Context‐dependent costs and constraints of begging and non‐begging activity by common grackle nestlings at the scale of the nanoclimate

Abstract

Summary Environmental factors such as heat and solar radiation directly affect the open‐cup nest environment and can impact nestling body temperature. The aim of our research was to understand how open‐cup passerine nestlings use behavioural thermoregulation to mitigate solar heat gain and to measure the metabolic cost of begging and non‐begging activity under three light treatments: room light (<2 Wm−2), simulated shade (500 Wm−2) and simulated sun (1000 Wm−2). Our study coupled behavioural (field) and physiological (laboratory) experiments to explore the adaptive behavioural response of passerine nestlings to nest cup nanoclimates (sun and shade), using the common grackle (Quiscalus quiscula) as a model species. Non‐begging behaviour was evaluated in the field by randomly assigning nestlings to a homogeneous (all sun) or heterogeneous (sun and shade) nest environment; measuring body temperature (Tb); and identifying behaviours from videotaped footage. The Tb of nestlings without access to shade was significantly higher, and nestlings spent more time moving and panting, suggesting an increased metabolic expenditure. To test this, we duplicated the insolation nanoclimate under laboratory conditions using a 1·0 KW Sciencetech Illumination System and measured the energetic cost of begging and non‐begging activity using open‐system respirometry on non‐irradiated and irradiated nestlings. Our results suggest that, in grackle nestlings, the relationship between activity (begging and non‐begging) and energy expenditure is context‐dependent. Our results identified: (i) a reduction in energy expenditure associated with accessing the shade; (ii) expenditure on non‐begging activity equalled or exceeded expenditure on begging activity; and (iii) a potential behavioural constraint on begging imposed by exposure to a homogeneous sun environment (sun without access to shade). The combination of context‐dependent costs and constraints suggests the potential for strategic costs associated with the heterogeneous nest environment.