The Choughs of Bardsey

P.J. Roberts n recent years, several studies on the Chough Pyrrhocorax pyrrhocorax Ihave shed light on the species' breeding ecology and distribution in Britain and Ireland (Bullock et al. 1983a), and given details of feeding and foods taken (Bullock 1980; Warnes 1982; Roberts 1982, 1983). This paper gives information on population structure, dispersal and biometrics: spheres in which diere are few reliable published data. It is based on observations of individually colour-ringed Choughs on Bardsey, Gwynedd, over a six-year period 1978-83. When studying a small and long-lived population of birds, data are sparse and slow in accumulating; the results are, therefore, preliminary and tentative in some aspects. They are presented here as part of a general overview of Chough dispersal and population dynamics throughout Britain and Ireland.

Background and study methods Bardsey is an island of 180ha, 3km off the tip of the Lleyn Peninsula in Gwynedd, North Wales. Choughs have traditionally bred on the island and adjacent mainland coastline. Recent national surveys (Rolfe 1966; Bullock et al. 1983a) have shown stable numbers throughout an unchanged breeding range, including about 50 pairs in Caernarvonshire (Gwynedd). In this period, numbers on Bardsey have increased from two to four pairs (1954-78) to five to seven (1979-83). Breeding numbers and ringing totals since the observatory was founded in 1953 are shown in table 1. During 1954-83, owing to some nest sites being inaccessible, and to the closure of the observatory in 1971-73, a maximum of 40-45 nestlings (about 20% of the total) may have fledged unringed. Since 1978, all Choughs ringed have been given three colour rings to enable individual recognition in the field. During 1978-83, a maximum of 10% of nestlings may have gone unringed. Within this study period, 29 full-grown Choughs were also ringed (see table 1); these, caught mainly in late autumn, represent about 30% of all full- grown Choughs ever ringed in Britain and Ireland. The island was manned annually from March to November or Decem­ ber; midwinter records are, therefore, scant or lacking. All casual observations of colour-ringed Choughs were recorded daily by visitors and

[Bnl. Birds 78: 217-232, May 1985] 217 218 The Choughs ofBardsey

Table 1. Ringing totals and numbers of nests of Choughs Pyrrhocorax pyrrhocorax on Bardsey, Gwynedd, since 1954 NO. RINGED Year No. nests Nestlings Full-grown

1954 2 8 1955 2 8 1956 2 3 1957 2 1 1958 2 3 1959 1 4 1960 2 7 1961 2 9 1962 3 9 1963 3 4 1964 4 3 1965 3 4 1 1966 4 4 1967 4 5 2 1968 2 3 1969 3 6 1970 3 8 1974 3 7 1975 4 5 1976 3 5 1977 3 7 1978 4 15 15 1979 5 11 11 1980 6 6 1 1981 7 20 2 1982 6 15 1983 6 10

staff. Additionally, I made weekly Chough 'forays' for at least half a day, specifically to observe and record the population. With so many observer- hours on such a small and discrete area, it was possible to gain an accurate picture of presence or absence of individuals. Any uncertain or incomplete records were excluded from this analysis. Thousands of further observa­ tions were made on all aspects of behaviour, and some of this anecdotal information is included where relevant. All data are put into a national context by reference to other Chough studies, British and Irish ringing recoveries, and county records. Results Mortality and longevity Fig. 1 shows recoveries by month of all Bardsey-ringed Choughs. Even from this small sample, it is clear that there is a high mortality of young in summer, when family groups have broken up, as noted by Holyoak (1971) for other members of the crow family (Corvidae). Death of older birds occurs mainly in winter months (the recoveries in October and April refer to long-dead individuals, whose actual date of death is unknown). A very similar pattern of mortality was shown by Bullock et al. (1983a) using data from the whole of the United Kingdom, although their winter peak was slightly greater than that for first-autumn Choughs. The Choughs of Bardsey 219 Bullock et al. (1983a) tentatively suggested a first-year mortality, based on all BTO ringing recoveries, of 85%, with a similar figure for second-year mortality. By using sightings of colour-ringed young surviving to adult­ hood, it was possible to calculate maximum mortality rates for each of five years in the study period, thus adding to and refining the calculations of Bullock et al. Although the Bardsey data are few and potentially unreliable, the large amount of time spent observing and searching for colour-ringed Choughs gives a little more validity to the figures. Table 2 shows that, in three of the five years, first-year mortality was below the figure given by Bullock et al. (1983a), and in 1979 was at most 45%. Obviously, such a small sample size can give widely varying results, but at least in some years survival was quite high in this population. The difference in first-year mortality between the years 1978-80 and 1981-82 is significant. This is reflected in fig. 2, where life histories of 50% of young ringed in 1978-80 continue over two and three years, compared with just one such individual for the period 1981-83 (observations continuing into 1984 to allow for further sightings). In addition, the number of fledglings known to have died in their first year varied from nil to two for each year in the study period, except 1981 when six were found, four of these within two or three months of fledging (see fig. 2).

