Sunday, August 7, 2011

Pancakes and Beavers (5/23/11 - 6/3/11)

Prior to May, and after learning that I had landed a position on the BTCU project, I spent considerable time digging through available literature on BTCU biology (and range maps, scouting prospective lifers).  One of the several interesting curlew quirks (tool use and flightless molts on wintering grounds, non-stop, pelagic migrations in excess of 4,000 km, and those mysterious bristle feathers -- to name a few) that really caught my attention was the sexual dimorphism in bill size.  On average, and with some overlap, female birds have much thicker bills that tend to curve much less than males.  Indeed, as it turns out, this was the most reliable means of identifying a bird's sex in the field.
Male BTCU profile -- very typical bill size/shape

Female BTCU profile -- actually, the mate of the fellow above (and possibly others!)

So, the question I had, and wanted to learn a bit more about, is why this dimorphism should exist.  Barring something like an epistatic effect of sex, it seems reasonable to assume that this trait represents some sort of adaptive significance.  And while it very well might be tied to some aspect of the bird's biology on the wintering grounds, where, for example, they use rocks and pieces of coral to crack open seabird eggs with their bills (the only known example of tool use by shorebirds!), I figured it was worth examining on the breeding grounds.

Every two days, after enduring exhaustive point counts that sometimes saw 13+ miles of hilly tundra by foot, Kevork and I would alternate roles and work on territory mapping and behavioral observations of curlews (way more interesting -- and easier).  During this time of the year, male curlews (and less often female curlews) stake claim to a slope of lichen-mat tundra via extraordinary aerial displays that entails the birds circling their territory while belting out their complex songs.  One of our primary duties was to construct a map of each bird's territory within the study area, as well as describe the various behaviors of birds in the context of apparent functionality, as so little is known about the life history of these dramatic creatures.

This offered me substantial time to observe what I figured might be a critical phenological time to uncover potential behavioral discrepancies between the sexes that might account for the bill dimorphism.  It seemed intuitively plausible that female birds, after logging some 2,500 miles of non-stop flight, might need to feed more than males in order to accumulate adequate resources for egg-production.  To my amazement, as intuition has not always served me well in science, this is exactly what we observed.  During these first couple of weeks, male birds tended towards the center of their territory, where they would perch conspicuously on an elevated hummock and viciously ward off conspecific intruders, if not already in the air displaying.  Females, however, could consistently be found quietly slipping through the vegetation of the territory's edge, ravenously foraging away and paying little mind to anything else.  How exactly this translates into divergent selective pressures on bill shape, I have little idea, though it is worth noting that at this time of the year, the tundra was still largely covered in snow, and exposed areas were all but frozen solid.  Male birds were not observed foraging heavily until later in the season, after nesting had started.  During this time, it appeared that females tended to incubate during the day, while males tended to work nights.  Again, I'm left uncertain as to how natural selection might be operating divergently in this situation, not to mention if this is even a 'real' phenomenom (this last observation is based only on the few nests we could find).  Nonetheless, I was interested in divergent behavior related to the different roles of each parent in the nesting process, where bills played dominant roles, and I found them.  Whether they have any relevance in explaining this interesting trait is pure speculation, but it kept my mind busy on some of the slower field days!

Interestingly, while bird activity generally started to pick up around this time (though much slower relative to last season, according to Kristine and Jake -- likely due to extensive snow cover over the low-lying willows that many songbirds nest in), with Whimbrels, Long-tailed Jaegars, Pacific-Golden Plovers, and Bar-tailed Godwits having arrived and settled down, Bristle-thighed Curlew display behavior seemed to vanish overnight around mid-May.  This is in stark contrast to last year, when the birds displayed well into the first week of June.  While the dawn chorus of Lincoln's Sparrows, Gray-cheeked Thrush, and Wilson's Warblers was consistently less impressive than last year, curlew activity was very high initially, leading some to speculate that they may have started nesting early despite a late arrival.

Long-tailed Jaeger -- king of the Nulato Hills!

No sooner had the recent influx of breeding birds and gushing snowmelt began to suggest spring that, without warning, the tundra summer was upon us.  The might roar of Allen Creek and her supporting drainages (which, in late May, make for one hell of an adrenaline rush when attempting to cross either by foot or holey inflatable raft) had been reduced to a dull moan, and the only glimpse of a dawn chorus was obtainable at the absurd hour of four in the morning!  Luckily, a maladaptive habit of evening coffee roused my bladder at this exact time most nights, allowing for half-conscious, yet thrilling observations of the awakening tundra.

Yellow Warbler singing, as all Alaskan individuals do, like a Chestnut-sided Warbler

On Sundays, our single day off from the rigors of field work, we all indulged in bountiful, southern-style pancake breakfasts, complete with coffee, sunny-side-up eggs, greasy bacon, and Old Crow Medicine Show.  Jake, being from Tennessee, and I, a nut for southern food and string-bands, conducted these all-too scarce gems of relaxation, which is where the crew's varying geographical backgrounds really seemed to stand out to me.  As a midwestern boy, I found myself at an intermediate position between the southern, conservative tendencies of Jake and the liberal, west coast influence of Kevork with respect to several cultural dimensions.  Jake and I meshed in our love of artery-clogging food and down home bluegrass, while Kevork and I had several agreeable discussions on sustainability and bands like Animal Collective and Girls.  Conversations on politics were rare, for a bit of tension was definitely present, especially with the inclusion of topics like climate change.  While I'm not one for politics (I find the whole ordeal overwhelmingly frustrating), I've always tended to view myself as maintaining a position somewhere between the two extremes with a slight liberal bias -- a position that seems relatively common here at home, which is likely where I derive much of that influence.  Likewise, Jake and Kevork maintained that their opinions were also common amongst their hometown fellows.  For the first time in my life, I actually found myself proud to hail from the corn-laden midwest.

