Parental care and offspring competition in Tree Swallows

TitleParental care and offspring competition in Tree Swallows
Publication TypeThesis
Year of Publication1986
AuthorsStein MF
Academic DepartmentSchool of Natural Resources and Environment
DegreeMaster of Science
Number of Pages44 pp.
UniversityUniversity of Michigan
CityAnn Arbor, MI

In the northern lower peninsula of Michigan, Tree Swallow females apparently invest more in parental effort than males. Females bring significantly more food to the nest. The percent of food brought by the male increased with larger broods. Additional data from this study and the literature indicated that incubation is done exclusively by the female; nest construction and brooding are either exclusively or primarily female tasks; and females invest more in gamete production than males. It is not clear whether territorial defense, guarding/defense of the young, and nest sanitation were performed more by one sex than the other. The only tasks performed exclusively by the male were generalizable rather than offspring-specific: guarding/defense of the eggs and provisioning the female. This supports Trivers' hypothesis that in monogamous bird species, males provide less care than females due to the possibility of obtaining additional matings. Tree Swallows appear to be using begging intensity as a proximate cue in determining an optimal distribution of food to their offspring. In 9 of 15 nests observed, there were differences among chicks in begging intensity. At 14 of 15 nests, though, there were no differences among chicks in total number of feedings received. Neither growth rate nor maximum size were correlated with begging intensity. In all cases of apparent starvation, however, dead chick(s) were the youngest in the brood. It appears, therefore, that while differences among chicks in competitive ability may always exist, these differences result in an uneven distribution of food within broods only during food shortages.