The increase in the Corncrake Crex crex population of the United Kingdom has slowed

Abstract

Capsule The UK Corncrake population increase has slowed, but has rapidly recovered from a large decline in 2013.

Aims To provide an update on the population size and distribution of breeding Corncrakes in the UK, including the results of the most recent full national survey and annual monitoring of the core population.

Methods A full survey of singing Corncrakes was undertaken in the UK and the Isle of Man in 2009, following the last full censuses in 1978/79, 1988, 1993, 1998 and 2003. Monitoring of singing male Corncrakes within the core range in northern and western Scotland has been undertaken annually since 1993.

Results The 2009 results show that numbers increased substantially since 2003, from 829 to 1166 singing males. Corncrake numbers in the core range have increased in 17 of the 21 years between 1993 and 2014, and peaked at 1274 singing males, in 2014. Numbers of singing males dropped by 24% between 2012 and 2013, probably due to a particularly cold spring in 2013 that inhibited growth of vegetation cover, but numbers recovered markedly in 2014.

Conclusion The recovery of the UK Corncrake population has continued since a low point in 1993. However there are no signs of range expansion into mainland UK, away from the core breeding areas in Scotland. The large decline in the core population between 2012 and 2013 highlights the sensitivity of the population to external factors, although the subsequent increase in 2014 shows a capacity for rapid recovery.

Corncrakes Crex crex are summer visitors to Eurasia and winter in sub-Saharan Africa. The species breeds in habitats with tall ground grass or herbage and few trees and shrubs. The tall grass in meadows managed for the cutting of hay or silage is the most important habitat in many countries (Green et al. 1997a). Declines of Corncrake populations and contractions of geographical range during the late 19th and 20th centuries are documented for nearly all parts of the range in which surveys were conducted (Tucker & Heath 1994, Green et al. 1997a). However, the species is still widespread in Central and Eastern Europe, despite declines there, and it is markedly more abundant there than in Western Europe (e.g. Green et al. 1997a, Mischenko & Sukhanova 2006, Boldogh et al. 2009, Budka et al. 2012). The species was formerly designated in the IUCN Red List of globally threatened species as Vulnerable and later as Near Threatened because of these declines, however, monitoring in Russia, which holds the majority of the world breeding population, indicates that earlier declines there have not continued in recent decades and the species is consequently now listed as of Least Concern (BirdLife International 2012). Grassland and savanna are preferred habitats on the wintering grounds in Africa, with birds also occurring in rank grass near rivers, sewage ponds and pools and in relatively short grass in wetter areas, moist sedgebeds and reedbeds and in tall grass within young conifer plantations (Barnes 2000). Most winter records are from grass-dominated habitats (Walther et al. 2013). There are potential threats to the extent of the Corncrake wintering range in Africa, from an ongoing change from low-intensity to high-intensity agriculture and an increase in the amount of land being converted to grow monocultures such as oil palm and biofuels (Walther et al. 2013). Most Corncrakes migrate through the eastern Mediterranean, where there is a continued threat from illegal hunting and trapping in some countries (Koffijberg & Schäffer 2006, Emile et al. 2014).

In the UK, the Corncrake is on the red list of Birds of Conservation Concern because of a large population decline and range contraction over a period of about 100 years (Eaton et al. 2009). The species was formerly widespread as a breeding bird throughout the UK up to the late 19th century (Holloway 1996), but then a marked decline began that continued through most of the 20th century (Green 1995). The timing of the rapid declines in the late 19th and early 20th centuries was correlated with the replacement of hay mowing by hand with horse-drawn mowing machines, the efficiency of which allowed a higher proportion of mowing of hay to occur during the Corncrake's breeding season (Norris 1947a, 1947b, 1948). Mowing during the breeding seasons leads to nests in hay and silage fields being destroyed. In addition, machine mowing leads to chicks being killed because mowing from the margins of the field inwards results in chicks becoming trapped because they are reluctant to escape across already-mowed open ground (Broyer 1996, Tyler et al. 1998). Recruitment of young adults to the breeding population was markedly reduced during this period of rapid decline, probably as a result of low breeding success caused by machine mowing (Green 2008). The population continued to decline until the late 20th century, by which time the Corncrakes became restricted first to Northern Britain (Norris 1948, Sharrock 1976) and later largely to the Northern and Hebridean islands of Scotland (Gibbons et al. 1993). By 1993, high densities of breeding Corncrakes were only found in parts of Britain and Ireland where substantial areas of suitable tall vegetation were present throughout the breeding season (late April to early September) and where mowing of hay and silage mainly occurred in late July and August (Green 1996). This was likely to be because of beneficial effects on breeding success and the survival of adults and chicks of having vegetation tall enough to conceal Corncrakes throughout the period when they are on the breeding grounds, combined with lower nest failure and chick mortality rates observed when mowing occurs late (Green et al. 1997b).

