Tree Swallows Trade off Immune Function and Reproductive Effort Differently across Their Range

Latitudinal differences in life histories are believed to be underlain by differences in trade-offs between current and future reproduction. I report differences in tradeoffs between parent and offspring across the range of a widespread avian species, the Tree Swallow (Tachycineta bicolor). I manipulated parental effort and found that in Alaska, where yearly adult return rates are low, breeding females increase their reproductive investment to maintain offspring quality, whereas in Tennessee, where yearly adult return rates are high, breeding females are either unable or unwilling to increase reproductive investment and, consequently, raise offspring of lower quality. I further investigated a critical mechanism of self-maintenance that may underlie differences in survival among sites: immunocompetence. Females breeding in Alaska mounted weaker immune responses when raising enlarged broods, whereas females in Tennessee did not, further suggesting a strategy in Alaskan females to incur costs (i.e., reduce future reproduction) in order to maintain offspring quality. Insect availability increased as the breeding season progressed in both sites, but more food was available in Alaska than in Tennessee. This is among the first studies to report geographic differences in immune function consistent with life history theory, which predicts that individuals with higher survival probabilities should invest more in self-maintenance.