Decline of the Orkney Hen Harrier Circus cyaneus population: do ...

Bird Study (2005) 52, 18–24

Decline of the Orkney Hen Harrier Circus cyaneus population: do changes to demographic parameters and mating system fit a declining food hypothesis? ARJUN AMAR1,2*, NICHOLAS PICOZZI2, ERIC R. MEEK3, STEPHEN M. REDPATH2 and XAVIER LAMBIN4 1Game

Conservancy Trust, c/o CEH Banchory, Hill of Brathens, Banchory, Aberdeenshire, AB31 4BW, UK, for Ecology and Hydrology – Banchory, Hill of Brathens, Banchory, Aberdeenshire, AB31 4BW, UK, 3Royal Society for the Protection of Birds, 12-14 North End Road, Stromness, Orkney, KW16 3AG, UK and 4Aberdeen Population Ecology Research Unit, School of Biological Sciences, Zoology Building, University of Aberdeen, Tillydrone Avenue, Aberdeen, AB24 2TZ, UK 2Centre

Capsule A decline in polygyny and lower secondary female breeding success have reduced male productivity and most likely led to the population decline. Aims To examine whether changes in demographic parameters during a population decline fit with a hypothesis that there has been a reduction in food supplies. Methods We examined changes in the number of young, numbers of successful broods and the size of these broods, produced on west Mainland, Orkney between 1953 and 2000 from ringing returns and original nest records. We also examined the changes in polygyny levels between 1967–74, 1976–81 and 1998–2000. Using more intensive data from 1980–81 and 1998–2000, we examined whether there were any changes in various breeding parameters of primary (monogamous or alpha) females or secondary (beta) females. Results Numbers of young and broods produced have declined dramatically in the west Mainland of Orkney since the end of the 1970s. In the 1970s, an average of 60 chicks fledged each year, whereas an average of only 16 chicks fledged in the 1990s. However, over this same period, the average brood size of successful nests did not change. The percentage of males breeding polygynously decreased from around 75% during 1967–81 to only 17% during 1998–2000, and the breeding success of secondary females also decreased. As a result, productivity of males on Orkney was significantly lower in 1998–2000 than in 1980–81. Conclusions The Hen Harrier population has declined dramatically on Orkney since the end of the 1970s. This was due to a decrease in polygynous breeding and a reduction in the breeding success of secondary females. Changes to demographic parameters were consistent with a hypothesis that the decline was an effect of reduced food availability during the early part of the breeding season during recent years.

Population declines are associated with changes in the balance of productivity, survival, immigration and emigration. Investigating changes to demographic parameters during a period of a decline can therefore elucidate the mechanism behind population change (Thompson et al. 1997, Siriwardena et al. 2000). In this manner we can explore the ‘declining-population paradigm’ outlined by Caughley (1994). Identification *Correspondence author. Email: [email protected]

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of which demographic rates have changed and how they have altered during a population decline can also be useful in designing remedial measures. The Hen Harrier Circus cyaneus population in Orkney is of great historical importance. Hen Harriers became virtually extinct in Britain as a breeding species in the early 20th century through intense persecution and were thought to have recolonized from the Orkney population (Watson 1977). Hen Harriers on Orkney have been monitored almost continuously since 1944

Hen Harrier decline on Orkney

(Balfour 1957), which is probably longer than any other raptor population in the world (Brown 1982). This monitoring has quantified a dramatic population decline over the last 20 years, with the number of adults declining by around 70% and overall productivity declining by 60% (Meek et al. 1998). One notable feature of Hen Harriers on Orkney is the former high level of polygyny in this population (Balfour & Cadbury 1979, Picozzi 1984a, 1984b). This was thought to result from a female-biased adult sex ratio and an abundant supply of food, enabling most males to sustain more than one breeding female (Balfour & Cadbury 1979, Picozzi 1984a, 1984b). Recent research has revealed that polygyny levels may have declined since the 1970s (Amar & Redpath 2002). The population decline on Orkney has been associated with reduced clutch size, hatching success and productivity (Kalejta-Summers 1995, Meek et al. 1998). However, both these studies suffered from a lack of knowledge regarding the degree of polygyny and specifically the status of the females monitored. Hen Harrier harems are structured hierarchically, with males favouring earlier-settling females by providing them with more food, which results in better breeding success for primary females (Picozzi 1984a, Simmons et al. 1986). The decline of polygyny in this population therefore creates problems when comparing current demographic rates with past data. Analyses that do not compare females of equal status may not be reliable indicators of the changes that may have occurred in the Hen Harriers’ demographic rates. Recent research has demonstrated that the population is currently limited by food, particularly during the early breeding season; levels of polygyny, the probability of a male breeding and the probability of a female hatching her clutch were all found to be influenced by the amount of food available (Amar & Redpath 2002, Amar et al. 2003). A decline in food supply has therefore been postulated as the most likely cause of the population decline. However, we do not have data on prey availability or provisioning rates during the period of the decline to verify this as the ultimate causal mechanism of the decline. In this study we attempt to overcome this by examining: (a) the changes that have occurred in various breeding parameters during the course of the decline and (b) whether the changes that have occurred are as we might predict, assuming our hypothesis was correct. In this way we attempt to provide a check on this hypothesis as suggested by Green (1999).

