Behav Ecol Sociobiol DOI 10.1007/s00265-006-0176-2
ORIGINA L ARTI CLE
Stephen M. Redpath . Fiona M. Leckie . Beatriz Arroyo . Arjun Amar . Simon J. Thirgood
Compensating for the costs of polygyny in hen harriers Circus cyaneus Received: 22 March 2005 / Revised: 23 December 2005 / Accepted: 8 February 2006 # Springer-Verlag 2006
Abstract In polygynous species, the adults are faced with a dilemma during chick rearing. Males must decide how to distribute food between their females and food allocation patterns are often highly unequal. In turn, the females that receive less food from males have to decide how much time to invest in additional hunting. If they spend more time hunting, then they leave their young exposed to weather and predators. However, if they stay at the nest, they increase the risk of their chicks starving. One way that birds may compensate for reduced provisioning is by increasing the size of prey caught. We tested this hypothesis by comparing prey deliveries to nests of hen harriers, Circus cyaneus, with females of different breeding status. As expected, male harriers delivered less food items to the nests of polygynous females, and especially their secondary, or β females. However, both sexes were able to compensate by delivering larger items and there was no difference in the overall mass of food delivered to nests. Moreover, females spent a similar amount of time at the Communicated by C. Brown S. M. Redpath . F. M. Leckie . B. Arroyo CEH Banchory, Hill of Brathens, Banchory, Aberdeenshire, AB31 4BW, UK S. M. Redpath (*) Instituto de Investigacion en Recursos Cinegeticos, Ronda de Toldeo s/n, 13005 Ciudad Real, Spain e-mail:
[email protected] Tel.: +44-1330-826334 Fax: +44-1330-826330 A. Amar RSPB, The Lodge, Sandy, Bedfordshire, SG19 2DL, UK S. J. Thirgood Game Conservancy Trust, Crubenmore, Newtonmore, Inverness-shire, PH20 1BE, UK S. J. Thirgood The Macaulay Institute, Craigibuckler, Aberdeen, AB15 8QH, UK
nest, irrespective of status, and there were no overall differences in breeding success. Our results show that polygynous female harriers can compensate for the costs of polygyny, but we suggest that their ability to do so will vary according to the abundance of both large prey and predators. Keywords Polygyny . Provisioning . Parental care . Harriers . Circus cyaneus
Introduction During the breeding season, many animals have to balance the need to provide their offspring with food against the need to stay and defend their young against predators and adverse weather. Species commonly deal with this conflict by dividing the two roles between the sexes, with one sex tending to stay with the young, whilst the other sex assumes the role of principal food provider. In such species, however, this balance between the sexes may be altered when adults switch from monogamy to polygyny (CluttonBrock 1991). Males in polygynous species breed with more than one female, and when compared to monogamous systems, males often show a reduced provisioning rate to one or more of their females (e.g. Alatalo et al. 1981; Yasukawa et al. 1990; Johnson and Kermott 1993; Sejberg et al. 2000). This inequality in provisioning may result in unequal breeding success between females (e.g. Webster 1991; Slagsvold and Lifjeld 1994). The extent to which breeding success is reduced will depend partly on the ability of males to compensate for the increased demand from two or more females and partly on the ability of females to compensate for the reduced provisioning by the male. Compensation could occur either through increased provisioning rate by the female or through the provisioning of larger prey (Sejberg et al. 2000). However, even in situations where females can fully compensate for reduced male provisioning, breeding success may still be lower, as it depends on other factors such as the quality of prey or the
reduced time females spend at the nest (Alatalo et al. 1988; Duckworth 1992; Wolf et al. 1990). In this paper, we quantify parental care and breeding in the socially polygynous hen harrier, Circus cyaneus, to determine whether birds are able to compensate for reduced provisioning. Whilst polygynous breeding systems are relatively common in some families of birds amongst birds of prey, the harriers (Circus spp.) are the only known genera where polygyny is thought to occur frequently (Newton 1979). In the hen harrier, the number of offspring per female is generally lower in polygynous than in monogamous matings (Balfour and Cadbury 1979; Picozzi 1984; Hamerstrom et al. 1985; Simmons et al. 1986a). Simmons et al. (1986b) showed that polygynous female harriers received fewer food items from males and that nest predation was more common in these birds. They argued that reduced male provisioning forced females to spend more time away from the nest, leading to increased predation. In these Canadian populations, voles were the main prey of harriers and prey size varied little (Barnard et al. 1987). In other areas such as our study areas in southern Scotland, voles still formed an important component of the diet, but a wide diversity of avian and mammalian prey were also taken (Redpath and Thirgood 1999; Redpath et al. 2002). We hypothesise that this diversity may provide polygynous breeders with the opportunity to compensate for the decrease in provisioning rate by increasing the size of prey caught. To test this hypothesis, we compared the number and size of prey items delivered to harrier nests of different breeding status. We also quantified the amount of time females spent at the nest and the number of offspring they produced.
Materials and methods Breeding hen harriers were located from 1993 to 1999 on one estate managed for red grouse, Lagopus lagopus scoticus, in southwest Scotland (55° 10′N, 2°58′W). This estate was managed by gamekeepers, who controlled the numbers of other predators such as foxes, mustelids, and crows (Redpath and Thirgood 1997). All harriers nested in heather Calluna vulgaris dominant vegetation (Fig. 1.) and the distribution of heather was obtained from the Land Cover Map of Great Britain 1990 (LCM 1990) (Fuller et al. 1994). It was possible to determine the breeding status of individuals by watching birds from vantage points in the spring. Females were classified as monogamous (M), alpha females (α) of bigamous males, or beta females (β) of bigamous males. Only on one occasion were more than two females seen associating with a male (in this case three females to one male in 1988), but the breeding attempt of the tertiary female failed before hatching her eggs. The position of females in hierarchies was determined from hatch date, with α females hatching their eggs first. Harrier nests were watched during the breeding season from hides set 6 to 10 m away from the nest. No nests were
Fig. 1 Map of study area, showing the distribution of nesting habitat (heather) in grey shading and the location of watched nests. Nests are split into monogamous (filled squares) and polygynous (open circles)
watched in 1997. The hides were set up once the first egg had hatched, and nests were monitored up to the point of chick fledging. We recorded behaviour at nests of 13 monogamous pairs, 11 α females and eight β females during 562 watches lasting 3,101 h. Hide watches typically lasted 5–6 (mean±SD, 5.5±0.7) hours and enabled us to identify the number of prey, their size and species (Redpath and Thirgood 1999). During each watch, we recorded the age and number of harrier chicks, the amount of time females spent at the nest and the number and size of items provided by males and females. Female harriers usually left the nest when the male approached with food and called constantly to males when they received food (Cramp and Simmons 1980). If the female was away hunting, then the male dropped the food at the nest. Estimates of prey weight for all main prey were available from Schipper (1973), Picozzi (1978), or our own measurements. Prey that could not be identified were invariably small items, which were rapidly eaten and were estimated as a typical small prey of 20 g. Average prey size was estimated as 47 g (range 9– 500 g), so prey
