INDIVIDUAL AND GEOGRAPHIC VARIATION IN LIFE HISTORY TRADEOFFS IN TREE SWALLOWS AND EUROPEAN STARLINGS
This work investigated individual and geographic variation in life history tradeoffs in tree swallows (Tachycineta bicolor) and European starlings (Sturnus vulgaris) by examining how breeding females make tradeoffs between current and future reproduction. In tree swallows, timing of breeding is a strong predictor of reproductive success, with early laying birds tending to have higher fitness than do later laying birds. Experimental manipulation of offspring numbers in Ithaca, NY, revealed that later-laying females raising enlarged broods showed significantly decreased cell-mediated immune responses to phytohaemagglutinin (PHA) and humoral responses to sheep red blood cells (SRBC) relative to higher-quality females. Nestlings in enlarged broods grew at slightly slower rates, but otherwise nestlings did not differ in offspring quality among groups, suggesting tree swallows are willing to trade immune self-maintenance for offspring quality. The short-term stress of raising enlarged broods had long-term consequences. Females raising enlarged broods in year 1 mounted weaker secondary antibody responses to SRBC than did control females. Most importantly, females in year 2 that showed stronger secondary responses were more likely to return to breed for a third year. A brood manipulation experiment was conducted at the extremes of the tree swallow range: Alaska and Tennessee. In Alaska, breeding females raising enlarged broods increased feeding effort, decreased immune responses and raised offspring in similar quality to unmanipulated broods. However, in Tennessee, females raising enlarged broods did not increase feeding effort and, consequently, did not show decreased immune function and raised lower quality offspring. Tennessee females returned to breed at higher rates than did Alaska females and maintained higher cell-mediated responses than did Alaska females, suggesting different resource allocation patterns. To investigate offspring quality in more detail, a study was conducted to examine factors affecting nestling immune response in European starlings breeding in New Zealand. A split-nest cross-fostering brood manipulation revealed that nestling immune response was influenced by female genetic quality (in part through spleen size), maternal effects (in part through temperature conditions), and parental resource allocation (through residual body mass). Overall, this research adds to our understanding of how individuals differ in the tradeoffs associated with reproduction and how these responses can lead to geographic differences in life histories.
dissertation or thesis