I don’t know about you, but I can be lazy sometimes. If I have to choose between walking across campus to get a decent cup of coffee or walking across my office to make instant, I’ll probably go with the instant. We make hundreds of these small choices daily, weighing the benefits of different options against their energetic costs.
For us, the outcome of these tradeoffs might be suffering through a bad cup of coffee. But for wild animals, these choices can mean life or death. Take migration. It’s a risky, demanding process, involving moving long distances over inhospitable terrain. For some species, migration is necessary—their habitat becomes too cold or too hot, or doesn’t provide the resources they need year-round. But what about species that live in climates where weather and resources are acceptable, though not ideal, during the winter? These species are often partial migrants, meaning that some individuals spend the entire year in one place while others from the same breeding area travel to distant wintering grounds. But how do these individuals decide whether or not to migrate? Are their decisions hard-wired, or do they change from year to year? Can we predict which individuals will migrate? These are some of the questions motivating my research.
Over the past two years, my team and I have attached GPS transmitters to Brown Pelicans from across the northern Gulf of Mexico. Like the GPS in your phone, these transmitters determine the location of each pelican to within a few meters, and they use solar panels to recharge and continue transmitting over several years. We’ve also gathered individual data on tracked pelicans, including body size, physical condition, sex, and breeding success.
At each breeding colony we’ve found a range of different migration strategies, from year-round residency within the breeding area to migrations of up to 3,000 kilometers. Individuals from colonies with more breeding pairs and more local competition for food are more likely to migrate than individuals from smaller colonies, but this doesn’t explain the high level of variation within colonies.
Since geography alone isn’t enough to predict whether or not pelicans will migrate, we constructed mathematical models to test how individual factors influenced migration. We found that variable characteristics, like body condition and breeding success, weren’t good predictors. The individuals we’ve tracked tend to follow the same migratory routes and winter in the same spots each year. However, fixed factors, like sex and body size, do have predictable effects on migratory behavior. Female birds and smaller pelicans are more likely to migrate than males and larger birds. Breaking down the effects of body size by sex, we see that smaller males are more likely than larger males to migrate, while for females, migratory behavior doesn’t change with size. This pattern exists in territorial birds, where males benefit from arriving early to secure a high-quality breeding territory. Seabirds aren’t usually considered territorial, since it’s hard to control food resources in a marine environment. Our study is the first to suggest that territoriality might actually be a motivating factor for pelicans.
In the future, we hope to test whether there’s a genetic basis for migration. For now, our results can be used to help conservation planning. Understanding how species are distributed is important when preparing for extreme ecological events, especially in sensitive areas like the Gulf of Mexico. We now know that during the winter, female pelicans are vulnerable to events such as oil spills in the southern Gulf, while males might experience high mortality during periods of extreme cold in the northern Gulf under climate change.
So, are some pelicans lazy? Maybe! But I hope that, as a result of my research, we now know just a little more about the reasons behind their behavior.