Molt-breeding overlap in birds: phenology and trade-offs at the individual and the community levels

Abstract

Life history cycles in organisms represent evolutionary adaptations to selection pressures that act both through environmental and intrinsic factors. As a result, life history cycles are adjusted to temporal patterns in resource availability, in predation and in competition, among others, to maximize individual fitness. For species that live in different environments and that undergo fewer life history stages within a given cycle, the organization of annual schedules may deviate from the widely studied temperate zone species that typically undergo a number of different life history stages in strict sequence and temporal separation.

The study of life history schedules and evolution is mostly concerned with the timing of each life state and the factors that regulate the schedule as well as each separate event. However, it has been noted in various species that not all states are fully separated and that some overlap may occur between them, with only full co-occurrence of states reported to date in birds. I explored the interaction, regulations and consequences of tropical molt- breeding overlap by studying the occurrence of such an overlap stage at individual and community levels as well as comparing controlled laboratory conditions and with the wild setting. I addressed several major questions: How common is the overlap of reproduction and molt in this bird community as a whole? What environmental factors are associated with the overlap? During the overlap, are both life history stages indeed slowed down? Do any trade-offs that may affect fitness arise from the overlap?

By assessing reproductive and molting condition in all individuals, I was able to show the occurrence of overlap between these events as a frequent phenomenon in montane tropical birds, with approximately half of the species studied displaying the overlap not only at a population level, but also within individuals. I outline some environmental factors that might increase the frequency of the overlap in the community, while encountering yearly variations based on precipitation, which could point to phenotypical plasticity in the regulation of the life cycle.

Based on captive work, I show some of the changes and alterations in molt dynamics and behavior that individuals incurring the overlap might face. By comparing zebra finches (Taeniopygia guttata) that were allowed to overlap with those that just molted, I was able to show differences in individual feather growth rates, molt intensity and time budgets. Overlapping individuals undertake slower flight-feather replacement, with out the clear sex-difference that was initially predicted.

Cost and consequences were explored in a wild population of Slaty-brush finch (Atlapetes schistaceus) in Colombia, over a period of four years. In this population individuals in all major states of the life cycle can be found at a given time. For overlapping individuals, I have determined a decrease in feather quality, as well as in flight performance that could affect survival, and therefor fitness, in this long-lived species. Contrary to the captive experiment, and in accordance with prior expectations, sex differences were present in the wild population, with females showing lower quality flight feathers.

The frequency of molt-breeding overlap in montane cloud forests poses significant questions to regulation and mechanisms of control and integration of the life-cycle stages. In this work I have pointed to a clear trade-off when both events occur simultaneously, and I have shown how some individuals might cope with it. Molt-breeding overlap offers a unique opportunity to study life history evolution in both mechanistic and ecological aspects, given its variation in a wide range of organisms and environments.