Survival/return rates and juvenile dispersal in an increasing population of Lesser Whitethroats Sylvia curruca

Some 1200 Lesser Whitethroats, including 340 adults, were ringed during 1981–93 at Crook Bank, on the Saltfleetby‐Theddlethorpe Dunes N.N.R. Overall annual catches, and CES catches, of both adults and juveniles increased during the period. There was also a marked increase, from about 1987, in the numbers of breeding adults captured. The average ratio of juveniles: adults captured (1.88:1) was much lower than for Whitethroats studied on the same site (3.08:1), (Boddy 1993). This might be because many more Whitethroats than Lesser Whitethroats breed north of the study site. Juveniles appeared to disperse during mid post‐juvenile moult, though a few remained until moult was complete. The average annual survival rate for adults, estimated from between‐year recaptures during 1982–83 to 1992–93, was 43.5%. The overall ‘return’ rate for adults was 16.4%, similar to the 17.1% reported from Cleveland (Norman 1992). Juvenile ‘return’ rates increased in the later years of the study to give an overall average of 3.5%, compared with only 0.8% in Cleveland (Norman 1992). Juveniles ringed in June and July were significantly more likely to be recaptured in later years, than were birds ringed from August onwards. Returning juveniles were also significantly more likely to be males than females. There was, however, no difference in sex ratios for birds known to be returning for their second or later breeding season.