Based upon studies of habitat requirements and the factors affecting breeding productivity, it was considered that effective conservation measures should (1) increase the area of suitable tall vegetation (tall grass and herbs), including habitats that will provide cover at times when vegetation in grass fields is too short for Corncrakes (early and late cover), (2) reduce the overlap between the breeding and mowing seasons by delaying the date of mowing and (3) reduce mortality of chicks caused by mowing by adopting Corncrake-friendly mowing (CFM) techniques, that allow chicks to escape from mowed areas whilst remaining in cover (Tyler et al. 1998). It is expected that these measures would only be effective if implemented in the same area in combination.

A Corncrake recovery programme based upon these principles was established in Scotland in the early 1990s (Stowe & Green 1997). Since then, conservation measures have been introduced, including through government-funded agri-environment schemes, which have resulted in a partial recovery in the population (O'Brien et al. 2006). The initial objective was to reduce losses of nests and chicks to mowing, by paying farmers and crofters to delay mowing until after the end of July and to use CFM techniques in fields close to the singing places of male Corncrakes (Tyler et al. 1998). Subsequently, the recovery programme attempted to provide payments for farmers to establish areas of tall native vegetation to act as early cover for Corncrakes in the spring, following winter grazing, and as late cover in late summer when the grass in areas mowed for hay or silage is too short (O'Brien et al. 2006). Other initiatives have included the acquisition and management of nature reserves for Corncrakes, especially by the RSPB.

This paper reports on annual monitoring of the Corncrake population of core areas of Scotland between 1993 and 2014 and a UK-wide census in 2009. The results are used to generate a new estimate of the UK population of singing male Corncrakes, and to assess the extent of recent changes in geographical range.

METHODS

Fieldwork methods

Male Corncrakes produce a characteristic loud rasping song ‘crek-crek’. Males sing almost continuously on most nights when they are not in a short-term pair bond with a female (Tyler & Green 1996). Although females also sing on rare occasions, their song is different and distinguishable from that of the males (Ottvall 1999). Observations of radio-tagged birds shows that only males produce the loud rasping ‘crek-crek’ song (Tyler et al. 1996).

Since 1993, annual surveys of Corncrakes singing at night in May – June in core areas of Scotland have been conducted by RSPB staff, with assistance from National Trust for Scotland, and there have been similar annual surveys by RSPB of the re-introduced population of Corncrakes in Cambridgeshire, England, since 2004. The core areas of Scotland are shown in Table 1. The full survey in 2009 and the annual surveys of core areas followed the methods used in previous national surveys (Green & Gibbons 2000, O'Brien et al. 2006). All suitable habitats were surveyed systematically in the core areas between 20 May and 10 July each year. Observers made at least two survey visits during this period. For each survey area, a route was planned to go within 500 m of all potentially suitable Corncrake habitat, defined as herbs, grass, crops or marshland vegetation greater than 20 cm tall. Surveys were conducted at night, ideally between 00:00 and 03:00 BST. Windy conditions (Beaufort Force 5 or more) were avoided. Observers typically covered the area by car, stopping at regular intervals to listen for Corncrake song for one to two minutes. Two independent studies of radio-tagged Corncrakes in Scotland found that males were singing continuously on 81% (Hudson et al. 1990) and 79% (Tyler & Green 1996) of visits during the period and time of night specified by the survey protocol, so the proportion of males missed by two or three surveys would be expected to be small. Six-figure (100 m accuracy) Ordnance Survey grid references of the locations of singing Corncrakes were estimated from the direction of the calls.

