Annales Zoologici Fennici 39: 21-28
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ANN. ZOOL. FENNICI Vol. 39 • Breeding losses in red grouse 21 Ann. Zool. Fennici 39: 21–28 ISSN 0003-455X Helsinki 6 March 2002 © Finnish Zoological and Botanical Publishing Board 2002 Breeding losses of red grouse in Glen Esk (NE Scotland): comparative studies, 30 years on Kirsty J. Park1*, Flora Booth2, David Newborn2 & Peter J. Hudson1 1) Department of Biological Sciences, University of Stirling, Stirling FK9 4LA, United Kingdom (*e-mail: [email protected]) 2) Game Conservancy Trust, Swale Farm Satron, Gunnerside, Richmond, North Yorkshire DL8 3HG, United Kingdom Received 5 April 2001, accepted 27 June 2001 Park, K. J., Booth, F., Newborn, D. & Hudson, P. J. 2002: Breeding losses of red grouse in Glen Esk (NE Scotland): comparative studies, 30 years on. — Ann. Zool. Fennici 39: 21–28. Hatching success, brood survival and predation rates of red grouse chicks were examined at four sites in north-east Scotland over two years (1994–1995). Two of these sites have previously been the focus of a large-scale population study on grouse during the late 1950s enabling a comparison to be made. A total of 85 hens were radio-tracked and their breeding success monitored over the two years. Compared with studies undertaken in the 1950s, mean clutch size had risen from 7.2 to 8.6 eggs. Of the 76 nests monitored, 17 (22.4%) broods were lost either through egg or chick predation or by the adult being taken by a predator during incubation. Stoats appeared to be responsible for the largest amount of egg predation. There was a signifi cant increase in predation levels, although hatching success was not signifi cantly different from the 1950s. Chick mortality was highest within the fi rst ten days, a similar result to that found in the 1950s. Overall, mean brood survival from hatching to 20 days was 55.1%. Possible reasons for larger clutch sizes, and the apparent increase in predation levels, are discussed. Introduction al. 1995). It is a bird of economic importance as a quarry species and has been the subject of Red grouse (Lagopus lagopus scoticus) are several long-term studies investigating its’ ecol- restricted to the heather (Calluna vulgaris) moor- ogy, population dynamics and behaviour (e.g. lands of the British Isles, an internationally Hudson 1986a, Hudson 1992, Hudson et al. scarce and valuable habitat type (Thompson et 1998, Jenkins et al. 1963, 1967, Moss et al. 22 Park et al. • ANN. ZOOL. FENNICI Vol. 39 1975, Watson et al. 1994, Watson et al. 2000). heather moorland to forestry and overgrazing, Although periodic crashes in grouse numbers has been cited as one of the causes of long-term were recorded as far back as the early 19th declines in red grouse numbers (Thirgood et al. Century (MacDonald 1883), it was not until 2000c, Robertson et al. 2001). Another change the 1950s that the pattern of cyclic changes in has been an apparent increase in predation pres- grouse numbers were fi rst investigated in detail sure following the observed increase in species (Mackenzie 1952). Since then there has been a such as the red fox (Vulpes vulpes) (Hewson series of detailed population studies followed by 1984) and possibly stoat (Mustela erminea), and experimental investigations that have examined carrion crow (Corvus corone corone) (Hudson the possible cause of the cyclic fl uctuations in 1992). At the same time legislation has provided numbers. There are currently two competing improved protection for raptors and most upland hypotheses to explain the cause of the cyclic species have shown recovery from the long-term changes in grouse numbers: (1) that changes in effects of keeper persecution (Thirgood et al. spacing behaviour drives variations in recruit- 2000a). The aim of this study was to document ment leading to changes in population size (e.g. hatch success and brood survival of red grouse Moss et al. 1994, Moss et al. 1996, Matthiopou- in an area where intensive grouse research was los et al. 2000); (2) that parasite induced reduc- undertaken 30 years previously (Jenkins et al. tions in grouse breeding production is the prin- 1963). This would allow us to investigate any cipal cause of the cycles (e.g. Hudson 1986b, changes that may have taken place in the breed- Hudson et al. 1992, Hudson et al. 1998). How- ing success and predation levels between the ever, these two hypotheses need not be mutually two time periods. exclusive and indeed both may be operating or even interacting to various degrees in different populations (Hudson et al. 2002). A crucial fea- Materials and methods ture in all the work is to examine in detail the fea- tures infl uencing recruitment including breeding Female red grouse were captured at night by success. The factors determining hatch success lamping (Hudson & Newborn 1995) during and chick survival