“Fitness consequences of interspecific nesting associations among cavity nesting birds”
James C. Mouton and Thomas E. Martin (Sep 2018)
Nest tree availability and nest predation risk affect the benefits and rate of nest tree sharing in cavity nesting birds
Some of the most visually striking and exciting sights in nature are large herds, flocks, schools, and colonies of animals. Animals may benefit from being in a group with other species because it can reduce their chances of being killed by a predator, but on the other hand, large groups can attract predators. Competition over resources (i.e. food or breeding sites) between similar species can also cause fighting within the group making it difficult for groups to form. This suggests that similar species that often fight should keep their distance and not form groups. However, birds that nest in holes in trees (cavity nesters) are well known for aggressively attacking one another, especially around nests, can be seen sometimes nesting together in the same tree. How commonly does this occur? Why do they share nest trees in the first place?
James Mouton and Tom Martin set out to answer these questions by looking at whether the number of nests sharing trees changed in years with different availability of nest sites and risk that a predator eats the eggs or offspring). They found that sharing nest trees was more common in years with more nest trees and in years with higher risk of nest predation. They also found that nests in shared trees were less likely to be depredated than solitary nests, but only in years with high risk of predation. Together, these findings suggest that birds may fight less when nest sites are abundant, allowing them to nest close together more often and that birds may nest close together even when then fight if predation risk is especially high. Still, experiments are needed to fully understand these patterns. Ultimately, it seems that even though they are known for fighting, cavity nesting birds tolerate nesting together in the right conditions.
Interspecific aggregations of prey may provide benefits by mitigating predation risk, but they can also create costs if they increase competition for resources or are more easily detectable by predators. Variation in predation risk and resource availability may influence the occurrence and fitness effects of aggregating in nature. Yet, tests of such possibilities are lacking. Cavity nesting birds provide an interesting test case. They compete aggressively for resources and experience low nest predation rates, which might predict dispersion, but we found they commonly aggregate by sharing nest trees across 19 years of study. Tree sharing was more common when aspen were more abundant and somewhat more common in years with higher nest predation risk. Nest success was higher in shared trees when nest predation risk was higher than average. Ultimately, the costs and benefits of aggregating (nest tree sharing) varied across years and we outline hypotheses for future studies.