Nest Inattentiveness and Its Influence on Development of the Young in the Superb Lyrebird

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Abstract
Incubation and brooding regimes of superb lyrebird females were studied over 6 winter breeding seasons at 2 wet sclerophyll forest sites in southeastern Australia. Internal temperatures of 2 eggs were monitored telemetrically over several days soon after laying. Core temperatures of young nestlings were measured at intervals after maternal exodus during the first 10 days post-hatching, and some similar readings were obtained for older, endothermic nestlings. Growth curves were obtained and growth rates determined for 5 known-age nestlings. The full incubation rhythm developed gradually and was characterized by a low (45%) attentive index and 3-6 h morning recess. Internal egg temperatures fell and stabilized around ambient levels (.apprx. 10.degree. C) for several hours daily during these long maternal absences. When incubation resumed, the egg reheated much faster than it had cooled. Daytime brooding ceased 10 days after hatching when nestlings became endothermic. Mean recess duration during this brooding period was 39 min, but 45-60 min recesses were common on days 1-6. Nestling temperature measurements showed that 4-88.degree. C drops must have been common during maternal absences and drops of 12-13.degree. C were characteristic of longer recesses early in the ectothermic period. Nestling growth curves were logistic, mean K [growth constant] being 0.117 and mean t10-90 [growth time from 10-90% of asymptote] 37.6 days. Comparative evidence suggests that retardation of embryonic growth is the main effect of daily egg neglect. One possible adaptive value of a single long recess rather than several shorter daily recesses is advanced. The effect of cooling on nestling growth rate is harder to assess and probably less pronounced.

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