are diverse and vary between March and April 1994 and 1995 at four sites in sites. Possible factors include: (1) female quality Glen Esk, Angus (NE Scotland; altitude 300–640 prior to breeding (Jenkins et al. 1963, 1967, m). Two of these study sites were the same as Moss et al. 1975); (2) parasitic infection of the those used by Jenkins et al. (1963) and referred female with the caecal nematode Trichostrongy- to as the High and Low study areas. Age of lus tenuis (Hudson 1986b); (3) chick infection the females was not known in either study. A with the louping ill virus (Hudson et al. 1997a); total of 85 hens were fi tted with necklace radio (4) predation of chicks by hen harriers (Circus transmitters weighing 15 g (Biotrack, Wareham, cyaneus) (Redpath 1991, Thirgood et al. 2000b); UK). Previous studies have found no evidence (5) availability of dietary invertebrates (Hudson that radio tags have a deleterious effect on 1986a, Park et al. 2001, Wilson et al. 1999). the survival or breeding success of red grouse Some of these factors may in turn be infl uenced (Thirgood et al. 1995). Of the 85 birds tagged, by habitat quality. 6 were taken by predators before incubation, Red grouse have been extensively studied and the radio tag failed on one bird. Radio- for nearly 50 years although the longest dataset tagged birds were usually located at least once from a single site is currently just 25 years a week until incubation, using a Telonics TSR-2 (Hudson et al. 2002). Over this period of time receiver (Telonics, Arizona, USA) and a 3-ele- there have been changes in land use practices, ment hand-held Yagi Antenna (Mariner, Lowes- wildlife legislation and ecological conditions toft, UK). If present at the nest, hens were that have altered conditions on heather moorland disturbed once during incubation in order to and infl uenced the population ecology of grouse record clutch size. Incubating hens were then and other birds (Hudson 1992, Robertson et located every two days to determine hatch date al. 2001). Habitat loss, with the conversion of (mid May to early June). Some nests were taken ANN. ZOOL. FENNICI Vol. 39 • Breeding losses in red grouse 23 4 100 = 3 2 10 Log 10 1 [Grouse density (adults per km Fig. 1. Breeding produc- 0 1 tion (= bars) and grouse 4 100 density (= line) on (a) > Invermark High and (b) Invermark Low, during the 3 period of which the two studies were conducted 2 10 (Game Conservancy, unpubl. 2 Breeding production (young:old birds) data). There is no observ- )] 1 able relationship between young:old ratios and den- sity for either site or with a 0 1 time lag of one year (see 1958 19601985 1987 1989 1991 1993 1995 1997 1999 text). Year by predators during incubation and these nests (number of eggs, proportion of clutches) to and the surrounding area were searched and any investigate differences between the two time remaining egg shells were examined carefully periods 1957–1961 and 1994–1995. Tests using to determine which species caused the preda- proportion of clutches are reported only where tion. Eggs taken and opened by crows are quite there is a difference between the results of the distinctive since they show clearly the pecking two methods. marks of the crow and leave a distinctive hole At two sites (High and Low Invermark), through which the contents were eaten. Some data were available from The Game Conserv- of the eggs showed bite marks and these were ancy Trust, which provided estimates of grouse compared with skulls of stoats and foxes to density and breeding production (1958–1961, identify the probable predator. It is possible that 1985–1999; Fig. 1). On each site, red grouse some predation events assigned to stoats may (numbers of hens, cocks and young) in two 1km2 infact have been due to weasels, although the blocks were counted in July with pointing dogs former are considered to be the more common to obtain estimates of post-breeding populations mustelid on upland grouse moors. Hatch success (Jenkins et al. 1963). The ratio of young:old is defi ned here as the number of eggs that grouse was used as a measure of breeding suc- hatched, i.e. eggs laid minus the number of eggs cess. taken by predators and those eggs that failed Where assumptions of parametric tests were to hatch. violated, data were transformed (arcsine square Jenkins et al. (1963) calculated hatch suc- root, arcsine, or sine + 1) to achieve normality and cess, failure of eggs to hatch, and predation homoscedasticity, or non-parametric tests were levels from the total number of eggs laid in the adopted. General Linear Models, and 2-sample study area. As eggs from the same clutch do not t-tests (2-tailed unless otherwise stated) were represent independent data points, we felt it was used to investigate differences between sites and more appropriate to examine the proportion of years with MINITAB release 13 (Ryan et al.