Timing of breeding and reproductive output in two Black-tailed Godwit Limosa limosa populations in The Netherlands

To get a better understanding of the current population decline of Black-tailed Godwits in The Netherlands, we determined reproductive parameters in two Dutch breeding populations over the period 2002-2005 and investigated the relationship between reproductive output and timing of breeding. Annual median laying dates ranged from 14 to 25 April, and median hatching dates from 11 to 28 May Sites differed in laying dates but not in hatching dates. Daily survival rate of nests was positively correlated to nest age and was affected by year and by the interaction of year and site, but not by laying date. The number of eggs hatching per successful nest also did not depend on laying date. Maximum chick survival (maximum estimate of the number of chicks fledged divided by the number of chicks hatched from the nest) and the probability of raising at least one chick to fledging declined significantly with hatching date, resulting in a decline of reproductive output with laying date. Minimum chick Survival correlated negatively with cumulative minimum temperature during the first week after hatching. Duration of rainfall during the chick-raising period did not affect chick survival. Our estimates of reproductive output were lower than found in previous studies, and in most years reproductive output was too low to compensate for adult and juvenile mortality. Five possible proximate causes for the seasonal decline in chick survival are discussed: parental quality, weather conditions, food availability predation pressure and mowing of the grassland habitat.