Date of this Version
THE ORIOLE VOL. 61 NO.1
The Wood Thrush (Hylocichla mustelina) diet consists of both insects and fruits. Insects provide birds with more crude protein (31-80 %, Robbins 1983) than fruit (e.g. cherry: 2%, Martin et al. 1951). Thus, Wood Thrushes eat little fruit material during spring (5% of diet) and summer months (35 %), but fruits comprise 77 % of the diet during the fall when insects are less available (Martin et al. 1951). Wood Thrush nestlings are fed largely on an insect diet and are not fed plant materials until they are well developed (Martin et al. 1951, Ehrlich et al. 1988). Of course, the type of food and amount delivered to nestlings is determined by the energy content of the food delivered and the adults' foraging ability and time constraints (Ricklefs 1974). We monitored 153 Wood Thrush nests during 1993-96 at the Piedmont National Wildlife Refuge (PNWR), as part of a research project for the University of Georgia. PNWR is located in Jones and Jasper Counties, north of Macon, Georgia. On 4 July 1995, we found 20-30 Black Cherry (Prunus serotina,) pits in the cup of a Wood Thrush nest that had fledged the previous day. We found some insect remains in several nest cups during 1993-96, but this was the only nest to exhibit evidence that nestlings are fruit. We believe the female, who fed the nestlings without the help of a mate, may have switched to a fruit diet to efficiently provide her brood with the necessary energy to complete their growth in the nest. We had been using radio telemetry to monitor the male and female Wood Thrushes since 10 May and 6 May, respectively. The pair were successful at producing fledglings from the first nest, and this was their second nesting attempt during 1995. The pair initiated the nest on 12 June with a clutch of 3 eggs, and the eggs hatched on 23 June. On 26 June, the male died during mist netting which was conducted in an attempt to attach a new radio transmitter. Both male and female Wood Thrushes feed the young (Brackbill 1958). Following the male's death, we did not observe any new males or helpers at the nest. Also, the female was only on the nest during 43 % (n = 3 of 7) of the telemetry observations, compared to 58 % (n = 7 of 12) of the observations during the nestling period of her previous nest (chisquare = 0.425, P = 0.52). So, the female apparently raised the nestlings alone, and she may have become more active as a single parent, although our limited numbers of telemetry observations during both nesting attempts did not show a significant change. Black Cherry fruits furnish less energy per unit mass than insects: fruits provide 0.64 kcal/g of metabolizable energy, compared to 1.18 kcal/g from insects (Rickkefs 1974). However, Black Cherries are locally common at the PNWR. In late June and early July this was an available food source which, once located, would provide a constant supply. In contrast, insects may have required more intensive foraging efforts by the lone female. Three fledglings were produced from this nest. We believe the female Wood Thrush may have changed foraging strategies after the death of her mate. She found a readily available source of food to facilitate her efforts to feed her brood in the late nestling stage. This underscores the current efforts of habitat managers at PNWR to provide diverse habitats for nesting songbirds.