Effect of abiotic factors on reproduction in the centre and periphery of breeding ranges: a comparative analysis in sympatric harriers

Variables such as weather or other abiotic factors should have a higher inf luence on demographic rates in border areas than in central areas, given th at climatic adaptation might be important in determining range borders. Sim ilarly, for a given area, the relationship between weather and reproduction should be dissimilar for species which are in the centre of their breeding range and those that are near the edge. We tested this hypothesis on two s ympatric ground-nesting raptors, the hen harrier Circus cyaneus and the Mon tagu's harrier Circus pygargus in Madrid, central Spain, where the hen harr ier is at the southern edge of its breeding range in the western Palearctic and the Montagu's harrier is central in its distribution. We examined the reproductive success of both species during an 8-yr period, and looked at t he influence of the most stressful abiotic factors in the study area (betwe en-year variation in rainfall and within-year variation in temperature) on reproductive parameters. In the hen harrier, low levels of rainfall during the breeding season had a negative influence on annual fledging success and thus on population fledg ling production. The relationship between rainfall and reproduction was pro bably mediated through food abundance, which in Mediterranean habitat depen ds directly on rainfall levels. In the Montagu's harrier, no negative effec t of dry seasons on productivity was found. Additionally, in the hen harrie r, the proportion of eggs that did not hatch in each clutch increased with higher temperatures during the incubation period. No such relationship was found in the Montagu's harrier. We interpret these between-species differen ces in terms of differences of breeding range and adaptations to the averag e conditions existing there. Hen harriers, commonest at northern latitudes, are probably best adapted to the most typical conditions at those latitude s, and have probably not developed thermoregulatory or behavioural mechanis ms to cope with drought and high temperatures in Mediterranean habitats, in contrast to Montagu's harrier. Thus hen harrier distribution might be cons trained by these variables, due to lower reproductive success or higher rep roductive costs. Accordingly, a logistic regression analysis of the presenc e or absence of both species in 289 random points throughout the western Pa learctic showed that the distribution of both species was related to temper ature, but the relationship was in opposite directions for the two species: hen harriers had lower probability of breeding in areas with higher temper ature (as expected in a species with a more northerly distribution).