BREEDING BIOLOGY OF SYLVIA WARBLERS

The nest record cards for blackcap [Sylvia atricapilla], garden warbler [S. borin], whitethroat [S. communis], lesser whitethroat [S. curruca] and Dartford warbler [S. undata] were analyzed. There was poor agreement between [British] nest record card totals for each species during 1961-70 and the Common Bird Census indices for farmland and woodland. More blackcaps than other species nested in suburban and urban areas. Garden warblers nested more frequently in scrub than blackcaps, which occurred more in high canopy woodland. Whitethroats nested mainly in scrub, while lesser whitethroats bred more often in hedgerows. The distribution of nest heights was different between the species, whitethroats nesting lowest and lesser whitethroats highest. All Dartford warblers nested in gorse or heather and most individuals of the other species nested in bramble, but there were differences in preferred secondary sites. These differences in habitat are discussed in relation to spatial separation and habitat refuges. The peak of laying for all species was in May, though Dartford warblers in the Channel Islands started most clutches in April. Blackcaps nested slightly earlier than the other species. The peak of laying of garden warblers was over a shorter period than in the other species. Clutches of 5 eggs were the most frequent for blackcap, whitethroat and lesser whitethroat, but in the garden warbler clutches of 4 were almost as frequent as clutches of 5. Four was the commonest clutch for the Dartford warbler. Significant differences in mean clutch size of the species were found. The seasonal decline in clutch size was 25-30%. Hatching and fledging success were generally high. Whitethroats had a lower hatching success but a higher fledging success than other species, except Dartford warbler. More nests appeared to fail when population densities were high, but the results were not conclusive. Whitethroat and lesser whitethroat had the highest overall productivity, garden warbler the lowest. The smaller clutch size, more synchronous laying and lower overall success of the garden warbler may be due to its relatively late arrival on the breeding grounds, so that food supplies are dwindling when the majority of young are in the nest.