Why didn't I think of that? Avian nest predation and parental activity.

Journal Club Every so often, a hypothesis comes along that contains an idea that makes you wonder why no one has thought of it before. A recent paper by T.E. Martin et al. 1 contains such a conceptual leap. In 1949, A.F. Skutch provided an original hypothesis regarding the evolution of parental care behaviour in birds. The hypothesis was that nest predation should increase with increasing parental activity at the nest, thus predicting that parental activity and nest predation show opposite proximate and evolutionary relationships. That is, nest predation is expected to increase with parental activity within species to give a positive proximate cost function, and species or populations in environmental conditions with higher ambient levels of nest predation should evolve lower parental activity, giving a negative evolutionary function across species or populations. There is much support for the negative evolutionary pattern, but apparently little support for the proximate relationship. In line with several previous studies, Martin et al. 1 found that nest predation was no greater during the nestling period than it was during the incubation period, when flights to and from the nest were fewer. This same pattern was seen in all of ten species studied. Previous workers had concluded that results of this kind argued against a proximate cost function between nest predation and parental activity: if increased parental activity during the period when nestlings need to be fed produces no significant increases in predation risk, then it seems unlikely that there should be selection to reduce parental provisioning rates. The great insight of the current work, however, is to see that such conclusions assume that parental activity is the dominant influence on the predation risk of a nest and, in particular, that the site of an individual nest does not influence its risk of predation. By using baited nests from the previous year whose predation history was known, Martin et al. show that the location of a nest has very clear effects on nest predation rates – nests that had a high risk of predation in one year also had a high risk in the subsequent year. Thus, high initial predation rates on easy-to-find nests should cause a higher predation rate during the incubation period than during the chick-rearing period (when predators have found most of the nests in vulnerable sites). The fact that neither Martin et al.'s study, nor the previous works found such an …