Prefledging energy requirements in shorebirds: Energetic implications of self-feeding precocial development

Understanding ecological consequences of avian developmental modes requires knowledge of energy requirements of chicks of different positions in the prococialaltricial spectrum, but those have rarely boon measured in birds with self-feeding precocial young. We studied prefledging energy budgets in chicks of Black-tailed God vit (Limosa limosa) and Northern Lapwing (Vanellus vanellus) in the field and in the laboratory. Lapwings show slower growth than godwits, reaching a 29% lower fledging mass (142 vs. 201 g) in a 32% longer period (33 vs. 25 days). Daily energy expenditure (DEE), measured by the doubly labelled water (DLW) technique, and daily metabolized energy (DEE plus energy deposited into tissue) increased proportionally to body mass at similar levels in both species. Total metabolized energy (TME) over the fledging period was 8,331 kJ in godwits and 6,982 kJ in lapwings, 39 and 29% higher than an allometric prediction ( Weathers 1992). That suggests that self-feeding precocial chicks have high energy requirements compared with parent-fed species, due to costs of activity and thermoregulation associated with foraging. Those components made up 50-53% of TME in the shorebirds, more than twice as much as in seven parent-fed species for which DLW-bascd energy budgets are available. In captive lapwings and godwits growing up under favorable thermal conditions with food readily accessible, thermoregulation and activity costs were 53-58% lower and TME was 26-31% lower than in free-living chicks. The proportion of TME allocated to tissue formation (13-15% deposited as tissue plus 10-12% synthesis costs) was low in the shorebirds, and reductions in food intake may therefore sooner lead to stagnation of growth than in parent-fed chicks. Furthermore, the need to forage limits potential for saving energy by reducing activity in periods of food scarcity, because that will further decrease food intake. Self-feeding precocial chicks thus seem to operate within fairly narrow energetic margins. At the same time, self-feeding may allow birds to use food types that could not be profitably harvested if they had to be transported to the young