Table 2. Mortality rates of Choughs Pyrrhocorax pyrrhocorax on Bardsey, Gwynedd, 1978-82 For subsequent years, one year is calculated as a 12-month period from time of fledging. Mortality 77.6% in year 1; 46.7% in year 2

NO. SEEN ALIVE IN Year of No. SUBSEQUENT YEARS First-year fledging fledged 1 2 3 4 5 6 mortality (%)

1978 15 4 4 2 1 1 73.4 1979 11 6 1 — — — 45.4 1980 6 2 2 1 — 66.7 1981 20 1 1 — 95.0 1982 15 2 — 85.7

TOTALS 67 15 8 3 1 1 0

Reasons for this large difference in first-year mortality between the two periods are not known. Personal observations, however, suggest that it is due at least partly to climatic conditions adversely affecting food supplies at the times of the usual peak mortality as shown in fig. 1. The early-autumn peak in first-year mortality is due largely to the juveniles' inexperience in finding food without parental help at a time when food availability is 'patchy'; staple prey items such as ants (Formicoidae) and beetles (Coleoptera) are often in poor supply (Roberts 1982, 1983). The winter mortality has already been attributed by Holyoak (1971) to the Chough's specialised feeding requirements, and Bullock et al. (1983b & in press) also suggested that hard winters could have serious effects on mortality if feeding became poor. These natural factors were thought to be exacerbated in the years 1981-83, when more severe and extreme weather conditions The Choughs of Bardsey

first-year mortality

Fig. 1. Monthly distribution of mortality of Choughs Pyrrhocorax pyrrhocorax on Bardsey, Gwynedd, based on 23 ringing recoveries disrupted the complex life cycles of invertebrate prey in summer, making food more difficult to find in winter. When data for all five years are combined, the average mortality in the first year is 77.6%, very close to the figures given by Bullock et al. (1983a). Calculations of second-year mortality are very spurious, but in one year produced a maximum of 83%, and averaged 46.7% for the whole study period. A male Bardsey Chough had the greatest proven longevity of any wild Chough. Ringed as an adult in March 1965, it bred successfully on the island until its death in December 1981, when at least 17 years old. Darke (1971) suggested that the last surviving Cornish Chough was one of a pair that last bred in 1947, remaining subsequently as non-breeders; it died in 1973, when at least 27 years old.

Social structure Colour-ringing of nestlings and full-grown Choughs has allowed preparation of life histories for 76 individuals, from which details of social structure and behaviour can be deduced. Fig. 2 shows the known histories of those ringed as nestlings and gives valuable facts about known-age Choughs. Fig. 3 shows life histories of Choughs ringed as full-grown in autumn (mainly October and November). As there seems to be no reliable way of ageing Choughs after about September, most of those caught were of unknown age; all that could be deduced was that they were not Bardsey- bred, as these were all ringed. Any first-years would by now have got past the critical early-autumn mortality period illustrated in fig. 1. Their chances of survival might be expected to be higher, and this is to some extent borne out by the greater proportion of birds in fig. 3 with longer histories. Of the full-grown Choughs trapped, seven were from the island's breeding population. The Choughs ofBardsey 221

