A new theory of clutch size evolution: Consequences of morphology at fledging on mobility and survival interacting with parental energy expenditure (FMR)

University Of Montana, Missoula MT

Investigators

Abstract

How hard should parents work to raise offspring, and how does this work effort influence the number and survival of offspring that parents attempt to raise? These are questions that have intrigued scientists for more than a century, and have major implications for population dynamics of wild animals. These questions have been highlighted among songbirds because species vary extensively in the number of young that parents attempt to raise. Even among species that breed in the same habitat, such as high elevation riparian habitat in Arizona where work will be conducted, species vary extensively in the number of offspring they attempt to raise. For example, a green-tailed towhee in that habitat typically lays 3 or 4 eggs, a red-faced warbler lays 4 or 5 eggs, while a mountain chickadee may lay 8 eggs. A new theory to explain this variation based on risk of mortality to offspring and parents is proposed and this grant will fund tests of the new theory. Species differ in the risk of their young being eaten by predators, with the open cup nests of the towhee or warbler experiencing high predation, while the cavity nests of chickadees experience very low risk of predation. These differences in predation risk affect how long the young stay in the nest; species like the towhee and warbler leave their risky nest 8 to 11 days after hatch, while chickadees do not leave their safe cavity nest until 20 to 22 days after hatch. As a result, the towhee and warbler young cannot fly when they leave their nest, while the chickadees are fully flight capable when they leave. These differences in developmental state of the young when they leave the nest also potentially affect how quickly they start feeding themselves, with the chickadees potentially feeding themselves within a few days after leaving the nest, while the towhee may take 3 weeks to develop this ability. Potentially, towhee parents must work much harder for each young to provide all their food compared with the chickadee where young almost immediately start obtaining some of their own food and can follow parents to reduce their travel. The extent to which parents are willing to work differs among species depending on their risk of mortality over their annual cycle. Taken together, these effects may explain why species differ in the number of young they attempt to raise, and has critical consequences for population dynamics of species. The proposed work will use observational and experimental approaches to testing these hypotheses. The fledgling period (after offspring leave the nest) is a critical period for demography, but interspecific variation in survival is unknown and documentation will aid understanding of population vulnerabilities to changing environmental conditions. Given the documented economic and aesthetic importance of birding to society, this conservation knowledge is important. Three graduate students whose thesis research will be supported by this project will be trained. Each year, 1 technician and 6 undergraduates will be trained in field techniques and the conduct of science, as well as 8 undergraduates from across disciplines (in and outside of biology) to transcribe videos of parental behaviors. The latter provides outreach to engage diverse students in scientific principles. Students will be educated through discussion sessions that will be used to teach the scientific principles for the work being conducted. To identify possible recruits, we will use broad advertising approaches to further enhance the diversity of social and cultural groups from which we can recruit. Finally, nests provide a rich source of video opportunities to demonstrate differences among species in social interactions, as well as intriguing videos of predation events. The researcher started obtaining short, high definition videos to develop an educational series on parental and offspring behaviors and predation events to put on YouTube for public outreach. PBS Nature is filming a documentary on nest architecture and avian family life that will include footage at the focal field site to further increase public outreach.