The invertebrate diet of Black Grouse Tetrao tetrix chicks: a comparison between northern England and the Scottish Highlands

ABSTRACT

Capsule: Conservation programmes for Black Grouse Tetrao tetrix need to consider managements that increase invertebrates for chicks.

Aims: To compare invertebrates in chick diet between habitat and region and to relate abundance of preferred invertebrates to Black Grouse breeding success.

Methods: Chick faeces were collected from roosts in the Pennines (northern England) and Perthshire (Scottish Highlands) to identify invertebrates eaten. Larval abundance was determined by sweep netting (Pennines) and related to Black Grouse breeding success.

Results: Invertebrates were recovered out of 98 faecal samples from 63 broods. Ants were more numerous in chick diet in Perthshire, with larvae (both sawflies and moths) more numerous in the Pennines. Sawfly larval abundance was positively correlated with Black Grouse breeding success in the Pennines.

Conclusions: Management for Black Grouse should promote high invertebrate abundance to maintain high breeding success. This may be achieved through creating low-intensity grazed moorland fringes to encourage Bilberry, and associated moth caterpillars, adjacent to areas of higher grazing intensity to retain grasses and rushes that host sawfly larvae.

Black Grouse Tetrao tetrix were widespread in the UK 150 years ago, occurring in virtually every county (Gladstone 1924, Holloway 1996). Since then, there have been large and widespread range retractions, totalling 29% in the 40 years since 1968–72 (Balmer et al. 2013). Estimates of UK population size from surveys of lekking males in 1995/96 and 2005 suggest a 22% decline between surveys, from 6510 males (95% confidence limits: 5000–8100) (Hancock et al. 1999) to 5078 males (95% confidence limits: 3920–6156). This represented a decline of 29% in Scotland, chiefly in southern Scotland, but only an 11% decline in England (Sim et al. 2008), where 86% of the population is now confined to the North Pennine Hills (Warren et al. 2015). Such long-term declines have occurred throughout most of temperate Europe south of the boreal zone, where small population sizes and geographic isolation are associated with habitat degradation and fragmentation following both changes in human land-use and increasing predator numbers, which appear to limit the chances of Black Grouse survival and recovery (Storch 2000).

In the UK, several Black Grouse conservation projects have concentrated on improving habitat, either in commercial forests by thinning trees and increasing open space, or on moorland edges by reducing sheep grazing or Red Deer Cervus elaphus browsing (Baines et al. 2000). Limited data available from forest based projects in North Wales and parts of Scotland suggest that expensive management intervention may limit habitat restoration within post-thicket forest stages to inappropriately small scales and hence does little to prevent continued declines (White et al. 2015). Conversely, appreciable benefits have been described for conservation management along moorland fringes in northern England and North Wales. Reductions in sheep grazing funded by UK agri-environment schemes in northern England, and improved heather management and instigation of predator control in North Wales have both been associated with increases in numbers of displaying males (Calladine et al. 2002, Lindley et al. 2003). Furthermore, extensive uptake of such Black Grouse friendly prescriptions across many of the North Pennine Hills in northern England have been linked to increases in the English population from 773 displaying males in 1998 to 1029 in 2006 and then 1437 in 2014 (Warren & Baines 2004, 2008, Warren et al. 2015). Low chick survival had previously been considered to be a key demographic factor limiting population growth in the North Pennines, where breeding success was lower than elsewhere in the UK (Baines 1991, Baines et al. 2007). However the introduction of low-intensity sheep grazing at sample sites was associated with improvements in breeding success (Calladine et al. 2002). The precise mechanism for this was unclear, but similar reductions in Red Deer in Scottish forests were linked with more invertebrates, particularly moth caterpillars (Baines et al. 1994), preferred by Capercaillie Tetrao urogallus chicks (Picozzi et al. 1999). Thus more invertebrates available to foraging chicks following a relaxation of sheep grazing pressure forms a plausible mechanism to explain locally improved breeding success in Black Grouse in the North Pennines. In this study, we compared the relative importance of different invertebrate groups in Black Grouse chick diet and compared the abundance of invertebrates in the diet with both their field availability, and with Black Grouse breeding success.