Table 1. Numbers of singing Corncrakes recorded in surveys conducted in the UK and the Isle of Man between 1978/79 and 2014. Years with full survey coverage of Great Britain, the Isle of Man or Northern Ireland are shaded grey. Scottish islands included in the core areas counted in every year since 1993 are marked with asterisks.

Area	1978/79	1988	1993	1998	2003	2004	2005	2006	2007	2008	2009	2010	2011	2012	2013	2014
Shetland	3–5	1	3	4	4	2	–	–	1	2	3	–	–	–	–	–
Orkney*	102–105	29–36	6	13	31	17	13	17	21	20	17	27	28	32	12	36
Lewis*	31–38^a	66–76	106	77	105	118	113	101	111	68	81	106	180	141	75	140
Harris*	7	5	4	3	11	24	10	14	12	8	2	2	12	12	4	10
Berneray*	5	1–2	2	2	1	3	3	4	7	7	6	6	6	6	3	3
North Uist*	75	56–59	66	73	77	112	104	100	117	102	93	107	113	131	80	120
Benbecula*	33	22–23	17	41	30	35	28	26	27	25	18	22	14	13	10	31
South Uist*	83	91–100	50	65	101	119	103	110	105	92	90	92	84	113	75	88
Barra, Vatersay & Sandray*	26–28^a	59	26	47	68	72	66	75	76	70	76	80	80	98	68	103
Pabbay, Mingulay & Berneray	–	–	0	0	2	5	3	–	2	2	1	–	4	–	6	4
Skye*	31–34	26–28	8	12	23	32	27	16	18	12	7	25	19	31	38	38
Canna*	9	1–2	3	2	4	4	1	2	3	5	4	0	5	0	0	0
Rum	1–2	0	1	0	0	0	–	–	–	–	0	–	–	–	–	–
Eigg	4–6	0	1	0	0	1	1	–	2	–	0	–	–	–	–	–
Muck	4	0	1	0	5	3	4	3	3	–	0	1	–	–	–	–
Coll*	28	20	20	40	90	134	159	171	180	118	122	116	121	103	64	91
Tiree*	85	99–103	111	136	184	260	310	316	391	408	389	391	385	371	348	396
Mull & Staffa*	1	2–3	1	2	0	5	7	7	5	9	2	8	6	4	4	4
Iona*	25^b	3	4	12	24	24	29	39	30	50	46	34	24	31	25	28
Treshnish Isles & Lunga	1	0	2	3	2	4	3	1	2	2	2	1	1	2	1	2
Colonsay & Oronsay*	22	17–18	10	14	32	46	50	58	71	67	62	72	58	71	53	86
Islay*	22–24	19	9	4	10	31	52	55	66	82	84	81	83	86	87	98
Jura	1	1	0	0	0	0	–	–	–	–	0	–	–	–	–	–
Gigha	1	4	0	0	1	0	–	–	–	–	1	–	1	–	–	–
MacCormaig Islands	0	0	1	0	0	0	1	2	3	2	3	–	2	–	–	–
Scottish mainland	94–103	24–29	18	24	17	12	16	16	24	19	21	23	16	8	11	11
England	11–17	4	8	13	2	1	3	5	8	16	34	21	14	6	8	27
Wales	1	0	1	2	0	–	1	–	–	–	0	–	–	–	–	–
Isle of Man	0–3	1	1	0	6	–	2	1	1	–	0	–	–	–	–	–
Northern Ireland	169^c	122–133^d	9^e	–	–	1	5	1	2	4	2	–	6	1	–	–
Scotland core areas total	585–602	516–556	443	543	791	1036	1075	1111	1240	1143	1099	1169	1218	1243	946	1272
GB total	706–743	550–595	479	589	824	1064	1107	1138	1285	1186	1164	1215	1256	1259	972	1316
UK total	875–912	672–708	488	589	824	1065	1112	1139	1287	1190	1166	1215	1262	1260	972	1316
Grand total	875–915	673–729	489	589	830	1065	1114	1140	1288	1190	1166	1215	1262	1260	972	1316

^aSurveys in this year known to be incomplete.

