| Summary: | Throughout North America, many species of aerial insectivorous birds – birds that capture flying insects – have exhibited sharp declines, with tree swallows (Tachycineta bicolor) experiencing a 2.8% annual decline in Canada since 1989. The timing of these declines coincides with major changes in agriculture, perhaps signaling a potential causal link. Declines may be influenced indirectly by agriculturally-driven reductions in the abundance of the swallow’s main diet of aerial insects through increased agrochemical use, wetland drainage and cropping intensity. My objectives were to determine how aerial insect abundance and biomass vary across an agricultural landscape in the Canadian Prairies, and determine how breeding tree swallows respond in terms of (i) foraging behaviours and return rates, (ii) body condition, and (iii) selected physiological responses (i.e. oxidative stress and feather corticosterone). Artificial nest-boxes were erected at each of 5 sites with varying levels of agricultural intensification in south-central Saskatchewan, 2012-2014. Insect abundance and biomass were monitored daily using passive aerial samplers at each of the sites throughout the breeding season. Tree swallows adults (n = 596) and nestlings (n = 1107) from each nest were captured, banded, measured and blood and feather sampled to determine condition and physiological status. In 2014, radio frequency identification (RFID) technology was employed at a subset of nest-boxes to determine adult foraging rates and durations.
Despite strong differences in land use characteristics among sites, insect abundance and biomass varied by year but were similar between heavily cropped and grassland references sites. Regardless, birds responded to temporal changes in prey abundance and differences were found in measurements of foraging behavior and physiological measures of oxidative stress. Adult foraging rate (number of nest visits/hour) was primarily influenced by wind and nestling age, but was also positively related to daily insect biomass. Foraging intervals (i.e., length of time spent on each trip) differed between agricultural intensities; birds on agricultural sites spent more time away from the nest box, presumably foraging, suggesting reduced nest attentiveness. Return rates were primarily predicted by body condition and brood size from the previous breeding season, but were also higher on grassland sites, suggesting that agricultural sites may be less favorable. Older adults (based on banding records) and females displayed a higher body condition than did younger individuals and males, respectively. Measures of physiology (i.e. total antioxidant counts and oxidative damage) in both adult and nestling swallows were primarily driven by temporal changes in insect biomass and abundance. Similarly, adult body mass and physiology were related to local insect availability, but not land use (agriculture/grassland). Measures of nestling feather corticosterone were unrelated to any of the explanatory variables. These results provide valuable information pertaining to components of individual quality, foraging behaviour, and return status in response to prey availability under differing agricultural land use regimes, which may help to inform conservation strategies for a variety of insectivorous bird species of conservation concern.
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