I

IMiW O B:-,Wrf

1978 1979 1980 1981 1982 1983 Fig. 2. Sightings of Choughs Pyrrhocorax pyrrhocorax colour-ringed as nestlings on Bardsey, Gwynedd, 1978-83. Life histories start in June each year, at the time of fledging. Black bars = bird present on island; open bars = bird absent from island; o = sighting on mainland; + = found dead on island; X = found dead on mainland; stipple = no observations BREEDING INDIVIDUALS Pairs remained together all year. They freely joined feeding flocks at all times, but were discernible as pairs within the larger group. This was suggested also from observations in other areas by Williamson (1959), Coombes (1978), Bracegirdle (1964) and Praz (1971). Breeding pairs appeared totally sedentary, remaining on the island throughout the year. Praz (1971), watching Choughs in winter in the Valais region of the Alps, noted breeding pairs wintering at or near the nest site, both alone and in flocks with other Choughs. This has possibly given rise to claims by many authors(e.g. Goodwin 1976; Coombes 1978) of sedentariness for the species as a whole. 222 The Choughs oJBardsey

94. Nestling Choughs Pyrrhocorax. pyrrhocorax, Gwynedd, May 1982 (P.J. Roberts)

At least during the study period, pairs were completely faithful to one another, and to their nest sites. Most sites are 'traditional', several being used annually. The farthest any pair moved was about 10 m within the same boulder-beach nesting site. Only one individual among the five to seven breeding pairs was known to have changed during the study period: this was due to the death of the 17-year-old male mentioned above; the female retained the nest site, attracted a mate, and bred successfully without missing a season. Not all pairs bred annually. One marked pair bred in 1980, then held territory at the same site for two subsequent years without any attempt to breed. This interrupted breeding has not been recorded previously, although Holyoak (1972) noted non-breeders holding territory. Apart from a 20% chance of an unringed Bardsey individual being present, observations showed that none of the breeding Choughs was reared on the island. Cowdy (1962) also noted a total absence of Bardsey-reared Choughs in the population in 1961.

DISPERSAL It is clear from fig. 2 that most Choughs were not seen beyond their first few months after fledging, although there is a great deal of annual variation (see Mortality and longevity). Observations of a total of 17 family groups over four breeding seasons showed that young stayed with parents as a family unit for up to 43 days after fledging (Holyoak 1972 recorded periods of 28-35 days on the Calf of Man); after 48-50 days, young were seen on their own, and some had left the island. This corresponds well with the time of known highest first-year mortality as shown by ringing recoveries. Fig. 2 illustrates the diversity of dispersal patterns of those surviving first-year individuals. Three birds (k, 1 & m in fig. 2) left the island during their first autumn The Choughs of Bardsey 223 (from August to October), were seen in mainland flocks, and never returned. Many other fledglings (b-j in fig. 2) left in their first autumn, but returned intermittently in that same autumn and in later years. Of 15 fledglings, one was absent only in August; 11 were absent in August and September, returning in October; and two were absent from August to October, returning in November. Several of these disappeared again, but returned sporadically in subsequent summers and autumns (e.g., c in fig. 2). At the other extreme, bird a in fig. 2 remained permanently on the island, in the non-breeding flock, for three-and-a-half years, before either dying or leaving the island. By the end of the study period, none of the colour-ringed young was still on the island, and there was no evidence of the presence of any earlier-ringed young either; this is in strong contrast to the highly sedentary nature of the breeders. The pattern of occurrence and the life histories of the unaged Choughs in fig. 3 are very similar to those of the young in fig. 2 which survived beyond their first autumn. The largest number of Choughs occur on the island in late autumn: representing a dispersal from the mainland to Bardsey at the same time as Bardsey young are dispersing away from the island. The sedentary nature of the breeding birds, and biometric data (see page 228), suggest that many of these unaged autumn arrivals are first-years dispersing from nesting areas on the mainland, mixed with a few older, non-breeding individuals. Two birds (a & b in fig. 3) remained in the island non-breeding flock for three years after capture; bird b returned inter• mittently in the fourth year, and was then seen back on the mainland. Birds d-h in fig. 3 remained in the non-breeding flock for one to two years, and were then seen sporadically in their third and fourth years, with bird g on the mainland later; birds c and i were seen on the island only in the autumn of capture, but were found on the mainland again respectively four and one years later.