Methods

Study areas

This study was conducted in two regions of the UK, Perthshire in the Central Highlands of Scotland and the North Pennines in northern England. The Scottish part of the study was conducted in 1997–98 in the upper reaches of Glen Errochty near Trinafour in north Perthshire. Here, wet heath moorland used for low-intensity walked up Red Grouse Lagopus lagopus scotica shooting and Red Deer staking had been partly afforested with Sitka Spruce Picea sitchensis, Lodgepole Pine Pinus contorta, Scots Pine Pinus sylvestris and hybrid Larch Larix decidua. Planting had been in two blocks, each of 150–200 ha between 1983 and 1985 (Baines et al. 2000). The study area in the North Pennines consisted of eight acid grassland sites, each of 150–300 ha, on upland sheep farms within the upper reaches of three river systems: Harwood and Forest-in-Teesdale in Upper Teesdale, Co. Durham (River Tees), Garrigill in Upper Tynedale, Cumbria (River Tyne) and St Johns Chapel and Stanhope in Upper Weardale, Co. Durham (River Wear). The farmland was on the fringe of heather moorland managed for driven Red Grouse shooting (see Baines 1994). Here, female Black Grouse had been radio-tagged between 1997 and 2004 as part of a study into population dynamics (Warren & Baines 2002, Baines & Richardson 2007).

Faecal collection

Searches for females with recently hatched broods were conducted in the Perthshire study area using pointer dogs in the second half of June in 1997 and 1998 within a 1.5 km radius of the main leks. Two chicks, estimated to be less than 10 days old, from each brood were equipped with radio-transmitters of weight 1 g (type BD-2, Holohil Systems Ltd), which were glued to the down on the chicks’ backs between their wings. Brood positions were triangulated whilst roosting at night, marked and relocated the following morning to collect chick droppings after the brood had moved (Green 1984). A total of 44 samples from roosts were collected from 21 broods. Similarly, in northern England, radio-tagged females with broods were tracked at night, roost positions triangulated and chick droppings collected the following morning. Between 1999 and 2004, 54 samples of chick droppings were collected from 42 broods when less than 14 days old. Invertebrate abundance in chick droppings was calculated allowing for the number of key parts making up each whole invertebrate following Moreby (1987).

Invertebrate availability

A measure of general invertebrate availability was obtained from sweep netting known brood habitats (Baines 1991) within the same areas where brood faecal material was collected. In Perthshire, the main brood-rearing habitats of damp mires dominated by Bog Myrtle Myrica gale, Cross-leaved Heath Erica tetralix and Purple Moor Grass Molinia caerulea, with some Eared Willow Salix aurita scrub and wet grass-sedge rich flushes, were sampled in 1990–96. In the North Pennines, equivalent samples were taken in 1990–93 and 1997–98 from areas of damp rough grassland, dominated by Soft Rush Juncus effusus, coarse grasses, typically a mix of Nardus stricta, Festuca and Agrostis spp., with Heath Rush Juncus squarrosus frequented by broods (Baines 1994). Samples were collected in late-June, extending into ealy-July in the North Pennines, where Black Grouse bred on average almost ten days later than in Perthshire (Baines et al. 1996), to coincide with when chicks were less than three weeks old and consumed most invertebrates (Starling-Westerberg 2001). Each sample consisted of 25 vigorous sweeps of the ground and ranged annually from 20 to 82 sets of 25 sweeps. The net contents containing the invertebrates were inverted into plastic bags for later sorting. All invertebrates over 2 mm in length were sorted into groups and counted.

Larval abundance and breeding success in northern England

Given the known importance of moth caterpillars and sawfly larvae in the diet of Black Grouse chicks (Picozzi & Hepburn 1984, Starling-Westerberg 2001), larval abundance of both groups were sampled by sweep netting the same eight sites within three valleys in the Pennines from which faecal material was also collected. Larvae were sampled in each of 14 years within the period 1990–2015. Data from individual sites were combined for each valley in each year, giving a total of 26 valley-years. The number of samples gathered per valley-year ranged from 20 to 480. Black Grouse breeding success was estimated in each valley by searching brood-rearing areas for females and accompanying chicks using a pointing dog in late-July or August when chicks were typically six to nine weeks old. Breeding success was expressed as the number of chicks reared per female and data were combined to provide an annual estimate for each valley.