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We first collate data from this population between 1953 and 2000, using ringing returns (1953–74) or original nest records (1974–2000), to give a more complete and accurate account of the decline. We then examine polygyny levels of males during two periods when the population was not declining (1967–74 and 1976–81) and compare these to current levels (1998–2000). If food supplies have declined we would predict that polygyny levels would have also declined between then and now, since we already know that polygyny levels are linked to food supplies in this population (Amar & Redpath 2002). We might also predict a decline in clutch size and productivity, and also that clutches may be produced later, as food supply often influences these parameters in raptors (Newton 1979, Redpath et al. 2001). Furthermore, because of the hierarchical nature of Hen Harrier harems, we might predict that breeding performance of secondary females will have suffered more through declining food supplies than primary females. We test these latter predictions by comparing demographic data collected in 1980–81 with data collected in 1998–2000, these being the periods when the additional information on female status is available. METHODS

Details on the numbers of young and broods produced on Orkney between 1953–74 were obtained from ringing details supplied to the British Trust for Ornithology by (the late) E. Balfour. However, the number of young obtained from these data are, in three years, lower than those presented in Balfour & Cadbury (1979: Table 16) suggesting that not all young were ringed in those years. In these years, the higher published figures were used. A further complication is that the area monitored did not remain constant, but from 1970 was expanded to an unknown extent and the number of nests monitored increased by around 50% (from 42 to 62 nests) (Balfour & Cadbury 1979). The number of young and broods produced during this period therefore, are not directly comparable with data collected between 1975–2000, and should instead be treated as a minimum figure. Data for the period 1975–81 were recorded by N.P., who surveyed all the moorland of west Mainland. Numbers of young for this period are often higher than those published previously (Picozzi 1980, 1984a) for two reasons: (1) broods from which young had fledged before the nest was found were omitted (Picozzi 1980: Table 8); (2) 17 nests from 1975–78 subjected to © 2005 British Trust for Ornithology, Bird Study,

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brood manipulations were excluded from previous publications. As excluding these data would underrepresent the number of young fledged on Orkney, we included them for analysis of the total number of young and broods produced, but excluded them from calculations related to mean brood size of successful nests. Data for the period 1982–97 were collected mainly by staff of the Royal Society for the Protection of Birds (RSPB). Although monitoring was less intensive than previously, all successful nests are believed to have been located (Meek et al. 1998). Additionally, more intensive monitoring was conducted in 1989 (Downing 1990) and in 1996–97 (Meek et al. 1998). During 1998–2000, data were collected on a full-time basis mainly by A.A. The same areas were monitored on west Mainland between 1975 and 2000. During 1998–2000, birds were not individually marked, so repeated, synchronous watches by multiple observers were made in adjacent valleys, until we were certain that polygynous males were identified. The breeding status of females within a polygynous group was classified according to the sequence of laying, with the earliest laying female classified as the α female. To compare breeding parameters between 1980–81 and 1998–2000, the breeding status of a female was classified as either primary or secondary. Primary females were either in a monogamous association or the α female of a polygynous group. Secondary (β) females laid their clutch second in a polygynous group. Nests for which clutch size or lay date could not be accurately determined were excluded from the comparisons of data between 1980–81 and 1998–2000 relating to these variables. For full details of the criteria for inclusion or exclusion of data on lay dates and clutch size, see Amar & Redpath (2002). Data from the few repeat clutches were excluded from analyses, but were included in the figures relating to the total number of young and broods produced and mean brood sizes of successful nests. Nests provided with supplementary food as part of a separate experiment (Amar & Redpath 2002) were excluded from figures related to brood size at fledging, and also from all comparisons of breeding parameters between 1980–81 and 1998–2000. Statistical tests were performed on data with normal error structures using general linear models (GLM) in Minitab version 13 (MINITAB 1997). Data with nonnormal error structure were analysed using generalized linear models (GENMOD) in SAS, version 6.12 (SAS Institute 1990), with the appropriately specified error structure and link function. For example, binary data, © 2005 British Trust for Ornithology, Bird Study,