Fig. 3. Sightings of Choughs Pyrrhocorax pyrrhocorax colour-ringed as unaged, full-grown, on Bardsey, Gwynedd. Life histories start in October/November each year, at time of ringing. Black bars = bird present on island; open bars = bird absent from island; o = sighting on mainland; X = found dead on mainland; stipple = no observations

PAIRING AND FIRST BREEDING Definite ages for first breeding by Choughs were unknown until recently. Bird 1 in fig. 2 was seen on three occasions over two years at Llangrannog, Cardiganshire; on the last occasion, as a three-year-old, it was breeding. On Islay, Argyll, J. M. Warnes (in litt.) proved breeding in the second year. 224 The Choughs ofBardsey Other evidence from Bardsey suggests that many Choughs may not breed until their fourth year or later: for example, bird a in fig. 2 could not have bred until at least its fourth year, when it disappeared from the non- breeding flock. Similarly, birds a & b in fig. 3 did not breed in the three years they were resident on the island, thus making them at least four years old before they could have bred (assuming that they were first-years when caught, and that they bred immediately after leaving the island). Bird c in fig. 3 was found breeding on the mainland four years after having been ringed: the site had been closely watched each year, and the bird had not bred there before; given the nest-site tenacity of breeding Choughs, it is unlikely that it had bred elsewhere in the intervening period, which further suggests an age of first breeding of at least four years. Holyoak (1972) suggested an age of first breeding of three years, on the assumption that the Chough is similar to other, better-studied, members of the crow family; he also noted that some first-years were already paired. With the individually-marked Bardsey Choughs, however, it was clear that 'first pairing' was not so simple as this. Known one- and two-year-olds were seen to go through pair-bonding and pairing activities, such as mutual feeding, allopreening and other displays. These individuals were not all paired, but were clearly promiscuous: several examples of one bird being apparently paired with another, then that other seen to be paired with two further, different, Choughs in later weeks, indicate more exploratory activity than pairing for life. That many Bardsey Choughs did not breed until four years old may reflect the more dense population on the island compared with some other parts of the British and Irish range: young Choughs may be unable to compete for territory, and nest-site availability may be limited, causing delayed breeding and perhaps greater dispersal.

95. Chough Pyrrhocorax pyrrhocorax feeding on beach, Gwynedd, November 1979 (P.J. Roberts) The Choughs of Bardsey 225

FLOCKING AND ROOSTING Few references have been made to size and function of non-breeding flocks, so much a feature of the Chough population. In the Abruzzi National Park, non-breeders were noted as forming 20% of the total population in 1975 (Lovari 1976a); in the UK, 30% has been quoted (Holyoak 1972; Rolfe 1966). During 1953-83, on Bardsey, the figure varied greatly from year to year, ranging from 20% to 60%. Within the study period 1978-83, the largest non-breeding flocks were recorded in 1978-80, when first-year mortality was significantly lower than in 1981 -83 (see pages 218-220). This suggests that numbers of non-breeders may be related in part to better first-year survival, resulting in more 'spare' birds in the population as a whole. In Italy, these non-breeding summer flocks were noted by Lovari (1976b, 1978) as the largest gatherings of the year, the birds tending to split into smaller groups and pairs at other seasons. On Bardsey, maximum annual flock size has occurred in all months, but with distinct influxes and flocking in September and October in most years. This is due partly to intensive feeding activity on a sandy beach rich in invertebrate foods, but such larger autumn and winter flocks are also recorded at several other UK localities which have less of a focal point for feeding. Roosting behaviour is very variable according to time of year and to age and status of individuals. The pattern of roosting over the year seems generally to follow the pattern below. Communal winter roosts holding up to'50 or more Choughs have been recorded at several localities (contra Coombes 1978). These are regularly used from October to March (I. Bullock in litt.), and can include individuals of all ages; they may be used sporadically by some birds of all ages from August onwards. On Bardsey, a large area of open, east-facing cliff was used at least from August to November, when observations for the winter ceased. Individually-marked Choughs could be picked out by torch­ light; these included first-years and variously aged non-breeders, but not breeders. During diis same period, nest sites and surrounds were also checked, but no breeders were found here either. During the breeding season, non-breeders used a variety of loose summer roosts. Breeders roosted at or near the nest site according to their stage of breeding; as chicks hatched and grew, so adults moved off the nest site to nearby ledges within the area. Cowdy (1962) noted that, on Bardsey, fledged young and their parents roosted at or near the nest site while they remained as a family unit into July. Williamson (1959) confirmed this for the Isle of Man, adding that, by late August, breeding Choughs had moved to ledges farther from the nest site.

