It's long been thought that many birds make decisions during the breeding season—such as whether to attempt nesting twice with their mate, or whether to cheat on their mate—based on the other birds around them, or social factors (breeding density, or how much competition there is for mates).
New research, published today in the journal Proceedings of the Royal Society of London B: Biological Sciences, now shows that Black-throated Blue Warblers are reading cues from their surrounding environment to make decisions on when to remain faithful and when to be promiscuous. The study found these decisions can be based on food availability.
The study found that a sudden increase in food resources (for example, caterpillars) can cause male Black-throated Blue Warblers to spend more time at home with their mate, guarding her from other males that seek to breed with her.
"Our study experimentally demonstrates that changes in food supplies affect how birds invest in extra-pair mating," said Dr. Sara Kaiser, who conducted the research as a graduate student at the Cornell Lab of Ornithology and was the study’s lead author. "In this case, when food constraints are reduced, males allocate more to behaviors that reduce cuckoldry at home at the expense of siring offspring with additional females, especially when abundant food increases the likelihood that their mate will attempt two broods."
Black-throated Blue Warblers are small Neotropical migrant songbirds that migrate from wintering grounds in the Caribbean to the forests of the northeastern United States/southeastern Canada to breed. Like clockwork, they arrive in the White Mountains of New Hampshire (where Kaiser conducted her research at the Hubbard Brook Experimental Forest) during the first week of May every year.
Kaiser's study examined whether black-throated blues were so rigid in other aspects of their breeding behavior. Specifically, she investigated whether Black-throated Blue Warblers adaptively adjust their breeding strategy to changes in food availability.
It is known that reproductive success for these birds is limited by food availability. When caterpillars are scarce, these warblers will stick to raising a single brood. In times of caterpillar abundance, males and females attempt a second brood after raising a first brood.
Males can further increase their reproductive success through promiscuity, or breeding with females outside their pair bond. But searching for these "extra-pair" mates requires time and energy and comes with tradeoffs—namely, taking away from time the male could be back at home feeding his chicks or guarding his female from other intruding males. In times of food abundance when lots of males can afford to spend their time carousing, the promiscuous male who’s not at home can end up being a cuckold.
Kaiser conducted a large-scale experiment over four breeding seasons where she provided supplemental food (a tray of mealworms and waxworms placed near nests) to Black-throated Blue Warbler pairs throughout the breeding season, to see if the increase in food availability would affect the breeding strategy.
Sure enough, males that began their breeding season in a caterpillar-scarce area—then received a bounty of mealworms and waxworms—had a different strategy than males without food trays. These males stayed close to home to guard their mate and defend their territory from intruders, rather than pursue other females, and were more likely to attempt a second brood with their mate, just as a male in a caterpillar-rich area would act. Females living on territories with plenty of food also appeared to be less promiscuous than females with little food.
In other words, in this unique scenario where a food-poor habitat suddenly becomes a food-rich habitat, the male and female Black-throated Blue Warblers adjusted their behaviors in a way that increased their reproductive success.
"In order to predict the future biotic consequences of environmental change, we need to know how species respond to dynamic changes in their environment that affect the quality of breeding habitat," says Kaiser. "Our findings indicate that this bird has the capacity to evaluate the resource conditions around them and respond accordingly. Our research also suggests that environmental change may have subtle consequences for reproductive behavior of these and possibly other similar Neotropical migrant songbirds."
The paper is co-authored by Dr. Scott Sillett, a Senior Research Scientist at the Smithsonian Migratory Bird Center, Benjamin Risk, a Ph.D. candidate in the Department of Statistical Science at Cornell University, and Dr. Michael Webster, a Full Professor in Department of Neurobiology and Behavior at Cornell University and Director of Macaulay Library at the Cornell Lab of Ornithology.
A PDF copy of this research paper, entitled “Experimental food supplementation reveals habitat-dependent male reproductive investment in a migratory bird,” can be viewed and downloaded. This research was funded by the National Science Foundation, the Environmental Protection Agency, the Smithsonian Institution, the American Ornithologists’ Union, and the Cornell Lab of Ornithology.