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such as hatching or breeding success were analysed with a binomial specified error structure and a logit link function. As our measure of productivity was a count of the number of young produced per breeding bird, we analysed these data with a Poisson-specified error structure and a log link function. Data on changes in polygyny were analysed using the number of polygynous males/total number of males in each year as the response variable, and each of the three periods (1967–74, 1976–81 & 1998–2000) as categorical explanatory variables; this analysis assumed a binomial error structure and was fitted with a logit link function. RESULTS Trends in the numbers of young, broods and brood sizes produced on Orkney: 1953–2000

Between 1953 and the late 1970s the number of young produced on Orkney appeared to increase. However, caution must be applied given that the monitoring effort was not constant between 1953–74. Less ambiguous is that the number of Hen Harriers produced on Orkney declined dramatically from the late 1970s (Fig. 1a). Five-year running means show that the decline started in the late 1970s and continued throughout the 1980s and 1990s. The numbers of broods produced also follow the same pattern (Fig. 1b), as expected from the lack of any trend in the average brood size of successful nests over time (Fig. 2). The average brood size of successful nests was 2.31 ± 0.04 (mean ± 1 se) and did not differ either between decades (F4,803 = 0.75, P = 0.56) or between the period before and after the start of the decline (i.e. 1953–79 versus 1980–2000) (F1,806 = 0.21, P = 0.65). Changes in levels of polygyny

The proportion of males currently breeding polygynously has decreased dramatically and significantly when compared with the other two periods (1967–74 and 1976–81) F2,14 = 4.04, P = 0.04). In these previous two periods ~75% of males bred polygynously, whereas currently only ~17% of males do so (Fig. 3). Furthermore, the degree of polygyny as measured by the number of females with which a male breeds has also declined. In 1998–2000, no male was recorded breeding with more than two females, whereas in the past over 30% of males were mated with three or more females (Table 1).


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a

Figure 2. Average brood size of successful nests in west Mainland, Orkney between 1953–2000. Data are presented as means ± 1 se.

b

Figure 1. Number of young (a) and broods (b) produced each year from 1953–2000 (). Also shown are the three years where ringing returns were lower than the numbers published previously ( ). The solid line is the 5-year running mean.

Figure 3. Percentage of adult males breeding polygynously in Orkney during three time periods. Data for 1967–74 are from Balfour & Cadbury (1979), for 1976–81 are from Picozzi (1984b). Data are presented as means ± 1 se.

Comparative breeding performance: 1980–81 versus 1998–2001

of secondary females (n = 6) from Orkney during 1998–2000 was too small for formal comparisons with data from 1980–81, for both clutch size and lay date. During 1980–81, secondary females fledged an average of 0.65 young per breeding attempt, whereas no young were ever fledged by a secondary female during

A comparison of data for primary females on Orkney during 1980–81 with data collected on Orkney during 1998–2000 suggested that clutch size may have declined on Orkney between these two periods (Table 2). There is also a suggestion that females are currently laying later than during the early 1980s (Table 2). However, both these trends only approached statistical significance. There was no difference in the hatching success of monogamous or primary females between the two periods (F1,84 = 0.005, P = 0.93), a point reflected in the fact that there was no difference in the average number of young fledged per breeding attempt (i.e. clutch produced) (Table 2). Because of the reduction in polygyny, the sample size

Table 1. Mean percentage of breeding adult males mated with 1, 2 and 3 or more females during the three periods. Data for 1967–74 from Balfour & Cadbury (1979), for 1976–81 from Picozzi (1984b). Proportion (%) of males with Period 1967–74 1976–81 1998–2000

