We compared the growth of late-hatched, competitively disadvantaged nestlings to that of their first-hatched, larger nestmates in 12 asynchronously hatching broods in a Wyoming population of House Wrens (Troglodytes aedon). Upon hatching, late-hatched nestlings weighed, on average, 48% as much as their heaviest nestmate (range 38–60%). Late-hatched nestlings gained significantly less mass per day (0.23 g, on average) than first-hatched nestmates between the ages of 3 and 7 days when mass gain is most rapid. Late-hatched nestlings also showed a strong tendency to grow tarsi more slowly than first-hatched nestmates over the same time period (mean difference: 0.16 mm/d). Primary feathers of late-hatched nestlings, however, grew slightly but not significantly more slowly those of first-hatched nestmates (mean difference: 0.05 mm/d). Reduced rates of mass gain and tarsus growth but normal rates of feather growth have been observed in competitively disadvantaged nestlings in several other passerine species in which broods hatch asynchronously. Maintaining normal feather growth may be an evolved strategy which allows late-hatched nestlings to fledge and travel with older nestmates and hence survive to independence. Alternatively, normal rates of feather growth may simply result from the nutrients required for feathers (i.e., proteins) being more available in the prey of small passerines than the nutrients required for other body components (e.g., calcium for bones).
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Vol. 71 • No. 4