Small birds in temperate climates are presented with the troubling paradigm of how best to survive harsh weather and low food availability in winter months. Optimal foraging rates of small passerines in winter have been extensively modeled with the assumption that birds must replenish fat stores to endure cold winter nights. Likewise, wintering birds have been shown to increase fat reserves to survive the coldest night of the season. Given that birds respond to temperature by increasing fat reserves, we sought to understand the relationship between winter weather conditions and foraging activity. We asked if birds changed visitation rates at feeders to cope with energetic demands during winter weather conditions. We investigated the effects of temperature, wind speed, barometric pressure, and humidity (via LNK weather data) on hourly feeder visitation rates (HVR) and the effect of lowest overnight temperature (LOT) on morning visitation rates (MVR). We recorded 5 species at 8 RFID logger-equipped feeders from January 26-March 10, 2019 in Lincoln, Nebraska, USA. RFID technology allowed us to measure fine-scale foraging effort to best test our hypothesis. We modeled relationships between weather conditions and feeder visitation rates at the individual, species, and community scales using generalized linear and additive models.
Results/Conclusions
We found that hourly weather conditions were significant predictors of HVR at RFID feeders at multiple scales. Barometric pressure, wind speed, and humidity models varied in significance between scales, while temperature remained a significant predictor of HVR at all scales. Temperature was negatively correlated with HVR and explained the most variance in the data of all weather conditions. Consistent with theory, we also found that MVR was significantly negatively correlated with LOT. Interestingly, individual response to LOT was highly variable within and between species. We concluded that although cumulative weather conditions affect HVR, temperature was the primary weather condition that birds responded to. Our results are consistent with theory that birds respond to temperature by increasing fat stores, here by means of increased foraging effort, but we show this relationship at finer time scales than past studies. We suggest that birds increase HVR to cope with low diurnal temperatures as well as replenishing fat reserves for cold nights. Lastly, although MVR was largely driven by LOT, individual response likely varied because of factors like group membership and body size. Our findings support the hypothesis that foraging rates are driven by the energetic demands of thermal stress for small overwintering birds.