1 mate

2 mates

≥3 mates

26 23 83

38 44 17

36 33 0

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Table 2. Comparison of demographic parameters, between 1980–81 and 1998–2000. Clutch size and lay date are of primary females (i.e. alpha or monogamous females). For lay date, day 1 = 1 April. Data are presented as means ± 1 se. Test statistics are the output from the general linear or generalized linear models with the two time periods analysed as categorical effects. 1980–81 Clutch size Lay date Male productivity Primary female productivity Secondary female productivity

5.15 37.46 2.29 1.34 0.65

± ± ± ± ±

0.14 0.73 0.28 0.20 0.15

1998–2000 4.68 40.26 1.40 1.40 0

1998–2000 (except for one repeat clutch, which was excluded from the analysis), a difference which was statistically significant (Table 2). A comparison of the productivity of all adult males (i.e. monogamous and polygynous) between these periods shows that between 1998 and 2000, adult males produced significantly less young than they did during 1980–81 (Table 2). DISCUSSION Population decline

There has been a dramatic decline in the number of broods and young produced on Orkney, a trend that appears to have begun at the start of the 1980s. This decline coincides with an increase in sheep stocking densities on Orkney (Fuller & Gough 1999). Between 1980 and 1998, sheep numbers more than doubled on Orkney as a whole and the same trend is also apparent if just those parishes on west Mainland with breeding Hen Harriers are examined (Amar & Redpath in press). Agricultural records from Orkney also show that during this time there has been a reduction in rough grazing and an increase in the amount of intensive pasture (Amar et al. 2005). These factors could have reduced the amount of food for Hen Harriers by reducing the amount of rough grass, known to correlate with the abundance of many of the important prey species, especially voles (Gorman & Reynolds 1993, Amar 2001), which are particularly important during the pre-lay and incubation period (Amar et al. 2003) although apparently less important during the nestling period (Picozzi 1980). No trend existed for the mean brood size at fledging of successful nests, a point illustrated by the very similar pattern in the number of young and the number of broods produced over the years. Thus, the total number of young produced has declined but, at an individual level, a successful nesting attempt still © 2005 British Trust for Ornithology, Bird Study,

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± ± ± ± ±

0.23 1.49 0.26 0.26 0

df 1, 1, 1, 1, 1,

66 77 81 85 44

F

P

3.49 3.42 4.30 0.02 5.44

0.06 0.06 0.04 0.88 0.02

produced a similar number of young to that before the decline. This suggests that, if a decline in food supply is the causal factor in the population decline and is the critical limiting factor, then it appears that it may not be acting during the nestling stage. In other words, food supply may have declined but there is still sufficient food available during the nestling period to rear as many young per brood as previously, but fewer broods are initiated. Food limitation appears to be acting on a different phase of the breeding cycle and evidence suggests that food limitation is particularly important during the pre-lay and incubation period (Amar & Redpath 2002, Amar et al. 2003). The apparent trend for more young to be produced between 1953 and the 1970s should be treated with caution. The increase in the population may have been due to a natural recovery of the population following the end of control by gamekeepers on Orkney and the introduction of stricter bird protection laws in the 1930s (Balfour 1968). However, this is unlikely to be the whole story as there was an increase both in search effort and the total area monitored for Hen Harriers over these years in Orkney. Changes in polygyny levels

The dramatic decline in the proportion of polygynous males on Orkney supports our predictions and is suggestive of a decline in food supply, particularly during the early stages of breeding. Polygynous breeding is often associated with habitats or years that are rich in food (Hamerstrom et al. 1985) and experimental evidence has shown that polygyny levels of Hen Harriers on Orkney are currently limited by food (Amar & Redpath 2002). During 1998–2000 we found a similar female-biased sex ratio in the adult population to that found during the late 1970s, when polygyny peaked (Amar & Redpath 2002). Despite this and even though polygynous associations were formed in early


spring, only rarely did males have more than one female that actually produced a clutch. As well as a decrease in the proportion of polygynous males, the degree of polygyny (in terms of the numbers of females in a harem) has also declined. During 1998–2000, bigamy was the only polygynous mating pattern observed, whereas previously trigamy was common and as many as six females were recorded breeding with a single male (Balfour & Cadbury 1979). Our results suggest that feeding conditions may have deteriorated throughout Orkney, and it seems probable that few areas are now sufficiently rich in food to support such high levels of polygyny. Even individual males with the best hunting abilities seem unable to supply adequate food to support a harem. In the femalebiased Hen Harrier population that occurs on Orkney, the polygynous breeding system, so prevalent in the past (Balfour & Cadbury 1979, Picozzi 1984a), enabled more females to breed than is currently possible under a predominantly monogamous breeding system. As a consequence, there is now a higher proportion of nonbreeding females in the population (Amar et al. 2003). Changes in demographic parameters

