Changes in moorland and heathland bird abundance in southwest England in relation to environmental change

Figures & data

Figure 1. Location of study area (unenclosed heathland and moorland; shaded areas) within Exmoor National Park in southwest England.

Table 1. Abundance of moorland and heathland birds on Exmoor, UK BBS trends (Harris et al. 2015), UK Birds of Conservation Concern (BoCC4, Eaton et al. 2015), European red list status (BirdLife International 2015) and wintering zone of each species (Hayhow et al. 2014). Shown are the maximum (max) count of individuals in 2008 and 2014 across survey area, percentage change in maximum count between 2008 and 2014 and percentage change between 1995 and 2013 from UK-wide BBS data (*significant change at P < 0.05). No BBS trend data were available for Merlin or Dartford Warbler; instead abundance estimates for these species were derived from UK-wide national surveys in 1993–94 (Rebecca & Bainbridge 1998) and 2008 (Ewing et al. 2011), and 1994 (Gibbons & Wotton 1996) and 2006 (Wotton et al. 2009), respectively. Max count figures for Meadow Pipit and Skylark in 2008 and 2014 are counts from early visits only to exclude juveniles, and max count figures for Northern Wheatear in 2008 and 2014 are counts from late visits only to exclude migrants.

Species	2008 Max count	2014 max count	% Change 2008–14	% Change 1995–2013 from UK-wide BBS	UK BoCC4 status	European red list status	Wintering zone
Merlin Falco columbarius	5	1	−80	−13	Red	Least concern	Short distance
Eurasian Curlew Numenius arquata	11	6	−45	−46*	Red	Vulnerable	Short distance
Common Snipe Gallinago gallinago	14	21	50	5	Amber	Least concern	Short distance
Common Cuckoo Cuculus canorus	72	131	82	−46*	Red	Least concern	Humid
Common Kestrel Falco tinnunculus	38	19	−50	−40*	Amber	Least concern	Short distance
Raven Corvus corax	172	72	−58	42	Green	Least concern	Resident
Skylark Alauda arvensis	2693	4096	52	−24*	Red	Least concern	Resident
Willow Warbler Phylloscopus trochilus	871	1014	16	−4	Amber	Least concern	Humid
Common Whitethroat Sylvia communis	127	265	109	38*	Green	Least concern	Arid
Dartford Warbler Sylvia undata	197	24	−88	70	Amber	Near threatened	Resident
Grasshopper Warbler Locustella naevia	327	179	−45	−16	Red	Least concern	Arid
Dipper Cinclus cinclus	10	24	140	−22	Amber	Least concern	Resident
Common Redstart Phoenicurus phoenicurus	68	152	124	47*	Amber	Least concern	Arid
Whinchat Saxicola rubetra	570	554	−3	−54*	Red	Least concern	Humid
Stonechat Saxicola rubicola	764	968	27	17	Green	Least concern	Short distance
Northern Wheatear Oenanthe oenanthe	20	17	−15	−6	Green	Least concern	Arid
Grey Wagtail Motacilla cinerea	74	105	42	−21*	Red	Least concern	Resident
Tree Pipit Anthus trivialis	181	178	−2	13	Red	Least concern	Humid
Meadow Pipit Anthus pratensis	3773	6759	79	−15*	Amber	Near threatened	Short distance
Linnet Carduelis cannabina	946	1145	21	−29*	Red	Least concern	Resident
Lesser Redpoll Acanthis cabaret	369	459	24	40*	Red	Least concern	Resident
Yellowhammer Emberiza citrinella	105	121	15	−15*	Red	Least concern	Resident
Reed Bunting Emberiza schoeniclus	275	410	49	19*	Amber	Least concern	Resident

Table 2. Environmental variables, variable definitions, recording categories, which visit the data were recorded on, number of 200-m cells in which a change in each variable was recorded, and median values (i.e. summary values of categorical variables) of the 21 environmental variables surveyed in the field in 2008 and 2014. The change 2008–14 column shows whether the change between years in the median value for each variable was significant, using Wilcoxon matched pairs tests. Key: blank = no significant year effect; ---/+++ = decrease or increase at P < 0.001; --/++ = P < 0.01; -/+ = P < 0.05.

