The energetics of nestling birds

Abstract

The nestling energy budget is examined with particular reference to the Dipper. Dippers showed an adaptive strategy of differential growth allowing premature fledging. Sex-specific differences in energetics and growth dynamics were observed which may result in differential mortality between the sexes. Field thermoregulation costs were lower than laboratory estimates, however heat loss did not obey the 0.67 exponent rule in the Dipper. Adults appear to adjust their brooding behaviour in response to nestling body temperature. Activity costs measured directly were only about 10% of previous indirect estimates. Brood activity costs increased exponentially with increasing brood-size thus offsetting any reduction in thermoregulation costs through huddling; implications of these results are discussed. Time-activity-laboratory estimates of daily energy expenditure provided excellent agreement with field measurements using doubly-labelled water on 'mature* Dipper nestlings. TAL estimates, however, progressively over-estimated daily metabolised energy (DME) in younger nestlings. Sources of this error are evaluated, and a predictive equation for nestling DME presented. Influences of brood DME on parental care are discussed. Energetic implications of hatching asynchrony were examined in the House Martin. Four hypotheses are discussed.

(1) Nest failure; (2) Brood reduction; (3) Peak load reduction, and (4) Reduced sibling rivalry.

The latter two were modelled and tested in the field. Little evidence was found for the hypotheses considered, lending support to the view that hatching asynchrony is an incidental trait, and moreover one in which costs may outweigh benefits.