Blog written by Suzanne Austin & Doug Robinson. Read the full paper here.
Why do lowland tropical birds have longer incubation periods than temperate species? This question has received substantial attention in recent decades with a large amount of research on a hypothesis known as the predation paradox. This hypothesis suggests that the longer tropical incubation periods are a consequence of parents reducing their own time at the nest to reduce chances predators will find the nests by seeing the parents coming and going. Nests of tropical songbirds are, on average, lost to predators more often and more quickly than those of temperate songbirds. For example, an average songbird nesting in the humid lowlands of Panama will have to nest about four times before they fledge offspring whereas a typical temperate songbird may need to nest only once or twice to fledge kids.
As a consequence of less parental time at the nest, egg temperatures drop, which then increases the time required for embryos to develop. Despite these ideas, there have been few data gathered on actual time spent incubating by most species of tropical birds. We documented incubation behavior by 112 species of tropical and temperate songbirds with video cameras plus some data from the existing literature.
To understand how adults were allocating time to incubation of eggs, we compared multiple variables that described adult incubation behavior including constancy (the total proportion of time that adults spend incubating), visit rate (the number of trips by parents to the nest in an hour), on-bout length (the amount of time parents spent incubating their eggs in an individual visit), and off-bout length (the time parents spent off of the eggs during an individual recess). We also compared each parental attendance variable to a set of environmental, natural history and life-history characteristics, such as nest predation rate, egg mass, adult mass, clutch size, adult incubation strategy (uni-parental or bi-parental), nest height, and nest type (open-cup, enclosed-cup, or cavity).
Despite longer tropical incubation periods, we found that the proportion of time (constancy) spent incubating was not different for tropical and temperate birds. When we adjusted constancy to reflect the differences in day length between Michigan and Panama, we still found no difference associated with latitude in constancy. This result indicates that birds were spending a similar proportion of time incubating regardless of latitude (median= 71-75%).
However, when we investigated how birds allocated their incubation time, we found that tropical songbirds typically stayed longer on their eggs during each incubation bout (on-bout length), took longer recesses away from their eggs (off-bout length) and made fewer trips to their nests (visit rate). Lowland tropical species were engaging in a different incubation strategy than temperate songbirds.
While our finding of similar constancy across latitudes was inconsistent with the predation paradox, our data indicated that visit rate and off-bout length followed the pattern predicted by the predation paradox, but on-bout length didn’t. The expectation of lower constancy and shorter on-bout length, which were proposed to explain the longer tropical incubation periods, did not occur in our sample of tropical birds.
Our data did reveal that nest predation was related to several attendance variables, but in directions inconsistent with those suggested by the predation paradox. Instead, birds subject to higher nest predation rate tended to spend more time on the nest rather than less time. We also found that incubation period was not related to any of our attendance variables, which suggests that how eggs were incubated didn’t explain the variation in incubation period length in the songbird species in our dataset.
We suggest that parents have optimized their incubation strategy to limit fitness costs to their offspring and to themselves, which may explain why constancy is fairly consistent within species. Yet, how birds allocate their time incubating their eggs varies across latitude. We suggest that there is an interaction between the environment and the needs of the embryo, which influences the strategy parents use to incubate their eggs. For example, the higher, more stable ambient temperatures of the lowland tropics likely allow parents to spend longer away from their eggs than temperate parents because the cooling rate of eggs is lower. These data add to the growing body of literature that suggests nest predation does not explain the observed differences in embryonic development across latitudes. Instead, much of the variation might be influenced by intrinsic differences in embryos during development, such as latitudinal differences in construction of immune systems, morphological attributes and degree of maturity at hatch.