^bNo survey in 1978–79 so 1977 survey used.

^cData for 1978 from O'Meara (1979).

^dData from Mayes & Stowe (1989).

^eData from Sheppard & Green (1994).

A summary form was completed for each area surveyed which detailed the birds recorded and identified those records on successive visits which were believed to be of the same individual. If calling was heard on different visits from places less than 200 m apart, the second location was usually assumed to be of the same bird. Records separated by more than 200 m were assumed to be of different birds, unless a particular Corncrake could be identified by a distinctive call or if there was a change in habitat at the first location expected to displace the bird, such as mowing of all suitable grass meadows. Previous studies in Scotland (Tyler & Green 1996) showed that 22% of radio-tagged males (n = 9) moved to a new singing place more than 200 m from their previous singing place after concluding a pair bond with a female, but the double counting resulting from this is balanced to some extent by the males that are not singing during any of the survey visits.

Coverage outside the core areas for the 2009 national survey

Outside the core areas in Scotland, suitable habitat was surveyed in 2009 within all 1-km British national grid squares that were known to have held calling male Corncrakes between 2003 and 2008. The database of previously occupied 1-km squares was collated from several sources, including the Rare Breeding Birds Panel, the British Trust for Ornithology's (BTO) 2007–11 Breeding Bird Atlas (Balmer et al. 2013) and the RSPB's species database. The same approach was also used in Northern Ireland to cover 1-km squares with recently-occupied sites, and selected other areas considered to hold potential habitat. In Northern Ireland there was increased coverage, compared to previous national surveys, in areas thought to be suitable for recolonization.

As with the previous full censuses from 1978/79, there was a publicity campaign in the UK to request records of singing male Corncrakes. This included a collaboration with the BirdGuides website (www.birdguides.com) and a telephone hotline for people to report sightings. Follow-up survey visits were made to all locations from which credible reports of singing birds were received, as soon as possible after the report.

Intensive study on the Isle of Coll

Male Corncrakes were captured for ringing on the Isle of Coll (Argyll & Bute, Scotland) in May in 2012, 2013 and 2014. The whole of an area in the Arileod – Totronald part of the west of the island, centred on 56.599° N 6.625° W and comprising the RSPB reserve and some adjacent areas, was searched intensively for male Corncrakes in each of the three years. Searches were conducted by day and night for 8–15 days per season. Birds were mostly located by hearing the song and were captured at night in a mist-net or hand-net having been attracted by playing a recorded lure of conspecific song. A few birds were captured by day by driving them into funnel traps. The study was conducted under a Schedule 1 licence issued by the BTO. Captured birds were marked individually with BTO rings and their sex was determined from measurements and behaviour (Tyler et al. 1996). Some birds avoided capture, usually because they did not respond vigorously to the song lure. The birds that were not captured were known to be males from their singing behaviour (Tyler et al. 1996). Their locations were mapped and it was established that they were different birds from the captured and other uncaptured individuals by ascertaining that they were singing on the same nights as the others. The numbers of males recorded in these intensive searches were compared with results of the standard census conducted a few weeks later in the same area using the method described above.

RESULTS

Population of Corncrakes in 2009

A total of 1164 singing Corncrakes was recorded in Great Britain in 2009. This is a 41% increase since the previous full survey in 2003. Of these, 1099 (94%) were found within the Scottish core areas, a 39% increase since 2003. On the Scottish mainland, 21 singing Corncrakes were recorded from 14 10-km squares in 2009, compared with 17 males from 18 10-km squares in 2003. The numbers of Corncrakes counted in 2009 are given separately by island or other geographical area in Table 1, together with results from the previous national surveys and the annual core area counts since 2004. Annual counts for 1994–2002 for the Scottish islands included in the core areas are omitted for reasons of space, but are available in Table 1 of O'Brien et al. (2006).