Movements Fig. 4 shows all recoveries of Bardsey-ringed Choughs, and sightings of colour-ringed individuals away from the island. Both types of record reflect a similar pattern. The majority of birds did not move far (up to 20km). Three were recovered at longer distances: two 46km along the Lleyn Peninsula, and one 142km away in Liverpool (the farthest recovery for any Chough). Ringing recoveries for other localities indicate generally shorter 226 The Choughs of Bardsey

► Liverpool »Llangrannog

Fig. 4. Dispersal of Choughs Pyrrhocorax pyrrhocorax ringed on Bardsey, Gwynedd. o = live (ringed as nestling);.* = dead (ringed as nestling); X = live (unaged); □ = live (ringed as fall-grown); ■ = dead (ringed as full-grown) movements: the majority from the Irish Republic, Islay and the Isle of Man are less than 10km, with a few up to 27km. Other notable recoveries are of an Anglesey Chough found 32 km along the North Wales coast, four in Europe 25km, 40km, 50km and 60km from the ringing sites, and one in Switzerland 130km from the ringing site (Busse 1969). Sightings of colour-ringed Choughs were up to 20 km distant, within the breeding range on die Lleyn Peninsula, and also 30 km and 45 km away in Snowdonia. Condry (1981) suggested that coastal-breeding Choughs in North Wales might well disperse inland to the smaller and slightly less successful Snowdonia population; these records add weight to this theory. The farthest definite sighting of a Bardsey Chough was 70 km south, at Llangrannog in Cardigan Bay (bird 1 in fig. 2). There are also two reports, not fully confirmed, of Bardsey Choughs on Islay and in Pembrokeshire, respectively 340 km and 160 km distant. Ringing recoveries all comprised dead or dying birds, mostly in their first autumn. This could be dismissed as an enforced dispersal of 'weak' or 'inferior' individuals unable to compete, and doomed to die. While this may be true in some cases, the sightings of colour-ringed Choughs give a more balanced and hopeful picture. Although they, too, were mainly of first- years, some were of surviving and healthy individuals which had success­ fully joined other populations. The three distant sightings were all of Choughs associating with breeding birds as 'helpers', showing some interest in pairing, or eventually breeding. The Chough seen at Llangrannog had spent its first autumn on Bardsey, having dispersed to Llangrannog by the following spring: it bred after two years and has remained there since. The Choughs ofBardsey 227

