THE BREEDING OF THE BLACKBIRD TURDUS MERULA AT OXFORD

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Abstract
Summary: 1. Breeding statistics are given for a garden population of Blackbirds, and less complete data for a woodland population. Comparison is also made with data provided by the British Trust for Ornithology nest‐record cards.2. The breeding season lasts from March to June. Temperature influences the timing of the start of breeding, and rainfall probably influences the ending of breeding. Yearling females tend to begin to breed later than old females and to stop a little earlier. Old females averaged 3·1 broods per year, yearling females 2·3. Breeding begins later in woodland than in gardens.3. Yearling females lay smaller clutches than old females. Clutches are on average smaller in gardens than in woodland. Smaller clutches are laid when the weather is cold and perhaps also when it is very dry.4. Incubation starts gradually while the clutch is being laid. Consequently hatching is usually spread over a period of a day or two. Hatching success was about 90% in gardens, old parents being more successful than yearlings. Hatching success in woodland was 92 to 95%.5. In gardens, nestlings are fed mainly on earthworms in the early part of the breeding season, later on a greater variety of food, especially insects. In droughts food is hard to find and many nestlings die. In woodland, nestlings are fed especially on green caterpillars and a variety of other invertebrates taken from the woodland floor, and few were found to die in the nest.6. The young grow from about 6 gm. to 70 gm. The last‐hatched are often much lighter than the first‐hatched. If food is short, the lightest young usually die one by one. The weight of eight‐day‐old nestlings is highest in the middle of the breeding season. Young from large broods are on average heavier than young from small broods. The flight feathers emerge at the age of seven or eight days, but may be delayed for up to two days by food shortage.7. Brood‐size at eight days has been higher in woodland than in gardens, partly because clutches are larger in woodland and partly because many nestlings die in the garden nests. Proportionately more young die in the nest from large than from small broods.8. After leaving the nest, the young are fed by the parents for about 20 days. Parents very rarely feed strange young, and do not always feed all their own. Especially the last brood of the season may be strictly divided between the two parents.9. The garden population has produced an average of 4·1 young per pair per year, and 66% of them have survived to independence. Post‐fledging survival seems to have no relation to weight at the age of eight days. It is probable that at least 62% of young that reach independence survive to the next breeding season. The woodland population has been much less successful on account of heavy predation.10. The adaptive nature of the breeding season and clutch‐size is discussed.