Only two singing Corncrakes were recorded in Northern Ireland in 2009. Singing Corncrakes were recorded in 19 10-km squares in Northern Ireland in the period between 2003 and 2014, but no more than six birds were found in any one year (in 2011, Table 1). Numbers of Corncrakes in Northern Ireland have been much smaller than they were in 1978 for more than 20 years and show no sign of recovery (Table 1, see also O'Meara 1979, Mayes & Stowe 1989, Sheppard & Green 1994).

In England, 23 calling males were recorded from the re-introduced population at the Nene Washes, Cambridgeshire in 2009, and a further 11 were recorded elsewhere in England, mostly in the north. In 2003, the year before the re-introduction began, only two singing Corncrakes were reported from England. There were no records from Wales or the Isle of Man in 2009.

Average annual rates of population change between pairs of full surveys

There have been full surveys of the British Corncrake population at regular intervals, in 1978/79 (Cadbury 1980), 1988 (Hudson et al. 1990), 1993 (Green 1995), 1998 (Green & Gibbons 2000) and 2003 (O'Brien et al. 2006). Average annual rates of population change for Great Britain, Northern Ireland and the Isle of Man combined were calculated for each interval between successive national surveys, using the method of O'Brien et al. (2006). After allowing for incomplete surveys of two island groups in 1978/79, O'Brien et al. (2006) reported that the Corncrake population declined at an average annual rate of 3.4% per year between 1978/79 and 1988. They found that the decline continued at a similar rate (3.5% per year) between 1988 and 1993. From 1993 to 1998 the population increased by an average of 4.2% per year, which accelerated to 7.2% per year between 1998 and 2003. Between 2003 and 2009, our results indicate that the population increased at a lower average rate of 5.8% per year.

Population trends in the Scottish core areas 1993 – 2014

The Corncrake total in the core areas of Scotland increased relative to the previous year in 16 of the 20 years between 1994 and 2014 (Table 1, Fig. 1). The most rapid increase in the core area population occurred between 2001 and 2004, when numbers rose from 596 to 1036 singing males, with an average annual rate of increase of 20%.

Changes in population varied among the Inner Hebrides, Outer Hebrides and Orkney (Table 1). Corncrake numbers in the Inner Hebrides increased markedly in the mid-2000s, mainly driven by increases on Coll and Tiree. In the Outer Hebrides, there was a steady increase in population from 1993. In Orkney, Corncrake numbers have ranged between 6 and 36 singing males between 1993 and 2014. Populations of Corncrakes in the Inner and Outer Hebrides now far exceed the populations recorded in the first national survey in 1978/79, but the population in Orkney has remained below one-third of the 1978/79 level.

Large population decline between 2012 and 2013 and recovery in 2014

The largest decline from one year to the next occurred between 2012 and 2013, when the core population declined by 24% from 1243 to 946 singing birds. However, the core population recovered between 2013 and 2014, increasing by 34% to 1272 singing birds, the highest recorded count in the core areas. There was a marked drop in numbers in 2013 in Inner Hebrides, Outer Hebrides and Orkney (Table 1), followed by immediate recovery in 2014. There was substantial variation among islands in the magnitude of the decline and recovery. Singing Corncrakes declined by only 6% on Tiree, although on nearby Coll the decline was 40% (Table 1). Numbers recovered on both Tiree and Coll between 2013 and 2014, by 14% and 42%, respectively. On Islay and Skye, numbers increased between 2012 and 2013 (Table 1). In the Outer Hebrides, there were declines on all of the main islands between 2012 and 2013. The largest were 47%, 39% and 34% on Lewis, North Uist and South Uist, respectively (Table 1). Numbers on these islands subsequently increased by 87%, 50% and 17% in 2014 (Table 1).

The intensive study on Coll found that the number of males present in the study area in May was higher in 2013 than in 2012 or 2014, even though the standard census conducted later, in June, was much lower than in 2012 and 2014 (see above). The number of singing males recorded in the same area in the standard census in June was lower than the May total in 2013, but numbers were higher in June than in May in 2012 and 2014 (Table 2). The variation among years in the number in May relative to that in June was not quite statistically significant (chi-squared test on the 2 × 3 table in Table 2; χ²  = 5.86, df = 2, P = 0.053).