Fig. 5. Records of Choughs Pyrrhocorax pyrrhocorax away from breeding range in Britain and Ireland since 1960. Black areas = breeding range; + = single record; += four records; hatching = several records in area Fig. 5 shows the present breeding range of the Chough in Britain and Ireland, and records away from this range since 1960. It illustrates clearly the species' dispersive ability. Bullock et al. (1983a) showed that this range had changed little over the past 20 years, since the last survey (Rolfe 1966) and compared with the Atlas (Sharrock 1976). Thus, the records collected from county recorders have all emanated from a closely similar breeding distribution, regardless of which year they relate to. All accepted county records are included, except those considered by recorders to refer to escaped birds. Some areas have several undated records adjacent to the breeding range. Records, all of live Choughs, occur in all months, with no apparent pattern, some individuals staying for several months in the same area. 228 The Choughs ofBardsey It is apparent that there are many sightings at considerable distance from the nearest breeding site. Of the Scottish records, the two Orkney ones are 360km from the nearest breeders on Islay. The east coast of England records are 210km from the nearest (Isle of Man) population. The Cornwall and Devon sightings are 100km from the nearest breeders in Pembrokeshire. The bulk of records, however, come from the east shores of the Irish Sea, bordering Lancashire and Cumbria. These birds are likely to be equally of Isle of Man or North Wales origin. The records from the east coast of Ireland are of interest, but it is difficult to know whether they relate to Irish breeders or not. All are nearer to the British breeding populations (i.e. Scottish, Manx or Welsh), up to 80km away by sea. Goodwin (1976) stated that the Chough is extremely sedentary, and mentioned its reluctance to colonise any of the other Canary islands 40 km across the sea from its stronghold on Palma. The individual on Barra, Outer Hebrides, however, must have made a substantial sea-crossing (fig. 5). Several Lancashire and Cumbria records involved Choughs which appeared at the same time as, and mixed with, other corvids from a westerly (Isle of Man?) direction. The Irish record from Houth Head, Dublin (the first for 70 years on the Irish east coast), was seen flying with auks (Alcidae) and Fulmars Fulmarus glacialis after a night of easterly gales, suggesting a crossing from Wales. Eight recent instances of BTO ringing recoveries of other crow species also support the idea that crossings of the Irish and other seas are not unusual for crows. Six Jackdaws Corvus monedula ringed in Wexford, southeast Ireland, were found in various parts of Wales and one in north Yorkshire. A Rook C.frugilegus, also ringed in Wexford, was found in North Wales, whilst two Ravens C. corax crossed in the other direction: one from Kin tyre, Strathclyde, to Northern Ireland, and the other from North Wales to Dublin. Long-distance movements of Choughs are not a new phenomenon: several older records relate to movements of at least 150- 300km over the past century, when the species' range in Britain and Ireland was more extensive. It is evident that Choughs will make sea-crossings, and that there is much more mobility, at least from first-years and non-breeders, in the British and Irish population than is at first apparent. Fears of inbreeding in a somewhat fragmented breeding range seem unfounded, and the possibil­ ity of recolonisation of former breeding areas is quite real, given that suitable habitat still remains or can be created. Biometrics When full-grown Choughs were trapped (mostly in October and November), data were collected on their weight and on their length of bill and wing. Both known-age and unaged individuals were caught, some retrapped several times over long periods. Bill was measured from tip to feathering, and wing was measured as 'maximum chord' (see Svensson 1984). Although the sample is small, the data are, I believe, the most comprehensive and reliable yet available. Table 3 compares these data with those from various other sources. Witherby et al. (1941) measured wings of eight males and five females by 'standard chord', a method rarely used on live birds today and one which The Choughs ofBardsey 229

Table 3. Wing lengths and bill lengths (mm) and weights (g) of male and female Choughs Pyrrhocorax pyrrhocorax All data refer to British race P. p. pyrrhocorax, except Schifferli & Lang (P. p. erythroramphus). Wing length measured by 'maximum chord' for Bardsey, Calf of Man, Svensson and British Museum (skins); 'standard chord' for Witherby et al.\ not known for SchifTerli & Lang and Vaurie. Bill length measured from tip to feathering in all except Schifferli & Lang (method not known), and Calf of Man (tip to skull). Bardsey, Calf of Man and Vaurie data from definite adults. Sample size not known for Svensson

Wing Bill Weight Wing Bill Weight

BARDSEY Mean 297 49.5 366 274 46.0 309 Range 289-304 46-52 350-380 266-278 44-47 285-325 Sample size 6 6 4 6 6 5 WITHERBY et al. ( 1941) Mean — — — — — — Range 257-281 41-49 — 245-271 39.5-48 — Sample size 8 8 — 5 5 — BRITISH MUSEUM Mean 276 46.7 — 260 44.5 — Range 260-281 42.50 — 249-269 41-51 — Sample size 7 7 — 10 10 — VAURIE (1954) Mean 276 54.7 — 271.5 52.0 — Range 268-293 51-59 — 266-278 50-53 — Sample size 8 8 — 5 5 SVENSSON (1984) 270 — — 258 — — SCHIFFERLI & LANG (1940) 309 52.5 350 295 50.5 293 Sample size 1 1 1 1 1 1

