Facultative adjustment of pre-fledging mass loss by nestling swifts preparing for flight

Abstract

Nestling birds often maintain nutritional reserves to ensure continual growth during interruptions in parental provisioning. However, mass-dependent flight costs require the loss of excess mass before fledging. Here we test whether individual variable mass loss prior to fledging is controlled through facultative adjustments by nestlings, or whether it reflects physiologically inflexible developmental schedules. We show that in the face of natural and experimental variation in nestling body mass and wing length, swifts always achieve very similar wing loadings (body mass per wing area) prior to fledging, presumably because this represents the optimum for flight. Experimental weights (approx. 5% body mass) temporarily attached to nestlings caused additional reductions in mass, such that final wing loadings still matched those of control siblings. Experimental reductions in nestling wing length (approx. 5% trimmed from feather tips) resulted in similar additional mass reductions, allowing wing loadings at fledging to approach control levels. We suggest that nestlings may assess their body mass relative to wing area via wing flapping and special 'push-ups' (on the tips of extended wings) performed in the nest. Thus, by facultatively adjusting body mass, but not wing growth, nestling swifts are always able to fledge with aerodynamically appropriate wing loadings.

Figure 1

Convergence of wing loading values (mean±s.e. body mass per wing area) during the last two weeks prior to fledging (at day=0) for unmanipulated control nestling swifts from broods of two (n=12) and three (n=8). Best-fit lines are shown for exponential declines in mean wing loading values (broods of two, r²=0.967, n=14, p<0.001, y=0.0037 e^−0.0404x; broods of three, r²=0.985, n=14, p<0.001, y=0.0042 e^−0.0588x).

Figure 2

The effect of within-brood experimental manipulations, involving the addition of 2 g (approx. 5%) to body mass of ‘weighted’ nestlings, and the reduction by 10 mm (approx. 5%) of wing length in ‘trimmed’ nestlings, as compared to their unmanipulated ‘control’ siblings, in brood sizes of 2 and 3, for: (a) pre-fledging mass change (between 31 and 44 days of age); (b) final body mass at fledging.