Another field season bites the dust

As the remnants of a thunderstorm trickle down upon our cabin and the sun reclaims its dominance in the subarctic sky, I am left to contemplate a field season that is drawing to a close. The past few weeks flew by in an instant. The females arrived in late May and, just over a month later, most have already gone through the nesting process and are in the final stages of raising independent offspring – a time we call the fledgling period. The breeding season is almost over, and the birds will soon begin molting (some already have) and otherwise preparing for the long migration south.


Of course, my thoughts about orange-crowned warblers in Alaska cannot stand alone; they are always with reference to Catalina Island. Our study is a comparative one, and with that in mind I will use the remainder of this post to ramble about the facets of the Alaskan population that interest me most.

Time. These birds are sure in a hurry. They build their nests quickly and – almost literally – place all of their eggs in one basket, with little opportunity for a successful re-nest if they are unlucky enough to lose the predation lottery. That means they have to breed as soon as they get here. That also means that the people who study them don’t have to sit around making bets about which month they will start breeding. Time is of the essence, and these birds don't mess around.

Daylength. Even though the number of days may be limited, the number of hours per day is much less constraining. Sure, the sun dips below the horizon for a few hours each night, but darkness never befalls these birds on their subarctic breeding grounds. As it turns out, this phenomenon affects graduate students and birds in a remarkably similar way: an increase in the number of work-hours and a decrease in the number of sleep-hours. Good thing the breeding season doesn't last long.

Clutch Size. Females lay more eggs in the north than they do in the south. Alaskan clutches typically consist of 5-6 eggs, while Californian ones are much smaller, with 3-4 eggs. Orange-crowned warblers here don't build larger nests, so as the nestlings grow the space they share becomes especially tight. It seems that this is a price the parents are willing to pay to further the odds that their genes will make it to the next generation. Not a bad strategy when you have to survive a journey all the way to the Gulf coast and back before you can breed again.

Food. Theory has it that more food is available during the summer months the further you get from the tropics. I observed a year of complete reproductive failure on Catalina Island last year, so surely I can attest to that. But I have to say that there was less food here than I expected. My fellow graduate students who have been here in previous summers tell me that this year was unusual. We haven't finished processing our insect samples, but our guess is that food abundance was low this year. As someone who is interested in population fluctuations and their affect on higher-levels of the food chain, this little factoid interests me greatly. But at this point I have no idea what would be causing the low insect abundance.

Predation. Warblers here don't have to worry as much about nest predators. The Alaskan climate is too extreme for snakes, but they do still have to contend with foes of the avian and mammalian variety - namely, jays, magpies, mice, and voles. Still, in the end the predation rate is lower - a nice reward for migrating all the way to a site with such a short breeding season.

Density. Birds in Alaska range over a much larger area than their southern counterparts. On Catalina, the birds are so packed that you can walk 20 m from one nest to another, but here a few hundred meters can separate the core of neighboring territories. In all likelihood, this has something to do with migration strategy. Adults here incur much higher rates of mortality during the non-breeding season, which limits the number that make it back to breed each year and in turn how densely packed they are on the landscape. For the birds this means that they have fewer neighbors to compete with, and for us this means that we have to survey a much larger area than on Catalina.

In the end, orange-crowned warblers at the two extremes of their breeding range are more alike than they are different - for example, in their reliance on leaf-gleaning for acquiring food - but to us it is the differences that are the most interesting. Warblers here are not the same as warblers in California. Geography matters. These are things I can attest to after five months following the same species through the cactus patches of Catalina Island and the marshes of central Alaska - a long and exhausting field season, but also a rewarding one.