June broods are of greatest benefit to sand martinsRiparia riparia

This work examines the retrap rate in subsequent years of Sand Martins Riparia riparia ringed as juveniles in colonies in Nottinghamshire front 1969 to 1991. It shows that the weight of juveniles fledging with primaries still partially in pin fell until the primaries were fully developed. This is a hazardous time and at least one‐third of them died during this period whilst they were still dependent on their parents until they learned to forage for themselves. 19/322 (5.9%) with primaries 6 or 7–9 partially in pin were retrapped in future years compared with 78/865 (9.0%) with primaries 8–9 or primary 9 alone partially in pin. The subsequent‐year retrap rate decreased dramatically for juveniles examined as the day length shortened. 38/ 368 (10.3%) of juveniles with primaries partially in pin when examined in June were retrapped in subsequent years but this proportion fell to 47/527 (8.9%) in July, 10/186 (5.4%) in August and 1/91 (1.1%) in September. This is probably because the juveniles fledging earlier in the season survived the immediate post‐fledging period better. Data from an intensive part of this study suggest that they stayed around their natal colony for a median of six days in June increasing to 12 days in September.