Breeding season and clutch size of Indian passerines

We studied the timing of breeding of passerines inhabiting India, Bangladesh and Sri Lanka using data provided in the Handbook of the Birds of India and Pakistan (Ali and Ripley 1968-74). The data were analysed for the whole area and separately for the ten zoogeographical subregions of India. Peak breeding months (defined as those in which 75% or more of the breeding species are laying eggs or rearing young in the nest) throughout India, including the Anandman, Nicobar and Lakshadweep islands, were May to June. In Sri Lanka the peak started earlier and extended to three months (April-June). The peak occurred a month before the arrival of the monsoon rains, so that the peak food demand of chicks coincided with the arrival of the monsoon. The mean body mass of the breeding taxa declined significantly from December to November. This trend resulted from a relatively large proportion of large birds, mainly corvids, starting to breed between December and March, earlier than most smaller birds. The early breeding of large birds resulted in the peak food demand of their chicks coinciding with the arrival of the monsoon rains. The median clutch size for the study area as a whole, as well as for all subregions of the subcontinent, was 3.5-3.7 eggs, while in Sri Lanka and the other islands it was 3.0 eggs. The smaller median clutch sizes of the birds of Sri Lanka and the other islands and in relation to those of most other subregions were significant, and as expected from the 'island syndrome'.