Data analysis

For analysis of the composition of the invertebrate diet of chicks, invertebrate taxa were combined into seven categories: (1) spiders and harvestmen (Arachnida), (2) beetles (Coleoptera), (3) flies (Diptera), (4) ants (Formicidae, Hymenoptera), (5) sawflies (98% larvae, 2% adults) (Tenthredinidae, Hymenoptera), (6) moths (95% caterpillars, 5% adults) (Lepidoptera) and (7) Others – chiefly plant bugs and other Hymenoptera. Multiple samples from the same brood were combined to form one value. Samples from three broods, two in Perthshire and one in the Pennines, contained no invertebrates and were excluded. The invertebrate diet of each brood was expressed as the proportion of each invertebrate category which, when summed, totalled unity and thus could not be considered independent of the others (Aitchison 1986). Accordingly, composition analysis was used (Aebischer et al. 1993), whereby data were made independent by calculating the ratio of proportions using one category as the denominator. The results of the analysis do not depend on which category was used as the denominator. The ratios were then log-transformed to normalize the data. Analyses of differences in diet between regions and years were carried out using multivariate analysis of variance applied to the log-ratios. Univariate analysis of variance was used to investigate relative differences in abundance of individual categories using as dependent variable the log-ratio ln(p/1 − p), where p = the proportion of the category. Differences in composition were considered between regions (Perthshire or Pennines) and within each region, between years. Differences in the median annual abundances of invertebrate taxa between regions were compared by Mann–Whitney U tests. To assess whether Black Grouse breeding success varied spatially between Pennine valleys or temporally between-years, annual breeding success was compared between valleys and years within a generalized linear model (GLM) framework by Poisson regression, with a log link adjusted for over-dispersion. Chicks counted in August was the response variable, the natural logarithm of the number of females observed was specified as an offset and valley and year as fixed factors. A similar analysis was conducted using the natural logarithm of the mean sawfly larvae value for each valley and each year. To determine whether Black Grouse breeding success was influenced by larval abundance, three measures of annual breeding success in each of the three valleys were compared with the equivalent values for first sawfly larvae only, then caterpillars only and then for total larvae combined. Breeding success measures were (i) overall breeding success expressed as chicks per female, (ii) brood size (chicks counted in relation to broods observed) and (iii) broods per female (the proportion of females that were accompanied by a brood). Breeding success and brood size were considered in Poisson regressions, with chicks counted as the dependent variable, the natural logarithm of the number of females (breeding success) or number of broods (brood size) as offsets, valley as a fixed factor and log sawfly larval abundance, log caterpillar abundance and then log lepidopteran larvae, as covariates in turn. Broods per female was modelled as above, but using logistic regression (binomial distribution and logit link), categorizing females as successful if they had one or more chicks and unsuccessful if they had no chicks. If valley was non-significant (P > 0.05) it was dropped from a subsequent minimal model that considered breeding success in relation to larvae only.

Results

Invertebrates in chick diet

A total of 7455 invertebrates were identified to taxa from the 98 samples, 32% of them from the Perthshire samples and 68% from the Pennine samples. The invertebrate component of chick diet differed between the two regions (Wilks’ Lambda = 0.394, df = 7,52, P < 0.001) with ants being more prevalent in Perthshire (53% of items at a mean of 40 ants per sample) than in the Pennines (6% of items at a mean of 13 ants per sample) (F_1,58 = 30.87, P < 0.001). Conversely, sawfly larvae and moth caterpillars were both more frequent in the diet in the Pennines than in Perthshire, with sawfly forming 67% of items in the Pennines (mean of 69 per sample) compared to 20% of items (mean of 7 per sample) in Perthshire (F_1,58 = 9.31, P = 0.003) and moth caterpillars 11% and 7% of items (F_1,58 = 5.95, P = 0.018 respectively; Table 1). Ants were found in 80% of Perthshire samples, but only in 41% of Pennine samples, whereas sawfly larvae were found equally regularly in samples from both regions, being found in 85% of Perthshire samples and 91% of Pennine samples. These groups aside, chick diet did not differ between regions and in order of relative abundance comprised beetles, spiders and harvestmen, flies and Hymenoptera other than sawflies and ants. Plant bugs (1% of items) were only found in the Pennine samples.

Table 1. The composition of the invertebrate component of the diet of Black Grouse broods in Perthshire and the North Pennines expressed as both the percentage of total items and the percentage of samples that contained specific taxa.

