As species shift their ranges and phenology to cope with climate change, many are left without a ready supply of their preferred food source during critical life stages. Food shortages are often assumed to be driven by reduced total food abundance, but here we propose that climate change may cause short-term food shortages for foraging specialists without affecting overall food availability. We frame this hypothesis around the special case of birds that forage on flying insects for whom effects mediated by their shared food resource have been proposed to cause avian aerial insectivores' decline worldwide. Flying insects are inactive during cold, wet or windy conditions, effectively reducing food availability to zero even if insect abundance remains otherwise unchanged. Using long-term monitoring data from a declining population of tree swallows (Tachycineta bicolor), we show that nestlings’ body mass declined substantially from 1977 to 2017. In 2017, nestlings had lower body mass if it rained during the preceding 3 days, though females increased provisioning rates, potentially in an attempt to compensate. Adult body mass, particularly that of the males, has also declined over the long-term study. Mean rainfall during the nestling period has increased by 9.3 ± 0.3 mm decade−1, potentially explaining declining nestling body mass and population declines. Therefore, we suggest that reduced food availability, distinct from food abundance, may be an important and previously overlooked consequence of climate change, which could be affecting populations of species that specialize on foraging on flying insects.
Long-term monitoring field methods
We monitored a box-nesting population of tree swallows at the Queen's University Biological Station in southeastern Ontario, Canada (44.521° N, 76.385° W) regularly during the annual breeding season (May to July) from 1975 to 2017. Nest-boxes in this population are arranged in grids to mimic the natural distribution of cavities . From 1983 to 2017, we captured adults during breeding. Adults were sexed based on the presence of a brood patch (female) or cloacal protuberance (male). In most years, we also measured adult body mass and wing chord. From 1977 to 2017, nestlings were ringed at 10–16 days old, at which time their body mass and wing chord were also measured. Across the dataset used in this study, we measured 1437 females, 585 males and 10 232 nestlings.
Field methods monitoring nestling development in 2017
We expected that slowed nestling growth and changing patterns of adult body mass evident in the long-term data might be explained by changing local weather patterns. To elucidate associations between nestling growth and daily weather conditions, we closely tracked the growth of 445 nestlings from 91 nests from 25 April to 25 July 2017. We measured nestling body mass every other day from 2 to 12 days old (i.e. at days 2, 4, 6, 8, 10 and 12, n = 1723). We did not measure nestlings on days when it was raining so heavily that we would be unable to keep them dry, resulting in some gaps in the measurement records. At 12 days old, we fitted nestlings with a numbered aluminium Canadian Wildlife Service ring.
We caught adults between day 10 and 12 of incubation using a combination of mist netting (males and females) and hand trapping techniques (females). At this time, we fitted each female with a passive integrated transponder (PIT) tag embedded in a leg band. On day 10 of nestling development, we used radio-frequency identification (RFID) at the entrance to the nest-box to determine parental provisioning rates using the set-up developed in . Provisioning rates (i.e. numbers of visits by the parent to the nest-box) accurately reflect the amount of food nestlings received because tree swallows bring similar amounts of insects each trip . We were unable to deploy RFID readers on rainy days. We were able to measure provisioning rates for 55 females.
We compiled wind speed data from weather stations at the Queen's University Biological Station and all other weather data from Environment Canada's Hartington Court weather station (approx. 30 km from the site) . Variables included maximum daily temperature, daily rainfall and mean wind speed during hours of active foraging (5.30–20.00).