A central tenet of ecoimmunology is that an organism's environment shapes its optimal investment in immunity. For example, the benefits of acquired (relatively pathogen specific) versus innate (nonspecific) immune defenses are thought to vary with the risk of encountering familiar versus unfamiliar pathogens. Because pathogen communities vary geographically, individuals that travel farther during seasonal migration or natal dispersal are predicted to have higher exposure to novel pathogens, and lower exposure to familiar pathogens, potentially favoring investment in innate immunity. During the breeding season, migratory animals’ exposure to familiar pathogens should increase, potentially favoring investment in acquired immunity. We hypothesized that song sparrows Melospiza melodia adjust their constitutive immune profiles in response to risk of encountering novel versus familiar pathogens. We predicted that individuals migrating longer distances (inferred from stable hydrogen isotope analysis of claws) and less philopatric individuals (inferred from microsatellite assignment testing) would rely more heavily on acquired than innate defenses. We also predicted that reliance on acquired defenses would increase throughout the early breeding season. Consistent with trade‐offs between acquired and innate defenses, levels of immunoglobulin Y (acquired) varied negatively with macrophage phagocytosis activity (innate). Levels of acquired relative to innate immunity did not vary significantly with migration distance or philopatry, but increased throughout the early breeding season. Macrophage phagocytosis was not significantly repeatable between years. Song sparrows appear to shift from innate defenses immediately after migration to acquired defenses with increasing time at the breeding grounds. These patterns highlight the plasticity of constitutive immune defenses in migratory animals.
Study site and field methods
Study subjects were 100 song sparrows breeding on land owned by the Queen's University Biological Station, at a site near Newboro, Ontario, Canada (44.633°N, 76.330°W). We captured subjects in seed‐baited Potter traps between April 15–May 14, 2013 (N = 54) and April 14–May 9, 2014 (N = 60, including 14 individuals captured in both years). Capture dates corresponded to the time shortly after spring migration (first return to the breeding grounds: April 3 and April 1 in 2013 and 2014, respectively) through early nesting (first egg date: May 14, and May 8 in 2013 and 2014, respectively). We ran traps between 07:00 and 10:30 each day, checking each trap at least once per hour.
We collected up to 200 μL of blood for immune and genetic analyses by brachial venipuncture, the first time each bird was captured. We collected blood samples using sterile techniques (Millet, Bennett, Lee, Hau, & Klasing, 2007), and to minimize effects of handling stress on immune response (Buehler et al., 2008a), we sampled within 8 min of researchers approaching the trap. We measured mass to the nearest 0.1 g using a spring‐loaded scale, measured tarsus length and unflattened wing length to the nearest 0.1 mm using dial calipers, and determined sex based on the presence (male) or absence (female) of a cloacal protuberance. We used previous years’ banding records to categorize birds to age class (i.e., second‐year, hereafter SY, or after‐second‐year, hereafter ASY; see Kelly et al., 2016 for details). We clipped a sample of claw tissue for stable‐isotope analysis of overwinter latitude (details below), outfitted the bird with a numbered USFWS aluminum leg band and a unique combination of three colored plastic leg bands if not already banded, and released the bird at the site of capture. All subjects were recaptured or resighted later in the season, suggesting that all were resident breeders. Animal procedures were approved by the Animal Use Subcommittee at the University of Western Ontario (protocol 2008–054).