The Breeding Biology of the Great and Blue Titmice.

Summary. The breeding biology of Parus major and P. caeruleus was studied in 1947–49 in a mixed deciduous wood, near Oxford, in which 200 nestboxes were erected (density about two per acre). The combined Great and Blue Tit population rose from about 43 pairs in 1947 (following a very severe winter) to about 100 pairs in 1948, and about 135 pairs in 1949. Of these, nearly all the Great Tits and about two‐thirds of the Blue Tits nested in the boxes. Pre‐nesting behaviour is briefly described; hole inspecting and courtship feeding were observed. The average time taken to build a nest differed in each season; both species built many more nests than were used for laying. The breeding dates differed by two to three weeks in different years. The exact season is linked to the brief period of abundance of the larvae of Cheimatobia brumata and Hibernia defoliaria , on which the nestlings are largely fed. The seasonal rise and fall in the larval abundance were estimated by measuring the volume of faecal pellets from the larvae. The average clutch‐size differed significantly from year to year. Clutches laid early in the season were on average larger than those laid later. Repeat clutches were smaller than first clutches, and genuine second clutches were smaller still. Individual egg‐weights of the Great Tit ranged from 1.3 to 2.2 grams, and averaged 1.75 grams (no significant difference between years). Eggs from average‐sized clutches were heavier than those from very small or very large clutches. The first four to six eggs laid in a clutch tended to be successively heavier, but in general eggs in each clutch tended to be of similar weight. Clutches of the Blue Tit were weighed in one year only; the average individual egg‐weight was 1.11 grams. Incubation tended to begin earlier, in relation to clutch completion, in late nests than in early ones. The mean incubation period of the Great Tit was 13.9 days, and of the Blue Tit 14.2 days. The mean growth rates of Great Tits differed significantly in 1948 and 1949. The growth curve of Blue Tits is similar to that of the Great Tit. Members of large‐sized broods averaged significantly lighter than members of small‐sized broods, and second‐brood nestlings were lighter than first‐brood nestlings. Great Tit nestlings aged 14 1/2 days ranged in weight from 10.2 to 21.4 grams. Less than 1% of the nestlings in first broods died from starvation in the nest, but there were many losses in second broods. Eggs laid after the start of incubation hatched later than the rest of the clutch. Late‐hatched nestlings averaged significantly lighter than the others. Frequency of parents' visits to the nest increases with number in brood, but not proportionately. Hence a member of a small‐sized brood receives on average more food visits than a member of a large‐sized brood. Feather development was independent of body‐weight, and is therefore a reliable indicator of age. The mean nestling period of the Great Tit was 18.9 days, of the Blue Tit 19.7 days. When losses due to man are excluded, the breeding success was about 90% for each species.