Growth of late-hatched, competitively disadvantaged nestling house wrens relative to their older, larger nestmates

We compared the growth of late-hatched, competitively disadvantaged nestlin gs to that of their first-hatched, larger nestmates in 12 asynchronously ha tching broods in a Wyoming population of House Wrens (Troglodytes aedon). U pon hatching, late-hatched nestlings weighed, on average, 48% as much as th eir heaviest nestmate (range 38-60%). Late-hatched nestlings gained signifi cantly 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 fir st-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 differenc e: 0.05 mm/d). Reduced rates of mass gain and tarsus growth but normal rate s of feather growth have been observed in competitively disadvantaged nestl ings in several other passerine species in which broods hatch asynchronousl y. Maintaining normal feather growth may be an evolved strategy which allow s late-hatched nestlings to fledge and travel with older nestmates and henc e survive to independence. Alternatively, normal rates of feather growth ma y simply result from the nutrients required for feathers (i.e., proteins) b eing more available in the prey of small passerines than the nutrients requ ired for other body components (e.g., calcium for bones).