Lead shot ingestion by marsh harriers Circus aeruginosus from the Ebro delta, Spain

Breeding marsh harriers disappeared from the Ebro delta in the 1980s. Altho ugh marsh harrier breeding populations have recently increased in other wet lands of NE Spain, recolonization by breeding birds has not been observed i n the Ebro delta, despite the fact that it is an important wintering area f or the species. Lead (Pb) poisoning in marsh harriers, a problem described in other European wetlands, has been studied as a possible cause of this br eeding impairment. Around 11% of harrier pellets (mainly with duck remains) contained Pb shot, and 52.5% of marsh harriers had blood [Pb] higher than 200 ng ml(-1). Maximum values for both parameters occurred in the middle of the hunting season. Blood [Pb] of wing-tagged marsh harriers classified as wintering birds was higher than those classified as migrant birds, and Pb levels decreased at the end of the hunting season. Body condition was not a ffected by blood [Pb], and none of the marsh harriers found dead had died o f Pb poisoning. Birds with blood [Pb] higher than or equal to 601 ng ml(-1) returned to the Ebro delta in the next winter in a lower proportion (1/3) than birds with blood [Pb] lesser than or equal to 200 ng ml(-1) (6/6), alt hough this difference was not statistically significant (P = 0.083). Pb poi soning may affect populations of wintering marsh harriers, but other ecolog ical and behavioural aspects have to be considered as well to explain the l ack of breeding recolonization of the Ebro delta. (C) 1999 Elsevier Science Ltd. All rights reserved.