There was a suggestion that primary females may now be laying smaller clutches and later in the season. This might indicate lower resource availability, although neither trend was statistically significant. This lack of difference might be explained by breeding being dependent on female condition, or on resource availability being above a certain threshold, below which breeding is not initiated. In other words, only females with sufficient resources attempt to breed, albeit producing somewhat smaller and later clutches than in the past, while those in poorer condition simply do not breed at all. This notion is further supported by the increased proportion of non-breeding females in the population. We found that for primary females the number of young produced per breeding attempt did not differ significantly between the two periods, however productivity for secondary females has decreased considerably, with no secondary females (except one relaying bird) rearing young in 1998–2000, nor any hatching a clutch, except those provided with supplementary food (Amar & Redpath 2002). This again provides useful evidence that food supply may have declined for this population. Secondary female Hen Harriers usually have poorer breeding success than primary females and this is most likely due to the lower rates at which they

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receive food (Simmons et al. 1986). If food supplies decrease we might expect that the breeding success of secondary females would be the first to suffer. Comparisons of adult male productivity between the periods showed that their productivity levels declined significantly. This was due to both the combined effect of reduced polygyny and the fact that secondary females currently usually fail to produce any young. The increase in males failing to breed at all will also have reduced the overall productivity of the population (see Amar et al. 2003). Population modelling using a male-based population model has shown that a reduction in the breeding output of males in this manner is sufficient to have caused the population decline witnessed on Orkney (Amar 2001). In conclusion, this study supports many of the findings of Meek et al. (1998) and Kalejta-Summers (1995). However, analysing information on primary and secondary females separately provided a better understanding of the changes in demographic parameters than previous analyses. For example KalejtaSummers’ (1995) finding, that hatching success had decreased, appears to be mainly attributable to the current poor hatching success of secondary females, as primary female hatching success appears not to have changed. Furthermore we have identified that the major change in breeding productively appears to be the lower overall productivity of males, caused by the reduction of polygyny levels and the lower breeding success of their secondary females. This study supports the use of behavioural ecology as a vital component of conservation biology (Sutherland 1996). In particular, it highlights that changes in breeding system can have considerable consequences for the population dynamics of a species and need to be taken into account when planning conservation monitoring programmes. The changes we found in the breeding parameters of this population were consistent with our predictions; polygyny levels, male productivity and breeding success of secondary females all declined as predicted and there was also a suggestion that primary females are laying smaller and later clutches. Our findings therefore support the hypothesis that the population has decreased due to a decline in food availability. Conservation management should aim to improve food supplies for this population, particularly during the early part of the breeding season. Scottish Natural Heritage (SNH) launched the Orkney Hen Harrier recovery project in 2003, which aims to increase the amount of rough grass habitat, which is the favoured hunting habitat of male Hen © 2005 British Trust for Ornithology, Bird Study,

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Harriers in the spring (Amar et al. 2003). We recommend the continued collection of data on breeding parameters (identified from this study as being susceptible to changes in food supply), during the recovery project in order to assess the effectiveness of the suggested management practices. ACKNOWLEDGEMENTS We are grateful to everyone who has helped collect data on the Hen Harriers on Orkney over the past 50 years and to all the landowners that have allowed us access to their land. In particular we remember the late Eddie Balfour who started the monitoring of this population and collected so much of the data. We are grateful to the Centre for Ecology and Hydrology (formerly The Institute of Terrestrial Ecology), Aberdeen University and RSPB who initially funded this work and the Game Conservancy Trust, which also partially funded the writing of this paper. A.A. thanks Kerry Lock, Rory Gordon, Brian Ribbands, Keith Fairclough, Andy Knight, Graham Rebecca, Jacqui Todd, Maxim Esnault, Gaetan Bottin and Sarah Burthe for help in the field in 1998–2000 and we also thank Beatriz Arroyo and two anonymous referees for helpful advice on this paper. REFERENCES Amar, A. 2001. Determining the cause of the Hen Harrier decline on Orkney. Unpublished PhD thesis, University of Aberdeen. Amar, A. & Redpath, S. 2002. Determining the cause of the Hen Harrier decline on the Orkney Islands: an experimental test of two hypotheses. Anim. Conserv. 5: 21–28. Amar, A. & Redpath, S.M. 2005. Habitat use by Hen Harriers Circus cyaneus on Orkney: implications of land use change on this declining population. Ibis 147: 37–47. Amar, A., Redpath, S. & Thirgood, S. 2003. Evidence for food limitation in the declining Hen Harrier population on the Orkney Islands, Scotland. Biol. Conserv. 111: 377–384. Balfour, E. 1968. Breeding birds of Orkney. Scot. Birds 5: 89–104. Balfour, E. & Cadbury, C.J. 1979. Polygyny, spacing and sex ratio among Hen Harrier Circus cyaneus in Orkney, Scotland. Ornis Scand. 10: 133–141. Brown, L. 1982. British Birds of Prey. Bloomsbury, London.

