| Summary: | Geographic location largely determines environmental factors such as photoperiod and primary productivity. Breeding effort in birds may reflect different responses to these geographic related conditions. We studied geographic variation in mean laying date (LD) and mean clutch size (CS) of the Eurasian Kestrel (Falco tinnunculus) in relation to photoperiod, seasonality of resources, temperature and rainfall during the previous winter and breeding season. We assumed that actual evapotranspiration (AE) varies in direct proportion to primary plant production and food resources. Using published data on 23 Western Palaearctic regions (28–65° N) we found that variation in LD was related to photoperiod. Also, Kestrels bred earlier where winter and spring temperatures were higher, spring rainfall was lower and winter AE was higher. The variables that most affected CS were nest type (box/natural), winter AE and spring temperature. CS was directly related to the ratio between spring and winter AE. Moreover, CS was unrelated with spring AE but inversely related to winter AE. The effect of scarce resources (low AE) during the previous winter on CS supports Ashmole's hypothesis (Ashmole 1961). The data available are not sufficient to verify the influence of density of breeding adults on CS.
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