Breeding ecology of the Clay‐coloured Robin Turdus grayi in lowland Panama

Abstract

The breeding ecology of Clay‐coloured Robins Turdus grayi was studied in 1979 in the former Canal Zone of lowland Panama. Two study areas were chosen close together, Summit Gardens (27‐3 ha), an ecological island of habitat suitable for Clay‐coloured Robin surrounded by forest, and Morgan's Gardens (3–6 ha), a similar island surrounded by deforested areas and settlements. Eighty‐three active nests were found and checked. The breeding season lasted from 2 March to 21 June. In Morgan's Gardens the first broods were raised in the dry season and second broods in the rainy season; in Summit Gardens most broods started at the beginning of the rainy season. In comparison to the closely related Blackbird T. merula in Poland, Clay‐coloured Robins build nests on isolated trees or bushes, usually in more conspicuous sites (interpreted as an anti‐predator adaptation) and on flexible, horizontal branches off the main trunk. Over 45% of broods were destroyed by predators but other factors causing brood loss were negligible. Nests built on palms were considerably safer. In the dry season the predation rate was low (5%) and increased during the course of the rainy season. Unlike Blackbirds, Clay‐coloured Robin nestlings were given a lot of fruit; their diet was diversified. Starvation occurred in 60% of nests although usually only the youngest nestling died. Breeding density in Morgan's Gardens at 50 pairs per 10 ha was much higher than in Summit Gardens at 15>8 pairs per 10 ha. Breeding losses were lower, the nestlings were fed more fruit, the average nestling weight was lower and the production of fledglings per breeding pair was twice as high in Morgan's Gardens. It was concluded that a strategy of settling in a rather overpopulated place with hard foraging conditions but lower predation was better than settling in a area with a better food supply but higher predation.