Last night a late season nor’easter slammed into Ithaca, dropping 6 inches of wet snow (with another 6+ predicted for today), breaking branches, bringing down powerlines, and creating hazardous travel conditions. In fact, the Lab of Ornithology had to close its doors today and send the masses home because the power went out in Sapsucker Woods, so this little nuthatch is now happily ensconced four feet from a wood stove pumping out heat. To a nuthatch or a chickadee, this storm is little more than a blip in a winter filled with single-digit temperatures, blizzards, and heat-stealing winds. But to the early season migrants this storm represents a real challenge.
(Song Sparrow catching freshies between wind gusts)
Early spring brings the first waves of migrants to central NY, starting with the partial or nomadic migrants (like American Robins and Eastern Bluebirds), a variety of blackbirds, a smattering of sparrows, and the venerable Eastern Phoebe. And there are three important factors that these early season migrants gamble on to make their migration successful:
(Male American Robin facing the wind)
There’s a reason why these first migrants are not small flycatchers or warblers–they have to be resilient in the face of a spate of cold, wintry weather, and able to eke out an existence when the weather turns sour. But let’s not fool ourselves: there’s also a reason why these first birds aren’t winter residents, and an unusually late storm of uncommon strength–or a series of late storms–can result in their demise. A graphic example involving swallows is described over at Monarch’s Nature blog, and we have received several calls at the Lab of O from concerned nestbox owners regarding the plight of early season tree swallows.
So, if it’s such a craps shoot, why take the chance?
Well, as the saying goes, the early bird gets the worm. The first birds to arrive get first crack at the available habitat, staking their claim and marginalizing the latecomers. The importance of these claims cannot be understated: in many species, females tend to choose the males that have the best territory, and the best territories tend to offer the greatest amount of resources to the adult birds and their rapidly growing fledglings. Since both of these influence a bird’s “fitness” (or how many offspring (i.e. genes) it passes on to the next generation), you would expect there to be strong selection on walking that razor-thin wire between arriving too early (and dying) and arriving too late and having less or no offspring because the prime real estate has been taken. Sort of a “damned-if-you-do, damned-if-you-don’t” situation.
(Male American Robin taking shelter from the storm in a black alder thicket)
This evolutionary trend can also interact with migration strategies and changes in resources over time to exploit new habitat. The best example of this is the Blackcap, a European bird that traditionally winters in northern Africa, migrating across or around the Mediterranean. Migratory behavior is inherited, and can show variation such that small percentages of many populations wind up migrating in the “wrong” direction, which is often a dead-end genetically (imagine, for example, the short lifespan of a Red-winged Blackbird that migrated north in the fall!)
In the case of the Blackcap, over the last 50 years there has been a new wintering population that has arisen to the northwest in England, and its continued persistence there is tied to bird feeders (or bird tables, as they’re quaintly referred to there). This migrating population has a shorter, easier migration than the continentally migrating Blackcaps, and as a result these English immigrants arrive on the breeding grounds earlier, in better condition than their North African counterparts, and produce more young (as scientist Stuart Bearhop, of Belfast, describes it: “It’s a bit like getting their towels on the best sun-loungers first.”) Interestingly, these Big Ben Blackcaps appear to prefer punctual Blackcaps from Brittania, and this reproductive isolation has been suggested to be the first step towards the divergence of one species into two.
(Eastern Phoebe basking in the calm before the storm)
In any case, it can be heartwrenching to watch a Phoebe pump his tail as the snow piles up and he patiently waits for something to eat. These early season ambassadors are like extreme sports junkies, living on the edge, and from time to time something more extreme occurs that millions of years of evolution hasn’t quite gotten dialed in. Although I know it happens, it won’t stop me from rooting for them anyway…