Photo © Phil Scheuer

Why Do Some Birds “Try Harder” to Raise Young?

by Amos Belmaker
PhD candidate, Cornell Lab of Ornithology

Here in the Northeast, the breeding season of 2013 was a hard one. In Ithaca, New York, the poor nesting birds were hit by one storm after another. They barely had a day or two to recover and feed their chicks. I study Tree Swallows, which catch flying insects–if it’s cold and rainy, there are no insects and no food to bring home to the young ones. This season, mortality was high and my spirits were low.

But throughout this whole ordeal I kept noticing something interesting. While some birds were being hit hard, others didn’t seem that affected by it all. Of course, it was hard for them as well, but their chicks were fed and grew nicely and many fledged at the end of the season. Why the difference? How can some birds weather the storm just fine, while others, facing starvation, are forced to abandon the nest? They had access to the same food supply and the same weather, but it just seemed like some birds weren’t “giving it all they’ve got.” I started to think whether there was some biological basis to “giving it all you’ve got”, or “reproductive investment.” My dissertation work, which I’m doing under the supervision of Dr. David Winkler here at the Lab, focuses exactly on that question: What tells a Tree Swallow to invest more or less in reproduction?

I believe the answer is hiding in structures called telomeres, which protect the birds’ DNA (actually, people have them too). They shorten throughout life and, basically, anything the doctor tells you is bad for you is a telomere shortener. When telomeres get too short, they can’t do their job anymore and risk of death increases. So the shorter the birds’ telomeres, the less likely it is to survive. If a Tree Swallow is not going to live long, she might give the current breeding season “all she’s got” because the next one may never come. If she had longer telomeres, she could save energy for later, essentially hedging her bets against a bad year. (We don’t really know how birds perceive this shortening, but it may be in the form of hormone changes.)

Perhaps the birds who are working hard to make a bad year successful are those who have short telomeres and can’t afford to lose? I am still looking for evidence to support this intriguing idea. To do this, I add chicks to Tree Swallow boxes and give the parents a choice: keep investing normally and risk raising fewer chicks, OR increase investment at a survival cost to yourself. If I’m correct, then only short-telomere birds will rise to the challenge and fledge the additional chicks. Through my research, I am asking a very simple question with profound repercussions: What would a Tree Swallow do if it didn’t have much time left?