Abstract
Tree swallow (Tachycineta bicolor) breeding success in Ithaca, NY, USA, over the past quarter century has shown generally healthy fledgling production punctuated by years of high nestling mortality. This study tested the potential effects that temperature may have on the food supply and breeding success of swallows. Data from 17 years of daily insect samples were used to relate flying insect abundances to daily maximum temperatures and to define “cold snaps” as strings of consecutive days when the maximum temperatures did not exceed critical temperatures. The distributions of cold snaps and chick mortality events were investigated both through detailed reconstructions of the fates and fate dates of individual chicks, focused on the three breeding seasons of lowest fledging success, and with less detailed brood-level analyses of a larger 11-year dataset including years of more moderate mortality. Mark–recapture analyses of daily brood survival rate (DSR) reveal very strong support for the effects of cold temperatures on brood survival rates, and all the top models agree on a critical temperature of 18.5 °C for insect flight activity in Ithaca. The individual-level analyses, focused on years of higher mortality, favored a 3-day cold snap definition as the most predictive of DSR effects, whereas the larger-scale brood-level analyses revealed 1- and 2-day cold snaps as having the most significant effects on DSR. Regardless, all analyses reveal that, in an age of generally warmer climates, the largest effect of weather on swallow fledgling production is from cold temperatures.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-013-2605-z) contains supplementary material, which is available to authorized users.
Highlights
Two of the most fundamental linkages in animal ecology are the effects on reproductive success from variation in a species’ food supply and the weather
The 9 and 8 best models at the individual and brood levels, respectively, all included a cold snap threshold of 18.5°, but the 2 best models at the broodlevel analysis included cold snap durations of 1 and 2 days instead of the three-day duration supported by the individual-level analysis. Analyses at both scales indicated that the critical temperature for defining cold snap effects on chick mortality was
It is interesting that swallows seem to be suffering the effects of reduced prey availability as soon as insect availability begins to decline: they are apparently dependent on constant high levels of prey availability, and any deviation from this high plateau in food supply throws a switch to reduced parental care and, at least at the Ithaca sites, emigration from the upland breeding sites to more reliable, and more distant, foraging over large low bodies of water
Summary
Two of the most fundamental linkages in animal ecology are the effects on reproductive success from variation in a species’ food supply and the weather. For many species of birds, the measurement of the food supply can be a significant empirical challenge (Haftorn 1956; Tinbergen 1960; Sherry 1984; Wiens 1984; Holmes and Schultz 1988). The problem of food measurement for Oecologia (2013) 173:129–138 these species that take flying insect prey from the air is made easier by the availability of suction samplers that produce representative samples of available prey at frequent intervals, but this guild of consumers has the parallel challenge that day-to-day fluctuations in air temperatures are associated with dramatic variations in food supply (Hails and Bryant 1979; Emlen et al 1991; McCarty 1995). Aerial insectivores are distinctive in that they are vulnerable to physiological, in addition to demographic, responses in their prey (Bryant 1973)
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