Immune activity in temperate and tropical house sparrows: a common‐garden experiment

We hypothesized that Neotropical passerines would invest more in costly immune function relative to north‐temperate passerines, due to differences in their respective life histories. We further hypothesized that latitudinal variation in immune activity would persist in common‐garden conditions. To test these hypotheses, we compared immune function, measured via phytohemagglutinin (PHA)‐induced wing‐web swelling in both wild House Sparrows (Passer domesticus) and House Sparrows kept under common‐garden conditions for 18 months. We found that wild Neotropical sparrows had relatively stable immune responses across the year, whereas wild north‐temperate sparrows showed substantial seasonal variation in immune activity, having lower responses than Neotropical birds during the early breeding season and significantly higher responses than Neotropical birds during the late and nonbreeding seasons. Latitudinal differences in immune responses were not related to mass, sex, or body condition, but were influenced by mass change 24 hours after immune challenge. Under common‐garden conditions, birds from both populations first decreased (after five months) and then increased (after 18 months) their nonbreeding immune responses relative to wild values, indicating condition dependence in the PHA response. Relative differences in the PHA response between the populations, however, were maintained in captivity: after 18 months in common gardens, north‐temperate sparrows exhibited stronger nonbreeding immune responses than Neotropical sparrows.