	Perthshire				North Pennines
	Items (n = 2387)		Occurrence (n = 44 samples)		Items (n = 5068)		Occurrence (n = 54 samples)
	No.	%	No.	%	No.	%	No.	%
Spiders	28	1.2	17	43	98	1.9	40	74
Harvestmen	36	1.5	18	45	57	1.1	32	59
Arachnida	64	2.7	24	60	155	3.0	45	83
Weevil	168	7.0	33	83	49	1.0	24	44
Staphylinidae	50	2.1	23	58	229	4.5	37	69
Elateridae	36	1.5	11	28	79	1.6	33	61
Carabidae	31	1.3	17	43	66	1.3	33	61
Chrysomelidae	25	1.0	10	25	45	1.0	16	30
Scarabidae	0	0	0	0	9	0.2	7	13
Other	2	0.1	1	3	0	0	0	0
Coleoptera (beetles)	312	13.0	36	90	477	9.6	46	85
Tipulidae	6	0.3	4	10	17	0.3	13	24
Chironomidae	17	0.7	8	20	33	0.7	18	33
Other	9	0.4	6	15	41	0.8	17	31
Diptera (flies)	32	1.4	16	40	91	1.8	34	63
Heteroptera (plant bugs)	0	0	0	0	69	1.4	17	31
Formicidae (ants)	1274	53.4	32	80	282	5.6	22	41
Sawfly larvae	459	19.2	34	85	3302	65.2	49	91
Sawfly adults	6	0.3	2	5	86	1.7	11	20
Sawfly (Tenthredinidae)	465	19.5	34	85	3388	66.9	49	91
Ichneumonidae	41	1.7	21	53	60	1.2	29	54
Braconidae	7	0.3	2	5	2	<0.1	2	4
Other Hymenoptera	48	2.0	21	53	62	1.2	31	57
Moth caterpillars	167	7.0	22	55	535	10.6	43	80
Moth adults	25	1.0	3	8	9	0.2	6	11
Lepidoptera (moths)	192	8.0	22	55	544	10.8	44	81

Within both of the two regions, composition of the diet differed between years (Perthshire: Wilks’ Lambda = 0.327, df = 7,11, P = 0.04; Pennines: Wilks’ Lambda = 0.123, df = 42,130, P = 0.009). In both regions, between-year variation in the composition of the diet was caused by differences in the proportion of moth caterpillars consumed (Perthshire F_1,17 = 15.06, P = 0.001, Pennines F_6,33 = 4.83, P = 0.001; Table 2). In the Pennines, the proportion of moth caterpillars in the diet varied from 0 to 0.15 between-years, whilst that of sawfly larvae varied between 0.45 and 0.76. In addition, the proportion of sawfly larvae also tended to vary between years in the Pennines, but not significantly (F_6,33 = 2.04, P = 0.09).

Table 2. Annual differences in the proportion of sawfly larvae and moth caterpillars in the invertebrate component of the diet of Black Grouse chicks in Perthshire and the North Pennines. Values are weighted in relation to the number of invertebrate items found in chick droppings rather than the unweighted means from each individual brood.

Region	Numerical proportion in the chick diet
	Year	Items	Broods	Sawfly	Moth	Larvae combined
Perthshire	1997	147	3	0.05	0	0.05
	1998	2238	16	0.20	0.09	0.29
Pennines	1997	38	1	0.45	0	0.45
	1999	610	6	0.57	0.07	0.64
	2000	590	3	0.73	0.07	0.80
	2001	416	10	0.59	0.14	0.73
	2002	864	6	0.76	0.07	0.83
	2003	1741	8	0.66	0.13	0.79
	2004	721	6	0.63	0.15	0.78

Invertebrate availability and selection by chicks

Overall invertebrate abundance did not differ between the two study areas, but the composition of the samples did. In Perthshire both moth caterpillars and ants were up to 50-fold more numerous and beetles and spiders/harvestmen 2- to 3-fold more numerous than in the Pennines (Table 3). In contrast, plant bugs were twice as numerous in Pennine samples. Sawfly larvae were evidently highly selected by foraging chicks at both sites, forming up to 3% of the content of sweep net samples, but 20% of items eaten by chicks in Perthshire and 67% in the Pennines. Similarly, whilst ants formed only up to 1% of the invertebrates sampled, they formed 53% of items consumed by chicks in Perthshire and 6% in the Pennines. In contrast, whilst moth caterpillars were abundant in sweep net samples in Perthshire, particularly within the Bog Myrtle habitats, forming 30% of the catch, they formed only 8% of items consumed, whereas in the Pennines, where they formed <1% of the catch, they formed 11% of chick diet.

Table 3. The relative abundance of different invertebrate groups sampled by sweep netting in the Perthshire and Pennine study areas. Values presented are medians (range) of six annual samples from each site. Comparisons are Mann–Whitney tests (U and P).