Table 2. Numbers of male Corncrakes captured and located in intensive searches in May in the Arileod – Totronald area of the Isle of Coll, compared with numbers of singing birds in a later standard census of the same area.

Year	Captured	Not captured	Total	Search period	Census	Census period	Change %
2012	50	5	55	19–27 May	67	5–27 June	+22
2013	46	20	66	11–25 May	44	1–20 June	–33
2014	40	12	52	24–31 May	58	3–30 June	+12

The spring of 2013 was noticeably colder than usual throughout the Scottish core areas. The mean spring temperature (March–May) of the UK Meteorological Office's North Scotland region was 4.5°C in 2013, compared with 6.5°C in 2012 and 7.4°C in 2014 (Meteorological Office 2015). This measure of spring temperature was lower in 2013 than in any other year in which Corncrake surveys have been conducted (Figs 1 & 2). The North Scotland mean spring temperature anomaly (temperature relative to the mean for 1961–1990) was –1.0°C, markedly lower than that for 2012 (+ 0.1°C) and 2014 (+ 1.2°C). The log of the ratio of the singing Corncrake totals in the Scottish core areas in pairs of successive years was significantly correlated with the North Scotland mean spring temperature in the second year of the pair (r = 0.512, t₁₉ = 2.60, P = 0.018), suggesting that Corncrake counts tend to decline compared with the previous year in cold springs (Figs 1 & 2). However, this correlation was strongly influenced by the result for the unusually cold spring of 2013. When the datum for 2013 was excluded, there was a weaker and non-significant relationship between log population ratio and spring temperature (r = 0.336, t₁₈ = 1.52, P = 0.147).

Figure 1. The number of singing male Corncrakes in Great Britain between 1978/79 and 2014. The bars show the number of Corncrakes recorded in the full survey years, while the crosses show the numbers recorded in the core range. Filled circles show the annual spring mean temperature (March–May) in North Scotland (Meteorological Office 2015). Counts in 1978/79 on Barra and Lewis were incomplete, so estimates for those islands were imputed by multiplying the count in 1988 by the ratio of the 1978/79 to the 1988 counts for the other islands of the Outer Hebrides, excluding Barra and Lewis (O'Brien et al. 2006). Where upper and lower limits were given for a count (see Table 1), their average was used.

Figure 2. Annual estimates of the rate of growth of the Corncrake population of the Scottish core areas between pairs of successive years in the period 1993 and 2014 in relation to the mean spring temperature (March–May) in North Scotland in the second year of each pair.

Changes in the extent of the geographical range

The extent of the broadly defined geographical range in Great Britain, in terms of 10-km squares occupied by Corncrakes in national surveys, decreased from 160 in 1978/79, to 90 in 1988 (Green & Gibbons 2000), but the number of 10-km squares recorded as occupied in national surveys has not changed much since then. The number of all occupied 10-km squares showed a modest increase from 81 to 92 squares between 2003 and 2009 (Figs 3 & 4). However, there has been a slight but consistent increase in the number of occupied 10-km squares within the core areas in Scotland between 1993 and 2014 (Figs 3 & 4).

Figure 3. Numbers of 1-km (filled symbols) and 10-km (open symbols) Ordnance Survey grid squares with singing Corncrakes in national surveys of Great Britain and the Isle of Man (triangles) and annual surveys of core areas of Scotland (circles), from 1988 to 2014.

Figure 4. Corncrake distribution by 10-km square in Scotland during the full survey years of (a) 1988 (88 10-km squares), (b) 1993 (74 10-km squares), (c) 1998 (79 10-km squares), (d) 2003 (76 10-km squares) and (e) 2009 (84 10-km squares).

The increase in the number of 1-km squares with Corncrakes in national surveys has been more pronounced, increasing from 324 to 407 1-km squares between 1988 and 2009, a 47% increase in occupied 1-km squares (Fig. 3). The number of occupied 1-km squares in the Scottish core areas increased by 19% between 2009 and 2014, despite a marked decline in the number of occupied squares in 2013 (Fig. 3).