Caughley, G. 1994. Directions in conservation biology. J. Anim. Ecol. 63: 215–244. Downing, R. 1990. Orkney Hen Harriers – 1989. In Booth, C., Cuthbert, M. & Meek, E. R. (eds) Orkney Bird Report 1989: 57–58. Fuller, R.J. & Gough, S.J. 1999. Changes in sheep numbers in Britain: implications for bird populations. Biol. Conserv. 91: 73–89. Gorman, M.L. & Reynolds, P. 1993. The impact of land-use change on voles and raptors. Mammal Rev. 23: 121–126. Green, R.E. 1999. Applications of large-scale studies of demographic rates to bird conservation. Bird Study 46(suppl.): S279–S288. Hamerstrom, F., Hamerstrom, F.N. & Burke, C.J. 1985. Effect of voles on mating systems in a Central Wisconsin population of harriers. Wilson Bull. 97: 332–346. Kalejta-Summers, B. 1995. Abundance and breeding success of Hen Harriers Circus cyaneus in the Orkney islands, Scotland between 1957 and 1994. Unpublished report to SNH. RSPB, Inverness. Meek, E.R., Rebecca, G.W., Ribbands, B. & Fairclough, K. 1998. Orkney Hen Harriers: a major population decline in the absence of persecution. Scot. Birds 19: 290–298. MINITAB 1997. MINITAB Reference Manual, Release 13. Minitab Inc., State College, Pennsylvania. Newton, I. 1979. Population Ecology of Raptors. T. & A.D. Poyser, Berkhamsted. Picozzi, N. 1980. Food, growth, survival and sex ratio of nestling Hen Harriers Circus c. cyaneus in Orkney. Ornis Scand. 11: 1–11. Picozzi, N. 1984a. Breeding biology of polygynous Hen Harriers Circus c. cyaneus in Orkney. Ornis Scand. 15: 1–10. Picozzi, N. 1984b. Sex ratio, survival and territorial behaviour of polygynous Hen Harriers Circus c. cyaneus in Orkney. Ibis 126: 356–365. Redpath, S.M., Thirgood, S.J. & Leckie, F. 2001. Does supplementary feeding reduce predation of Red Grouse by Hen Harriers? J. Appl. Ecol. 38: 1157–1168. SAS Institute 1990. SAS/STAT Users’ Guide, Version 6. SAS Institute Inc., Cary, NC. Simmons, R.E., Smith, P.C. & MacWhirter, R.B. 1986. Hierarchies among Northern Harrier (Circus cyaneus) harems and the costs of polygyny. J. Anim. Ecol. 55: 755–771. Siriwardena, G.M., Baillie, S.R., Crick, H.Q.P. & Wilson, J.D. 2000. The importance of variation in the breeding performance of seed-eating birds in determining their population trends on farmland. J. Appl. Ecol. 37: 128–148. Sutherland, W.J. 1996. From Individual Behaviour to Population Ecology. Oxford University Press, Oxford, UK. Thompson, D.L., Baillie, S.R. & Peach, W.J. 1997. The demography and age-specific annual survival of song thrushes during periods of population stability and decline. J. Anim. Ecol. 66: 414–424. Watson, D. 1977. The Hen Harrier. T. & A.D. Poyser, Berkhamsted.

(MS received 30 March 2003; revised MS accepted 22 January 2004)

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