Temperature and hen harrier productivity: from local mechanisms to geographical patterns

Abstract

Climate is an important factor limiting demography and distribution patterns in many organisms. For species with a broad geographical distribution, the mechanism by which climate influences demography is likely to vary dramatically from one end of the range to the other. In this paper we first assess, in a Scottish population of hen harriers Circus cyaneus, how temperature and rainfall influence adult behaviour and chick mortality patterns at the nest. We then test for associations between harrier productivity and weather across Scotland, towards the northern edge of the range, and Spain, towards the southern edge of the range. We show that during the nestling period, female brooding time increased in cold weather. Male provisioning rate was negatively related to temperature and rainfall. Chick mortality increased in cold temperatures and was most likely to occur at nests where male prey delivery rates were low relative to temperature. Annual values of harrier fledged brood size across Scotland were positively related to summer temperature suggesting that the patterns seen in one population held at a national scale. In Spain, however, the opposite patterns were observed with fledged brood size being negatively related to temperature. This shows that whilst the impact of weather on productivity may be equally strong at two ends of a geographical range, the mechanisms vary dramatically. Large‐scale predictive models need to take such patterns into account.

Does timing of breeding matter less where the grass is greener? Seasonal declines in breeding performance differ between regions in an endangered endemic raptor

“…This contrasts to what is usually observed in harrier species breeding in the Palearctic, for which the timing of breeding rarely exceeds 3 months [e.g., Pallid Harrier (Circus macrourus), Montagu's Harrier (Circus pygargus), Hen/Northern Harrier (Circus cyaneus/hudsonius), (Schipper 1979, Simmons et al 1986, Arroyo et al 1998, Simmons 2000, Amar et al 2005, Terraube et al 2009). These large scale differences in the extent of the timing of breeding are likely related to climate, the Northern Hemisphere being in general characterized by a more pronounced seasonality in rainfall and temperature regimes (Garcia and Arroyo 2001, Redpath et al 2002a) than the Southern Hemisphere, limiting in time the conditions that are suitable for successful breeding, and also suggest that conditions for breeding are suitable over a longer time for Black Harriers, as for other African raptors.…”

“…Productivity was defined as the number of young reaching 35 days of age (range 0-4) for pairs that laid a clutch. Black Harriers fledge at approximately 40 days old (Simmons et al 2005) but in many cases our last visit occurred before that age; however, in harriers, as in many other species there is usually little mortality during this late nestling stage (Redpath et al 2002a). Thus, we assume that any nestlings alive at 35 days old would have fledged.…”

“…Breeding sites were located by observing areas where Black Harriers were previously known to breed and/ or where perched adults were detected. As in other raptor species, the females take care of the chicks at the nests and perform all brooding, while the male captures and provides the food in the early nestling period (Simmons 2000, Redpath et al 2002a). Thus, nests were located by following prey-carrying males and observing where females landed after a food pass (Simmons 2000).…”

Parental differences in brood provisioning by Hen HarriersCircus cyaneus

“…Brood age is clearly important as young broods will be more at risk from adverse weather and predators, and so will benefit from greater nest attendance (Newton 1979, Redpath et al 2002. In contrast, older chicks can thermoregulate, can better defend themselves and have higher food demands, so will benefit from an increased rate of food delivery (Houston & Davies 1985, Newton 1979.…”

Timing of breeding, habitat preference and reproductive success of marsh harriers (Circus aeruginosus)

“…Moreover, early breeders have higher fitness than late breeders (Arroyo et al 2001). Some recent studies of variability in timing of breeding among mating individuals revealed other factors that are connected with geographical rate and that can initialize breeding (Redpath et al 2002). Such environmental factors include territory quality (Krüger 2002(Krüger , 2004, habitat structure (Moore 1980;Němečková 2005), or parental fitness and phenotypic quality of mating individuals (Simmons 1989;Verboven et al 2001;Childress & Bennun 2002;Krüger 2002).…”