Reproductive success and parental attributes of Great Tits Parus major nesting in good and poor habitats were compared to investigate whether they differed and, if so, whether parental or environmental variation contributed most to the differences.
Monitored over 3 years, clutches in mature woodland were started earlier, were larger and produced larger broods of heavier chicks than those in marginal habitats such as gardens and hedgerows. Fledging success was significantly higher in woodland in 2 of the 3 years although egg weights were lower. Parents nesting in different habitats were very similar in body-size although those in woodland were in better condition. The prey delivered to nestlings in woodland was of significantly higher quality than that delivered in marginal habitats. Furthermore, the heritability of body-size (tarsus-length) was higher in woodland, suggesting that the environment limits nestling growth in poorer habitats. Reproductive success was considerably lower in poor habitats and appears to be constrained primarily by environmental quality, although this may operate partly through its affect on parental body condition.