The three Breeding Bird Atlas projects of the BTO also provide information on changes in the geographical range of Corncrakes (Sharrock 1976, Gibbons et al. 1993, Balmer et al. 2013). The number of 10-km squares in Britain from which Corncrakes were recorded fell markedly by 72% between the first and second BTO atlases (1968–72 and 1988–91), but increased by 14% between the second and third atlases (1988–91 and 2007–2011) (Table 3).

Table 3. Numbers of 10-km grid squares with Corncrakes in three BTO Breeding Bird Atlas periods (A1–A3).

	1968–72 Atlas 1	1988–91 Atlas 2	2007–11 Atlas 3	% change A1–A2	% change A1–A3	% change A2–A3
Great Britain	660	161	184	–75.6	–72.1	+14.2
England	193	16	16	–91.7	–91.7	0
Scotland	399	142	166	–64.4	–58.4	+16.9
Wales	55	1	1	–98.2	–98.2	0
Northern Ireland	126	37	15	–70.6	–88.1	–59.5
UK	786	198	199	–74.8	–74.7	+0.5
Isle of Man	13	2	1	–84.6	–92.3	–50.0

DISCUSSION

A striking finding of our study was the marked fall in Corncrake population and range in the Scottish core areas in 2013 and the rapid recovery in 2014. If these changes are genuine, they illustrate both the sensitivity of the population and its resilience and capacity for rapid recovery. It seems probable that the low count in 2013 was associated with the unusually cold spring in that year. The March–May mean temperature in North Scotland in 2013 was 1.0°C below the long-term mean. Numbers of male Corncrakes present in a part of Coll surveyed intensively in May were actually higher in 2013 than in 2012 or 2014, but the standard count conducted in June 2013 was much lower than in 2012 or 2014. The mean temperature in June 2013 in North Scotland was not unusually low (0.5°C above the 1961–1990 mean), so a direct effect of temperature at the time of the June count is unlikely. Hence, the 33% fall in the count on Coll between May and June suggests that a large loss of males may have occurred during that period.

Usually it would be expected that the number of males detected would increase between May and June because some males probably do not arrive until late May (Green et al. 1997a) and a high proportion of males are in pair bonds in May, during which time they sing infrequently (Tyler & Green 1996). It was observed that the ground vegetation on Coll, used as cover by Corncrakes, was unusually short and sparse in May 2013, presumably because of the cold spring. The height of vegetation in the same 12 patches of mixed Nettle Urtica dioica, Cow Parsley Anthriscus sylvestris and grasses at field margins was judged by eye in each year by estimating the proportion of the areal extent of each patch that was more than 10 cm in height to the nearest 5%. The means of these percentages were 53% on 21 May 2012, 4% on 22 May 2013 and 78% on 25 May 2014. Radio-tracking studies in Scotland and Ireland have shown that Corncrakes rarely use vegetation less than 10 cm in height (Tyler 1996), so these results suggest that suitable cover was scarcer than usual in May 2013. It is possible that more Corncrakes than usual may have been killed by predators such as Hen Harrier Circus cyaneus, Buzzard Buteo buteo and Otter Lutra lutra, because of the lack of concealment provided by the early cover. Also, some birds may have moved from Coll to settle elsewhere. Tiree, which is adjacent to Coll, has a much larger area of suitable habitat and is known to have much more Yellow Flag Iris Iris pseudacorus than other Hebridean islands, including Coll (Green 1996). Iris tends to be taller in May than other cover plants such as Cow Parsley and Nettle and its stout leaves provide shelter that enhances the growth of grasses and other cover species. Hence, it is possible that the much smaller reduction in the Corncrake count for Tiree between 2012 and 2013 (a fall of 6%), compared with that for Coll (a fall of 38%) was due to some birds relocating from the poor early cover of Coll to the better cover of nearby Tiree. However, redistribution is unlikely to be the explanation of the marked decline in the total population of the Scottish core areas because long-distance movements of Scottish Corncrakes during the breeding season are infrequent (Green 1999) and the arrival of large numbers of displaced Corncrakes elsewhere in the UK would probably have been noticed.

