Trapped between darkness and heat : desert birds (Argya squamiceps) initiate daily activity earlier (and terminate it later) in response to heat

Abstract

Diurnal desert endotherms must adjust the timing of their daily activity to mitigate physiological stress from ambient heat. Understanding the limits of this behavioural plasticity is important for predicting how climate change may affect desert species. Here, we studied the onset and termination of daily activity in wild birds (Arabian babblers Argya squamiceps) inhabiting the Arava Desert. We found that the birds (i) began/terminated their daily activity within a 45 min window around morning/evening civil twilights, (ii) advanced/delayed the onset/termination of daily activity in response to rising ambient heat, yet (iii) were never active in darkness. These temporal adjustments may help to mitigate energetic deficits caused by heat stress and concurrently extend foraging times under cooler temperatures. Though, subsequent analyses of the climate in the Arava Desert demonstrate (iv) a ~2°C increase in summer temperature over the past 31 years, and (v) that Arabian babblers advanced the onset of daily activity by 8.8 min during the last 11 summers. We thus propose the existence of a ‘heat versus dark trap’: rising temperatures drive diurnal desert endotherms to increase activity during the cooler pre-dawn and post-dusk times, but the leeway of these temporal adjustments is strictly constrained by nocturnal predation risk and reduced foraging efficiency in poor light conditions.