Migratory birds move through multiple habitats and encounter a diverse suite of parasites. This raises concern over migrants’ role in transporting infectious disease between breeding and wintering grounds, and along migratory flyways. Trade‐offs between flight and immune defenses could interfere with infected individuals’ migratory timing and success, potentially affecting infection dynamics. However, experimental evidence that parasitic infection affects migratory preparation or timing remains scant. We hypothesized that birds encountering hematozoan parasites shortly before migration incur physical costs (reduced body condition) and behavioral costs (delayed migration), due to the infection itself and/or to the demands of mounting an immune response. We experimentally inoculated song sparrows (Melospiza melodia) with Plasmodium shortly before fall migration. We monitored infection and body composition for 2 weeks after inoculation, and used radiotelemetry to track timing of migratory departure for another 7 weeks after release. Inoculated individuals that resisted infection had lower lean mass 12 days post exposure, relative to controls and infected individuals. This suggests trade‐offs between body composition and immune defenses that might reduce migration success of resistant individuals. Despite group differences in body composition prior to release, we did not detect significant differences in timing of migration departure several weeks later. Thus, malarial infection did not appear to incur detectable costs to body composition or to migratory timing, at least when exposure occurs several weeks before migration. This study is novel considering not only the costs of infection, but also the costs of resisting infection, in an experimental context.
Study subjects were 38 adult (after‐hatch year) song sparrows (Melospiza melodia melodia) captured on their breeding grounds in southern Ontario, Canada. Previous research on nearby populations of song sparrows suggests that individuals breeding in southern Ontario vary substantially in their overwinter latitude, ranging from as far south as Florida to as far north as New York (Kelly et al., 2016). We captured sparrows on their breeding territories between July 5th, 2016 and August 24th, 2016, using mist nets and playback of conspecific song. Birds were captured at two field sites: Elginfield Observatory (43.191 N, 81.315 W; nine males, three females) and the University of Western Ontario campus (43.009 N, 81.282 W; 20 males, six females).
After capturing each bird, we determined sex based on the presence (male) or absence (female) of a cloacal protuberance, supplemented by measuring unflattened wing length to the nearest 0.1 mm with dial calipers. We weighed each individual to the nearest 0.1 g using a spring scale at the time of capture and collected a small (∼25 μL) blood sample by brachial venipuncture to assess hematozoan infection status as described below. We transported birds to the Advanced Facility for Avian Research at the University of Western Ontario, and housed them indoors in vector‐free rooms maintained between 20 and 22°C. Birds were kept in individual cages (39 × 34 × 42 cm) under a light schedule mimicking the natural photoperiod (ranging from 15 hr light:9 hr dark [15L:9D] on July 5th, 2016 to 12 L:12 D on September 29th, 2016) and had ad libitum access to water and food (parakeet seed supplemented with Mazuri Small Bird Maintenance chow). Birds were captured under a Scientific Collecting Permit from the Canadian Wildlife Service (CA 0244). All animal procedures were approved by Western University's Animal Use Subcommittee (protocol # 2016–017).