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 p rococialaltricial spectrum, but those have rarely boon measured in birds wi th self-feeding precocial young. We studied prefledging energy budgets in c hicks of Black-tailed God vit (Limosa limosa) and Northern Lapwing (Vanellu s 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 (D EE 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 2 9% higher than an allometric prediction ( Weathers 1992). That suggests tha t self-feeding precocial chicks have high energy requirements compared with parent-fed species, due to costs of activity and thermoregulation associat ed 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 u nder favorable thermal conditions with food readily accessible, thermoregul ation 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 (1 3-15% deposited as tissue plus 10-12% synthesis costs) was low in the shore birds, and reductions in food intake may therefore sooner lead to stagnatio n of growth than in parent-fed chicks. Furthermore, the need to forage limi ts potential for saving energy by reducing activity in periods of food scar city, because that will further decrease food intake. Self-feeding precocia l chicks thus seem to operate within fairly narrow energetic margins. At th e 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.