Group	Perthshire	North Pennines	U	P
	Median (range)	Median (range)
Spiders/harvestmen	3.5 (1.5–5.5)	1.5 (0.9–3.2)	4.5	0.032
Beetles	2.8 (1.3–7.6)	1.0 (0.8–2.8)	3.5	0.017
Flies	12.3 (3.3–21.2)	22.1 (8.7–29.8)	7.5	0.104
Plant bugs	13.5 (3.2–31.1)	29.6 (13.7–75.9)	7.0	0.093
Ants	0.8 (0.3–1.8)	0.0 (0–0.05)	0.0	0.002
Sawfly larvae	0.2 (0.0–2.4)	1.5 (0.5–2.7)	8.0	0.132
Moth caterpillar	17.2 (6.6–30.8)	0.3 (0.0–0.5)	0.0	0.002
Others	1.6 (0.0–8.5)	0.5 (0.1–4.5)	11.5	0.331
Total	57.9 (40.9–71.5)	59.9 (35.7–113.9)	17.0	0.937

Larval abundance and breeding success in the Pennines

Neither Black Grouse breeding success nor sawfly larval abundance differed between Pennine valleys, but both differed between years (breeding success: F_13,12 = 4.84, P = 0.005, sawfly larvae F_13,12 = 2.85, P = 0.039). Outputs from models including both valley and larval abundance inserted in place of year are provided in Table 4, but to avoid repetition only results involving sawfly larvae and grouse breeding success are provided. Dropping the non-significant valley from the model resulted in a significant positive relationship between overall Black Grouse breeding success (chicks per hen) and both sawfly larvae (F_1,24 = 7.23, P = 0.013) and total larvae, that is, also including moth caterpillars (F_1,24 = 7.60, P = 0.011), but not with caterpillar abundance alone (F_1,24 = 0.04, P = 0.84): Black Grouse bred more successfully in years when larvae were more abundant. Splitting grouse breeding success into brood size and broods per female gave similar results, with sawfly larvae being positively related to both measures of success (brood size: F_1,24 = 4.39, P = 0.047, broods per female: F_1,24 = 6.86, P = 0.015). An analysis using total larvae gave qualitatively similar results.

Table 4. Output from GLMs examining with a Poisson error distribution and logit link examining the importance of valley and log sawfly larvae in determining three measures of Black Grouse breeding success in the North Pennines; (a) overall breeding success (chicks per female), (b) brood size and (c) broods per female (number of valley-years = 26). Valley estimates are relative to Valley: Teesdale.

	Estimate (se)	t (22)	P
a) Chicks per female
Constant	0.37 (0.16)	2.42	0.024
Valley: Teesdale	0	–	–
Tynedale	−0.27 (0.39)	−0.71	0.487
Weardale	−0.27 (0.64)	−0.43	0.672
Sawfly larvae	0.54 (0.22)	2.45	0.023
b) Brood size
Constant	1.13 (0.06)	20.67	<0.001
Valley: Teesdale	0	–	–
Tynedale	−0.13 (0.14)	−0.98	0.340
Weardale	−0.05 (0.22)	−0.25	0.807
Sawfly larvae	0.16 (0.08)	2.04	0.053
c) Broods per female
Constant	−0.07 (0.22)	−0.34	0.737
Valley: Teesdale	0	–	–
Tynedale	−0.33 (0.57)	−0.57	0.576
Weardale	−0.37 (0.81)	−0.45	0.655
Sawfly larvae	0.82 (0.36)	2.31	0.031

Discussion

The diet of Black Grouse chicks has been described from several European studies and tends to differ between habitats. Broods using forested habitats or wet heaths in Scotland, Central Europe and Scandinavia ate more ants (Picozzi & Hepburn 1984, this study) or moth caterpillars (Wegge & Kastdalen 2008), whilst those sampled on grass-dominated moorlands in northern England ate predominantly sawfly larvae and relatively few lepidopteran larvae or ants (Starling-Westerberg 2001, this study). Moth caterpillars were numerous in the brood-rearing habitats in the Perthshire study area, where although commonly taken in the diet, they were not selected relative to their availability, but here birds bred well and reared more chicks than in the Pennines (Baines et al. 2007), where caterpillars were scarce and highly selected by chicks. Along forest margins and on low-intensity managed moorlands in the Scottish Highlands, and in the boreal forests of Scandinavia, moth caterpillars are numerous on Bilberry Vaccinium myrtillus (Baines et al. 1994, Lakka & Kouki 2009). These Bilberry rich habitats are widely used by foraging forest grouse chicks (Picozzi et al. 1999, Ludwig et al. 2010), but high browsing intensities by deer can reduce not only larval abundance (Baines et al. 1994), but also cover, which may provide important concealment from predators (Baines 1996, Signorell et al. 2010). Such degradation of brood-rearing habitats, either by browsing deer or by high-intensity sheep grazing may, through lowering invertebrate abundance, lead to lower breeding success by Black Grouse (Baines 1996, Calladine et al. 2002).