CALF OF MAN 286 56 335 261 53.5 302 Sample size 1 1 1 1 1 1 gives shorter results. Their figures do not fall into the same range as that from Bardsey; their birds are also unaged, hence perhaps the overlap in range between the sexes, but there is still a clear difference in size between male and female. Measurements taken by myself from ten female and seven male skins of the British race P. p. pyrrhocorax in the British Museum (Natural History) again show this clear size difference; as with Witherby et al.'s (1941) and more recently with Svensson's (1984) data, however, the range is quite different from my own on Bardsey, as it fails to take into account size differences between first-year and older birds. Mayaud (1933) stressed the difference in size between adult and first-year Choughs, but quoted only Witherby et al.'s measurements for British individuals. Data from Vaurie (1954) were for fully adult British birds (method of measurement unknown). His data for females fit my own almost exactly, but those for males are much smaller, for which there is no apparent explanation. Fig. 6 shows wing length and weight of adult Choughs trapped on Bardsey and the Calf of Man; fig. 7 shows wing and bill lengths of all Choughs trapped on Bardsey. I found no overlap in wing length or weight 230 The Choughs of Bardsey 310r

-4-» bo a bo' c

260L 280 300 320 3A0 360 380 Weight (g) Fig 6. Wing length and weight of adult Choughs Pyrrhocorax pyrrhocorax on Bardsey, Gwynedd, and Calf of Man, Isle of Man. o = unsexed; • Calf of Man between the sexes, males all being distinctly larger, although there was a 1-mm overlap in bill length. The latter is contra Goodwin (1976), who suggested that sexual size difference in Choughs is most apparent in bill length. Measurements (by unknown methods) of a breeding pair of the race P. p. erythroramphus in Switzerland by Shifferli & Lang (1940) (table 3) support my findings of a big difference in weight between the sexes; a smaller, but noticeable difference in wing length; but little difference in bill length. Measurements from the Calf of Man are the most comparable with those for Bardsey, being taken recently from live British Choughs. They give closely similar results to those from Bardsey, the unsexed adults falling into two clearly differentiated size groups, and the two sexed birds equally widely separated. Measurements of juveniles (caught from fledging up to late August, not shown in fig. 7) showed an enormous range in size and weight, indicating very variable rates of growth. One caught just after fledging had a wing of 247mm; while four caught in August had wings averaging 272mm (263- 283mm), and bills averaging 41 mm (36-46mm). Clearly, recently fledged Choughs are far from fully grown, and bill length seems to develop more slowly than wing length. Weights of these juveniles were very variable, and appeared to reflect condition of health rather than size. Seven had weights of 237-360g, and one found freshly dead weighed 220g. The two lightest of the live young weighed respectively 23 7 g and 245 g: both were in poor condition, yet both survived to the next year, one being the bird established at Llangrannog. The unsexed first-winter Choughs shown in fig. 7 seem to split into two size groups, the larger of the two being in the adult female range. This suggests a sexual size difference in first-winter Choughs that are not fully grown. On the assumption that, on average, 50% of the birds should be male, it would seem that the larger ones are first-year males (equal to adult female in size), growing to adult male size in the following years, and that the smaller ones are first-year females. The Choughs of Bardsey 231 55r

1h

C m 4

35k 250 260 270 280 290 300 310 Wing length (mm) Fig. 7. Wing and bill lengths of Choughs Pyrrhocorax pyrrhocorax on Bardsey, Gwynedd. o = unsexed adult; • unaged; X = first-autumn The unaged individuals show a similarly large size range, although 85% fall into the first-year range, but with no clear split into two size groups. Accepting that adults appear to be very sedentary, and given the known dispersive pattern of these unaged birds, combined with their biometrics, it seems very probable that most are first-years dispersing from the mainland. The few larger individuals may be older non-breeders of adult size. In support of this theory, an unaged Chough at the lower end of the size range, and thus theoretically a first-year female, was retrapped two years later as an adult, in the middle of the female size range.

