Geographical variation in plasma bactericidal ability of breeding tree swallows (Tachycineta bicolor)

Immune defenses may vary across environments due to differences in life history or abiotic and biotic characteristics, but few studies have compared the strength of immune responses across broad geographical gradients. Here we investigated how bacteria killing ability (BKA), a measure of constitutive innate immune function, varies among populations of tree swallows (Tachycineta bicolor) inhabiting different environments. We hypothesized that populations living in unpredictable environments, which also have shorter breeding seasons and thus fewer opportunities to renest in the event of reproductive failure, would invest in reproduction at the expense of immunity. To test this hypothesis, we measured BKA and monitored reproductive activity in four populations located across most of the breeding range of tree swallows, in Tennessee, New York, Wyoming and Alaska. Consistent with our predictions, BKA was lowest in Alaska and Wyoming and highest in Tennessee. In most populations BKA was also lower during incubation than during nestling provisioning, and lower in males than in females. These results suggest that the high reproductive investment seen in populations experiencing more unpredictable breeding conditions may come at a cost to immune function. While reduced immune investment could increase susceptibility to parasites, this potential cost is likely offset by lower parasite pressure in these environments, limiting the need for maintaining strong constitutive innate defenses like BKA.