After food comas, a weekly shower, and boastful stories of respective hometown avifaunas (spring wood warbler migration won this category, hands-down), we spent most evenings birding at the legendary Beaver Pond.  Just about every drainage in the study area seemed to have a beaver dam at roughly mid-slope, some of which were the most recent works in a drainage that has hosted multiple dams over many centuries, evident by large mounds of sticks and mud now colonized entirely by various grasses and forbes.  The Beaver Pond was such a place.

The Beaver Pond in all its water-pooling glory

As a birder, one of the most frustrating aspects of the Nulato Hills' undulating topography was the inability of this landscape to hold water.  An absence of large pond-dappled tundra meant an absence of breeding loons, wagtails, and many species of shorebirds.  Though not enough to alter these circumstances, these beaver dams did provide the only exception to this general feature of the landscape, and, in doing so, facilitated the formation of a truly unique habitat that has become essential to many species of this area.  Overtime, the industrious habits of these amphibious rodents have resulted in large, stagnant accumulations of snowmelt, flooding parts of the tundra for the entire summer.  This is just long enough for colonization of these areas by various species of marsh flowers, sedges, and shrubs, the last of which serves as the sole breeding habitat in this area for Lincoln's Sparrows.  The edges of these ponds also contain the only mudflats in the area, which provides critical feeding grounds for migrating shorebirds and resident species, most notably Bar-tailed Godwits.  Additionally, these ponds also allow several species of dabbling ducks, mostly American Wigeon, Green-winged Teal, and Northern Pintail, to breed in these hills.  Without beavers, none of these species, and probably hundreds of aquatic invertebrates that I'm missing, could possibly call the Nulato Hills home.

Marsh Marigold

American Wigeon

Definitively illustrating the pivotal role of beavers in this ecosystem was their absence this year.  For unknown reasons, possibly predation or trapping, not a single beaver was observed, and several of the dams confirmed the lack of sightings with unrepaired leaks.  As a result, water slowly flowed through these blemishes, leaving historical ponds reduced to small puddles -- so much so that few dabbling ducks were able to breed this year, a very different scenario compared to last year when beavers were common.

Untended dams result in...

...great access to to wildflowers!

With the arrival of summer, the focus of our work shifted from territory mapping and point counts to extensive behavioral observations and nest searching.  Nests of Bar-tailed Godwits, Pacific-golden Plovers, Whimbrels, Bristle-thighed Curlews, Savannah Sparrows, Lapland Longspurs, White-crowned Sparrows, Willow Ptarmigan, Short-eared Owl, and Long-tailed Jaeger were all found within the first week of June.  While nesting birds meant a void of courting behavior, it also meant the emergence of anti-predatory behavior.

Rough-legged Hawk, Northern Harrier, Short-eared Owl, Glaucous Gull, Parasitic Jaegar, and Red Fox are all potential nest predators of curlews and other large, ground-nesting shorebirds.  Luckily, however, most of these species were reasonably scarce (with the exception of harriers and fox).  Unfortunately, the sparse nature of these predators was over-compensated by increases in local populations of the most efficient nest predator of the land -- the Common Raven.  It is thought that recent increases in human populations in the surrounding villages and the subsequent expansion of landfills has facilitated the growth of raven populations, allowing them to venture into the tundra en masse

Common Raven in Bethel, AK

On a daily basis, the presence of a pair of ravens hunting low over the tundra was revealed by the cries of agitated Whimbrels, the fearless leaders of mixed shorebird squads devoted to preserving their genetic legacies.  Among these groups, Bristle-thighed Curlews, Bar-tailed Godwits, Hudsonian Godwits, and Long-tailed Jaegars were also present.  In fact, it has been suggested that these species intentionally nest near each other so as to reap the personal benefits of mob-attacks.  Indeed, previous research has suggested that curlews are particularly fond of nesting near Long-tailed Jaegars -- one of the most aggressive species in these flocks, yet not enough of a threat to curlew eggs or young.  As such, gathering data on species nesting near curlews was a top priority in the field, and all but one nest was within 100 meters of a jaeger nest (making curlew nest monitoring a very exhilarating event).

A well-observed trend that was noted was the different responses of Bristle-thighed Curlews and their closest relatives, Whimbrels, during these attacks.  While Whimbrels seemed to burst from the earth with an explosive energy, heading straight for the ravens with menacing clamor and dives, curlews tended to arrive on the scene last, where they generally trailed their squad mates at a considerable distance, rarely taking active dives at the adversaries.  However effective these efforts were at initially thwarting the advances of ravens, these cunning corvids almost always returned for multiple passes of the area, apparently testing the intensity of aggression from the angry parents, leading me to suspect that the illicited response to their presence was being used to find nests.  As testament to the genius of these birds, I must point out that we would later use the exact same method when searching for curlew nests.

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