Testing the Limits of Food Limitation Theory

Why do some birds lay fewer eggs than others? Beginning in the 1940s, ornithological studies have investigated the importance of food in limiting the number of young a bird can raise. However, in an upcoming issue of the Proceedings of the Royal Society of London, Cornell Lab of Ornithology researchers Valentina Ferretti and Paulo Llambías and their colleague Thomas Martin, suggest that nest predation is more important in determining clutch size and other life history traits in the Rufous-bellied Thrush (Turdus rufiventris), a common species in subtropical forests throughout Bolivia, Paraguay, Argentina, and Brazil.

From 1997 to 2001, Ferretti and her collaborators studied two populations of this songbird, which has a bright orange-red belly and is closely related to our familiar American Robin. They noticed that the population in subtropical forest in western Argentina’s El Rey National Park had an average clutch size of 2.7 eggs, notably smaller than the 3.2 eggs per clutch found in another population more than 600 miles away in subtropical forest savanna near Buenos Aires.

According to the food limitation hypothesis, when food is scarce, birds should lay fewer eggs per clutch, spend more time looking for food, and feed their nestlings less often. Their nestlings would also be expected to grow more slowly. If instead the number of young is limited by nest predation, clutch size would still be small and feedings few (to avoid attracting attention from predators), but nestlings would be expected to grow quickly, enabling them to avoid predators by leaving the nest sooner.

Based on rates of starvation among nestlings, the researchers found that Rufous-bellied Thrushes in the population with larger clutch sizes—the population near Buenos Aires—were more limited by food than the population in El Rey National Park. Additionally, the researchers conducted experiments by controlling for clutch size, then comparing nests at the two locations. They found that at El Rey National Park the nestlings grew faster—matching the expectations if nest predation were the driving force in limiting clutch size.

According to Ferretti, the results of the study are significant because the researchers compared two populations of the same species and used experiments to test the food limitation hypothesis, whereas previous studies focused on evidence from a single population in different habitats or different species at the same site. These results and other recent studies suggest that factors other than food may be much more important in influencing the lives of breeding birds than previously thought.

—Lindsay Watkins

For permission to reprint all or part of this article, please contact Miyoko Chu, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-2451. email: mcc37@cornell.edu