It is more likely that the low count of Corncrakes in 2013 did not reflect the numbers of birds present. The proportion of males present that were detected during the standard counts may have been unusually low in 2013. This might occur if male Corncrakes were to sing less frequently during the main survey period in late May and June if the vegetation is short and sparse after a cold spring. They might do this because of the risk of predation in short vegetation. An effect of vegetation height might occur even when the temperature in June was normal, as it was in 2013, because plant growth would have been delayed by cold conditions earlier in the year. There are insufficient studies of singing behaviour of male Corncrakes in relation to vegetation height to test this hypothesis.

The population and geographical range of the Corncrake in the UK declined markedly throughout most of the 20th century (Green 1995), but a partial recovery began in 1993, co-incident with the beginning of actions to reduce the impact of hay and silage mowing on breeding success and to encourage the provision of suitable habitat (O'Brien et al. 2006). The population recovery was most rapid between 2003 and 2008. Since then, the Corncrake population has continued to increase at a lower rate, but it now appears to be levelling off. The Corncrake population might be reaching a plateau because population densities are approaching a carrying capacity set by the area of habitat with suitable vegetation structure and plant species composition. Densities were probably formerly kept well below carrying capacity because of the impact of mowing on breeding productivity (Green et al. 1997b). Conservation action has reduced the impact of mowing on breeding success substantially since 1993 (O'Brien et al. 2006), but the extent of suitable habitat has not changed much. In particular, the area of land per male Corncrake that is managed as early or late cover under agri-environment schemes has remained low (O'Brien et al. 2006). Hence, the Corncrake population may now be reaching a limit set by the area of suitable vegetation.

O'Brien et al. (2006) showed that the introduction of land management agreements and initiatives, mainly through agri-environment schemes, have been associated with the recovery of the Corncrake in the core range in Scotland. Agri-environment schemes for this species under the existing Scottish Rural Development Plan (SRDP) ended in 2014, but the Corncrake should continue to be a high priority for conservation action through the new SRDP from 2015. Delivery through the new SRDP in Scotland will be crucial, both in terms of population recovery and potential range expansion.

The population increase and geographical spread of the Corncrake since 1993 has occurred almost entirely within the core areas of the Hebridean islands and Orkney. The population and range of the Corncrake in mainland Scotland has changed little since 1993. It may be necessary to establish larger areas of late-mowed or late-grazed tall grass with associated areas of early cover vegetation than currently exist in western parts of the Scottish mainland if Corncrakes are to re-establish there.

Few Corncrakes have been recorded in Northern Ireland in the last decade. Annual fluctuations in numbers and changes in location indicate that there is no secure population there. In England, a re-introduction project began at the Nene Washes (Cambridgeshire) in 2002 and most of the singing males recorded in England since 2009 have occurred here. Numbers of singing males at the Nene Washes have varied among years because of fluctuations in the number of zoo-bred young available for release and breeding success in the wild. Numbers fell markedly in 2012 because of an unusual flood in spring and early summer. Breeding success in the wild was also low in 2012 because of the flood at the Nene Washes, but releases of captive-bred birds continued and breeding in the wild was unusually good in 2013, leading to a large number of singing males in 2014. Despite this recent encouraging sign, it remains uncertain whether the Nene Washes population will become self-sustaining in the long term.

ACKNOWLEDGEMENTS

The additional survey work in 2009 was funded by the Northern Ireland Environment Agency and RSPB under the Statutory Conservation Agency and RSPB Annual Breeding Bird Scheme (SCARABBS). Surveys were carried out by regional RSPB staff and volunteers, and we particularly thank Lisa-Marie Newth, James Homer and Laura Smith for their contributions to surveys of non-core areas. Annual monitoring of the core population is undertaken by RSPB staff and volunteers and the National Trust for Scotland. We thank Will George of the RSPB Conservation Data Management Unit for collating the annual survey data, Simon Gillings for the data from Bird Atlas 2007–11, which is a joint project between BTO, BirdWatch Ireland and the Scottish Ornithologists' Club, Mark Holling for access to the Rare Breeding Birds Panel records, Kendrew Colhoun for data from Northern Ireland and Ben Jones for help in surveying Corncrakes on Coll.