Ardea
Official journal of the Netherlands Ornithologists' Union

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Cavé A.J. (1983) Purple Heron survival and drought in tropical West-Africa. ARDEA 71 (2): 217-224
Survival rates for the first-year birds, the after-first-year birds and a reporting rate were estimated from ringing and recovery data of Purple Herons ringed as nestlings in The Netherlands in the period 1960-1977. These estimates were obtained by analysing the recovery categories shot and found separately. The sum of the annual maximum monthly discharges of the rivers Niger and Senegal as given by Den Held (1981) were used as a measure of drought in the wintering area during the birds stay there. The survival and reporting rates obtained from the shot and found recoveries were related to this measure of the drought. It was found that both the survival rate of the after-first-year birds and the reporting rate obtained from the shot recoveries are low under dry and high under wet conditions. The survival rate obtained for the first-year birds was only weakly related to the severity of the drought. A possible explanation for the latter finding is that a large part of the first-year mortality takes place before the birds arrive in their wintering area. The relation between reporting rate and drought in the wintering area can be explained by the low probability of dead, ringed birds being reported in tropical Africa as compared to Europe. In years with a high mortality in tropical Africa the reporting rate is low since a large part of the mortality takes place in an area with a low reporting probability. The survival rates and reporting rate obtained from the found recoveries show no relationship with the drought conditions in tropical Africa, This can be explained by the poor representation of such recoveries in tropical Africa. It could be shown with the help of a simulation model of breeding bird numbers that the variation in these numbers is caused by a variation in survival of the after-first-year birds due to the severity of the drought in the wintering area. This explains the negative correlation between these numbers and the severity of the drought in the wintering area, as has been found by Den Held (1981).


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