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.

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