Heat-conserving postures hinder escape: a thermoregulation-predation trade-off in wintering birds

Wintering birds may conserve body heat by adopting postures with minimal leg exposure or significant ptiloerection. However, maximally heat-conserving postures may hinder a bird's ability to escape attack, leading to a trade-off between predation risk and thermoregulation. Such a trade-off implies that birds should use the most heat-conserving postures only at very cold temperatures. Feeding in a relatively low-risk environment should also facilitate the use of such heat-conserving postures. In this experiment, we examined the effect of thermoregulatory postures on the time to initiate escape in feeding dark-eyed juncos (Junco hyemalis) exposed to a sudden sharp noise. Birds using progressively more heat-conserving postures required significantly more time to initiate flight, with a 50% increase or more for certain cumbersome postures when compared with the most "flight-ready" posture. This increase in the time needed to become airborne likely leads to an increase in the predation risk during an attack. Although this increase in risk could not be quantified, the delays associated with the most heat-conserving postures could allow a hawk to approach approximately 1.7 m closer to its prey during an attack. Postures that limit escape were used to a greater degree when feeding near cover and in cold conditions, lending additional support to a predation-thermoregulation trade-off. However, the effect of flock size was inconsistent and no effect of the nearest neighbor distance on thermoregulatory posture was detected. Overall, the results of this study provide insight into the seemingly intuitive observation that birds use maximally heat-conserving postures only under very cold winter conditions.