dedicated to birds, it wouldn't be right to publish this without a single bird photo.

Eastern Phoebe

Don Henise found this yesterday, which is pretty early for this species.  Spring is here!