In moorland fringe habitats used by Black Grouse in the Pennines, moth caterpillars and their main host plants such as Bilberry and Bog Myrtle were scarcer than on moors in the Scottish Highlands (Baines 1996). Consequently, moth caterpillars formed a lower proportion of the diet in the Pennines and their abundance was not related to breeding success. Instead, sawfly larvae formed the majority of the diet and their annual abundance was positively correlated with breeding success. Black Grouse breeding success is lower in years when June rainfall is relatively high (Summers et al. 2004), and a 100-year climate time-series for the Scottish Highlands suggests a shift towards wetter Junes (Summers et al. 2010). It is uncertain to what degree weather affects chick survival directly through chilling or reducing the time available for foraging, indirectly by influencing invertebrate abundance and activity, or a combination of both (Green 1984). This being the case, then managing ground vegetation to promote higher abundance of preferred larvae, both of caterpillars and sawflies may, in part, help mitigate against the negative impacts of increased rainfall on grouse productivity.

Sawfly larvae feed on a range of grass and rushes Juncus (Barker 1998), often of low palatability to grazing sheep. Although a move to lower intensity sheep grazing was associated with 41% more invertebrates overall, including 77% more moth caterpillars, the abundance of sawfly larvae did not differ with grazing intensity (Baines 1996). Accordingly, a mosaic of moorland fringe habitats with different levels of livestock grazing or deer browsing may be optimal for successful brood-rearing, with low-intensity grazing on the one hand to encourage Bilberry and Bog Myrtle as host plants of moth caterpillars and higher intensity grazing on the other to promote rushes and coarse grasses that host sawfly larvae. Our findings provide a mechanism that may help explain the positive association between changes in Black Grouse numbers between successive national surveys in Scotland and a heterogeneous mix of vegetation types on unenclosed moorland (Pearce-Higgins et al. 2016).

In many parts of their European range Black Grouse are in steep decline (Storch 2000). The survival of juvenile grouse species from hatching until recruitment is a critical determinant of population growth (Hannon & Martin 2006), with juvenile mortality rates often highest during the first two weeks after hatching (Ludwig et al. 2010). During this period, likely causes of chick mortality include adverse weather (Moss 1985, Summers et al. 2004, Ludwig et al. 2010), predation (Fletcher et al. 2010, Signorell et al. 2010) and availability of food, including arthropods (Hagen et al. 2005). The protein provided by invertebrates is important to gamebird chick growth, with higher invertebrate abundance associated with better chick survival (Park et al. 2001). Invertebrate abundance can be highly patchy and dependent upon herbaceous cover and vegetation structure, which in turn can be influenced by the intensity of management, either fertilizing and mowing (Di Giulio et al. 2001), livestock grazing (Calladine et al. 2002) or deer browsing (Baines et al. 1994) and climatic differences.

Our results illustrate how the needs of chicks of a single species may be met from foraging on different invertebrate groups, whose abundances vary regionally between vegetation and habitat types. Understanding these regional and habitat-related differences is essential when developing and implementing species recovery plans. A better understanding of breeding requirements, particularly those of chicks, may help overcome demographic bottlenecks. We propose experimental management of mixed livestock grazing to provide a range of vegetation heights and structures to determine how this affects invertebrate abundance, especially the moth and sawfly larvae preferred by young Black Grouse broods, and how this in turn affects Black Grouse productivity.

Acknowledgements

We thank the various land-owners and their gamekeepers for providing access. Field assistance was provided by Mark Andrew, John Calladine, Richard Francksen, Susan Haysom, Jackie Longrigg and Susan Raven. Analysis of invertebrate fragments was by Robin Foster under the guidance of Steve Moreby. The manuscript was improved by comments from an anonymous reviewer.

Additional information

Funding

The work was funded by the Game & Wildlife Conservation Trust.