| In species that are susceptible to mass-dependent flight costs, particularly seabirds and aerial insectivores, mass recession is a crucial aspect of the nestling period, ensuring fledglings will have an appropriate wing loading. My objectives were to determine if mass recession by Chimney Swift ( Chaetura pelagica) nestlings is intrinsically controlled or facultatively adjusted by nestlings, and if mass recession is driven by changes in parental behavior (i.e., reduced provisioning rates) or nestling behavior (i.e., reduced solicitation of feedings). Nestling swifts (N = 69) were divided into three groups: controls, half-weighted, or weighted. Half-weighted and weighted nestlings had 0.6-0.7-g or 1.2-1.3-g lead weights, respectively, glued to the tips of the body feathers in the middle of the back during the period from 16 to 24 days post-hatching. Weighted nestlings lost more mass than both controls (t 48 = 2.4, P = 0.009) and half-weighted nestlings (t40 = 1.8, P = 0.04). Control nestlings had a higher average wing loading than both half-weighted (t44 = 1.9, P = 0.03) and weighted nestlings (t48 = 1.9, P = 0.03). Video recordings of nests showed that provisioning rates of adult swifts did not vary throughout the nestling period. The percentage of time nestlings spent begging increased slightly with age, approaching significance (F6, 128 = 2.0, P = 0.07). Changes in body mass among nestlings allowed those in each treatment to converge toward a similar wing loading value, which is likely optimal for flight efficiency. The mechanism(s) involved in this process remain unclear because parental provisioning rates were similar throughout the latter part of nestling period (days 12-26 post-hatching) whereas percent begging time by nestlings tended to increase with nestling age. Because I was not able to distinguish individual nestlings, weighted nestlings may have solicited less food from adults than control nestlings, allowing them to lose more mass. Additionally, weighted nestlings may have been more active, exercising more and thus losing more mass than controls. Finally, because of their greater mass (i.e., more tissue), weighted nestlings may have also lost more water due to tissue maturation. |