Environmental variable	Definition	Recording categories	Visit data recorded	Number of 200-m cells present	Median 2008	Median 2014	Change 2008–14
Short vegetation	Change in % cover of short sward (0–5 cm)	<1%, 1–20%, 21–40%, 41–60%, 61–80%, 81–100%	First	3029	0.50	1.50	+++
Short to medium vegetation	Change in % cover of short to medium sward (6–15 cm)	<1%, 1–20%, 21–40%, 41–60%, 61–80%, 81–100%	First	3059	1.50	2.00	+++
Medium to tall vegetation	Change in % cover of medium to tall sward (16–30 cm)	<1%, 1–20%, 21–40%, 41–60%, 61–80%, 81–100%	First	2940	2.00	2.00
Tall vegetation	Change in % cover of tall sward (>30 cm)	<1%, 1–20%, 21–40%, 41–60%, 61–80%, 81–100%	First	3068	2.50	2.00	---
Variation in vegetation height	Change in number of vegetation height categories present	Scored from 1 to 4	First	2523	3.00	4.00	+++
Ericaceous vegetation (Calluna and Erica spp.)	Change in % cover of ericaceous vegetation	<1%, 1–20%, 21–40%, 41–60%, 61–80%, 81–100%	First	2036	1.50	1.50	---
Molinia caerulea grassland	Change in % cover of M. caerulea grassland	<1%, 1–20%, 21–40%, 41–60%, 61–80%, 81–100%	First	2568	2.00	2.50	+++
Non-Molinia grassland	Change in % cover of non-Molinia grassland	<1%, 1–20%, 21–40%, 41–60%, 61–80%, 81–100%	First	3026	2.50	2.50	+++
Bracken Pteridium aquilinum	Change in % cover of bracken	<1%, 1–20%, 21–40%, 41–60%, 61–80%, 81–100%	First	1675	0.50	0.50
Burnt ground	Change in % cover of ground burnt within the last year	<1%, 1–20%, 21–40%, 41–60%, 61–80%, 81–100%	First	355	0.00	0.00	+++
Tracks, footpaths and bridleways	Change in occurrence of paths or tracks which had received human foot or vehicle traffic	Presence (1) or absence (0)	First	988	0.50	0.50	+++
Medium grass tussocks	Change in % cover of medium (15–25 cm) tussock material, excluding this years growth	0, 1–25%, 26–50%, >50%	First	1887	0.50	1.00	+++
Large grass tussocks	Change in % cover of large (>25 cm) tussock material, excluding this years growth	0, 1–25%, 26–50%, >50%	First	1103	0.00	0.00	+++
Grazing pressure	Change in grazing pressure from sheep and ponies	Light, light to moderate, moderate, moderate to heavy, heavy	Second	2753	3.00	3.00	+++
Gorse Ulex europaeus and U. Gallii	Change in % cover of gorse	Absent, < 1%, 1–10%, 11–30%, 31–50%, >50%	Second	1313	0.50	0.50	---
Scrub	Change in % cover of broadleaved scrub	absent, <1%, 1–10%, 11–30%, 31–50%, >50%	Second	1415	0.50	0.50	---
Wall/bank or bank	Change in occurrence of wall >1 m in height	Presence (1) or absence (0)	Second	814	0.00	0.50	+++
Hedge/woodland edge	Change in occurrence of hedge or woodland edge	Presence (1) or absence (0)	Second	536	0.00	0.00	--
Wire fence	Change in occurrence of wire fence	Presence (1) or absence (0)	Second	589	0.00	0.50	+++
Rush Juncus spp.	Change in % cover of rush Juncus spp.	Absent, <1%, 1–5%, 6–15%, 16–30%, 31–50%, >50%	Second	2510	1.00	1.50	+++
Ground wetness	Change in score of ground wetness	dry, moist or damp, 1–10%, 11–50%, >50% waterlogged	Second	2594	2.00	2.00	+++

Table 3. Summary of GLMs testing for associations between change in bird abundance and change in environmental variables between 2008 and 2014. Data are presented only for environmental variables which showed significant associations with at least one bird species. Key: blank = no significant association; ---/+++ = negative or positive relationship at P < 0.001; --/++ at P < 0.01; -/+ at P < 0.05. No significant associations between Common Cuckoo, Grey Wagtail or Northern Wheatear abundance change and change in environmental variables were found in 168, 134 and 27 200 m sections, respectively.

Environmental change variable	Common Snipe	Common Kestrel	Raven	Skylark	Willow Warbler	Common Whitethroat	Dartford Warbler	Grasshopper Warbler	Whinchat	Stonechat	Dipper	Common Redstart	Tree Pipit	Meadow Pipit	Linnet	Lesser Redpoll	Yellowhammer	Reed Bunting
Short vegetation (0–5 cm)			+	+++		--	-			-
Medium to tall vegetation (16–30 cm)				+++	+									++
Tall vegetation (30+ cm)								++							+	+
Molinia grassland																	---
Non-Molinia grassland				+++										---
Burnt ground								-						--
Tracks, footpaths and bridleways												-
Medium grass tussocks																	+
Large grass tussocks														++
Grazing pressure														-
Gorse Ulex europaeus & U. Gallii					+				-							++
Scrub		+			+			-									++
Ground wetness				---	+													+++
Altitude	+					---	--			---			+				++
Mean slope				++							-
Variation in slope									+
Aspect (N, S, E, W)									++ (S)
Number of 200-m sections	30	54	162	2685	1029	241	161	431	616	894	28	160	268	3234	838	465	146	496

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