TIELEMAN, B.I.; WILLIAMS, J.B.; RICKLEFS, R.E.; University of Missouri – St. Louis; Ohio State University, Columbus; University of Missouri – St. Louis: Why do incubation periods vary in length? A test in the tropics

The paradoxical observation of long incubation periods in environments with high nest predation has puzzled scientists because a clear benefit of an extended embryonic development period has not been identified. A potential explanation for this paradox can be deduced from life history theory that predicts that parent birds accept an increased risk of mortality to their clutch from longer incubation if this results in a reduced risk of mortality to themselves. A critical assumption underlying this idea is that parents increase their risk of mortality when they spend more time at the nest. We exploited the large variation in incubation periods in the tropics to test the hypothesis that reduced nest attentiveness (% of daytime on the nest) results in lower average egg temperatures and longer incubation periods. Studying 14 species of birds in the humid lowland tropics, we found that increased nest attentiveness correlated with higher average egg temperatures. However, neither nest attentiveness nor average egg temperature were associated with the length of the incubation period. Therefore, we found no support for the idea that the costs of an extended incubation period may be balanced by an increased survival of incubating parents, as predicted by life history theory.