Acknowledgments Thanks are due to Mrs J. Stroud, P. Thomson and D. Owen for their comments on their own studies of Choughs in other areas. Most of the regional recorders were extremely helpful in their replies to my requests for information, as were the BTO staff. The British Museum (Natural History) kindly allowed my examination of the skin collection. The Calf of Man Bird Observatory kindly allowed use of their biometric data for this analysis. On Bardsey, many of the visitors, as well as Mrs J. Strick and assistant wardens G. Tucker, C. Rowley, M. Peacock and D. Suddaby, contributed greatly to this study by their detailed observations. Special thanks are due to Ian Bullock, who has encouraged the work and its publication from the start and provided valuable criticism and comment on the study and on an earlier draft. Summary Data are interpreted from a colour-ringing study of Choughs Pyrrhocorax pyrrhocorax during 1978-83 on Bardsey, Gwynedd, and these are combined and compared with analyses of national ringing recoveries, county records, and reference to other literature. Breeding Choughs are sedentary, with great nest-site and pair fidelity. First-years can show much more mobility, dispersing up to 70km within the breeding range and 140km outside it; extreme distances wandered by unaged individuals are up to 360 km. A first-year mortality rate of 75% is suggested, although this figure is shown to vary greatly annually. Adult longevity was in one case 17 years. First breeding is generally in the fourth year, but instances of breeding in the second and third years are known away from Bardsey. Notes on flocking and roosting behaviour are made. Biometric data show clear size differences between the sexes and between first-year and older Choughs. References BRACEGIRDLE, R. C. 1964. The Chough. Joum. of the Manx Museum 6: 80, 194-196. BULLOCK, I. D. 1980. Aspects of ecology of the Chough. MSc thesis, University of Wales, Bangor. 232 The Choughs of Bardsey

, DREWETT, R. R., & MICKLEBURGH, S. P. 1983a. The Chough in Britain and Ireland. Brit. Birds 76: 377-401. , , & . 1983b. The Choughs of the Calf of Man. Peregrine 5: 5. , , & . In press. The Chough in Wales. Nature in Wales. BUSSE, P. 1969. Results of ringing European Corvidae. Acta Orn. Warsz. 11: 263-328. CONDRY, W. 1981. The Natural History ofWales. London. COOMBES, C.J. F. 1978. The Crows. London. COWDY, S. 1962. Post-fledging behaviour of choughs on Bardsey Island. Brit. Birds 55: 229-233. DARKE, T. O. 1971. The Cornish Chough. Truro. GOODWIN, D. 1976. Crowsqfthe World. Brit. Mus. (Nat. Hist.), London. HOLYOAK, D. 1971. Movements and mortality of Corvidae. Bird Study 18: 97-106. 1972. Behaviour and ecology of the Chough and Alpine Chough. Bird Study 19: 215-227. LOVARI, S. 1976a. The Chough in Abruzzo N. P. Italy. S.O.S. Fauna: 189-214. World Wildlife Fund. 1976b. Population trends and seasonal flock size variation of Alpine Chough, Chough and Raven in the Abruzzo N P. Italy. Gerfaut 66: 207-219. 1978. Flocking behaviour of the Chough and Alpine Chough. Gerfaut68: 163-176. MAYAUD, N. 1933. Notes et remarques sur quelques Corvides. Alauda 2: 195-196. PRAZ,J.-C 1971. Ou hivement les Craves du Valais? Nos Oiseaux 31: 11-13. ROBERTS, P.J. 1982. Foods of the Chough on Bardsey Island, Wales. BirdStudy 29: 155-161. 1983. Feeding habitats of the Chough on Bardsey Island, Gwynedd. BirdStudy 30:67-72. ROLFE, R. 1966. Status of the Chough in the British Isles. BirdStudy 13: 221-226. SCHIFFERLI, A., & LANG, E. M. 1940. Beitrag zur Naturgeschichte der Alpenkrahe. J. Orn. 88: 550-575. SHARROCK,J. T. R. 1976. The Atlas of Breeding Birds in Britain and Ireland. Berkhamsted. SVENSSON, L. 1984. Identification Guide lo European Passerines. Stockholm. VAURIE, C. 1954. Systematic notes on Palearctic birds. No. 4. The Choughs (Pyrrhocorax). Amer. Mus. Novitates. No. 1658. WARNES, J. M. 1982. The ecology of the Chough on the isle of Islay. Unpublished report, University of Stirling/Islay Museum Trust. WHITTAKER, I. 1947. Notes on Welsh Choughs. Brit. Birds 40: 265-266. WILLIAMSON, K. 1959. Observations on the Chough. Peregrine 3: 8-14. WITHERBY, H. F., JOURDAIN, F. C. R., TICEHURST, N. F., & TUCKER, B. W. 1941. The Hand• book of British Birds, vol. 1. London.

P.J. Roberts, 3 Childscroft Road, Rainham, Kent