Previous studies of Woodcock diet have used stomach contents and focused predominantly on winter because of the relative ease of obtaining material (e.g. Hirons & Bickford-Smith 1983, Granval 1987). Relatively little is known about spring diet composition and, to our knowledge, there is no published information on chick diet. It is clear from our analysis that in May, when Woodcock spend more time feeding in woodland (Hirons & Owen 1982, this study), earthworms remain the most important dietary component in terms of biomass ingested, but spiders, harvestmen and beetles assume greater importance than in winter. The high proportion of earthworms in the diet of chicks is not surprising because they receive food from the female for about the first week and occasionally longer (Workman 1954, Marcström & Sundgren 1977, Watson 2004). The difference in the proportions of prey biomass contributed by earthworms between Whitwell Wood and Millden might be explained by the greater abundance/activity of surface invertebrates at Whitwell Wood.
The range of prey items found in the diets of the Woodcock at Whitwell Wood and Millden was similar to that found by Bettmann (1961) and Koubek (1986) based on the examination of stomachs in spring, with the exception that Bettmann also found Earwigs Forficula auricularia and snails, and fly larvae comprised 13% of prey items in Koubek's analysis. The absence of fly larvae in our samples may be because larval densities were low relative to earthworm densities (20 per m2 at Whitwell Wood and 45 per m2 at Millden) or because our sample size was too small to detect them if they were only eaten in small quantities. The absence of molluscs from the diet of adult Woodcock at our study areas was unexpected. This may be a consequence of the time of season that the samples were collected, because Gruar et al. (2003) found a seasonal decline in earthworms in Song Thrush Turdus philomelos diet and a concurrent increase in the relative abundance of snails, which was most pronounced after mid-June. Other Song Thrush studies have concluded that snails are only taken in large quantities when more preferred prey, such as earthworms or Lepidoptera larvae, are unavailable (Davies & Snow 1965, Török 1985).
Plant material was considered unimportant in the diet of Woodcock in this study because the items found consisted solely of small amounts of leaf fragments, presumed to have been ingested incidentally during probing. However, Koubek (1986) reported the presence of seeds of six species and pine needles, with plant fragments comprising 21% of all the items identified in stomachs. The American Woodcock is also considered to take some plant material as food (Gregg 1984).
Without larger numbers of faecal samples and statistical comparison of prey ingested with their availability, it is difficult to assess whether Woodcock feed selectively on invertebrates other than earthworms. This comparison will always be limited by the poor measure of relative abundance of different taxa in ground invertebrate assemblages provided by pitfall samples (Luff 1975, Adis 1979) and the fact that certain species are active mainly at night, although pitfall traps may provide a reasonable measure of likely encounter rates with some diurnally active taxa for birds. Spiders and harvestmen were the most numerous taxa in our pitfall catches, suggesting that the Woodcock were probably not selecting them over other taxa, but were responding to their seasonal abundance/activity. The difference in the proportion of millipedes in the diets of birds at Whitwell Wood and Millden is also related to the numbers caught in pitfall traps and is suggestive that the birds were simply taking items that were easily caught in relation to their abundance/activity.
Use of fields
Woodcock regularly feed on fields at night in winter (Hirons & Bickford-Smith 1983, Wilson 1983). Recently, nocturnal activity has been shown to be inversely correlated with air temperature and with daylight foraging activity (Duriez et al. 2005). Thus, birds in woodland patches with low food availability or in cold weather, or both, flew to fields at night, where there was a relative superabundance of earthworms, to satisfy their energy requirements. Because earthworms are such an important food for Woodcock, it is not surprising that this behaviour continued into spring. March and April are likely to be months of high energy demand for both males and females because most males will be roding and females will be attempting to maintain or improve body condition prior to egg-laying. The preference for pasture fields at night is consistent with birds maximizing their energy intake, especially because the flights to pasture fields at Whitwell were no further than those to the few winter wheat fields that were used. The relative stability of earthworm densities in the woodland provides a likely reason for the seasonal change in foraging behaviour by the Woodcock, although our data were not particularly convincing. It is possible that the effectiveness of the formalin sampling method differed between woodland and pasture fields, perhaps as a result of soil compaction in pastures, and soil cores would have provided better estimates of absolute earthworm densities in the two habitats. Our analysis of seasonal changes in earthworm availability would also be more convincing with larger sample sizes, particularly for pasture in late May and late June. However, a decline in earthworm availability in pasture fields in summer due to progressive desiccation of the topsoil is well documented (Edwards & Lofty 1977, Peach et al. 2004). Hirons and Owen (1982) present activity data for radiotagged Woodcock demonstrating that actual feeding time in woodland and pastures changes from early spring to mid-summer.
Compared with other soil invertebrate specialists breeding in woodland, the Woodcock's diurnal range is large (e.g. Song Thrush 3.3 ha, Peach et al. 2004). This is not surprising given its body mass (270–330 g in the breeding season), but it may also be due, at least in part, to the Woodcock's polygynous breeding system (Hirons 1983), which means that males are not required to defend territories. Although daily ranges were small, birds presumably had the freedom to change feeding areas more frequently and to explore larger areas for food-rich patches.
Woodcock home ranges at Whitwell Wood were established predominantly in Sycamore stands. It was apparent, however, that only small parts of home ranges were used intensively for feeding and that this took place predominantly in Sycamore stands where Dog's Mercury was the dominant ground cover. There are quite likely to have been subtle changes in habitat use over the 15-year period of our study at Whitwell Wood, related to changes in habitat structure and/or predator abundance. However, our analysis of diurnal habitat use was based on habitat categories which were probably too crude to detect this. Nevertheless, the fact that time period was not significant in the compositional analysis suggests that the combinations of tree species and dominant vegetation used to define categories remained important despite any changes over time in the structure of the understorey.
Our upland study area at Millden consisted of relatively simple but more fragmented habitats compared with Whitwell Wood and provided the first information on breeding Woodcock use of upland margins. Birch woodlands cover less than 2% of the Scottish uplands (Milne et al. 1998), but comprise the most abundant class of semi-natural woodland (MacKenzie 1987) and are important for their landscape and biodiversity value (Fenton 1984, Forestry Commission 1994). The Woodcock at Millden made greatest use of young regenerating birch, including high proportions within their home ranges and spending a large proportion of the daytime within it. Young birch was typically found encroaching onto unimproved grassland or acid flush at the edges of mature birch, and sapling-stage patches consisting of stems about 5–10 cm in diameter and 5–8 m tall seemed to be used most.
Habitat use represents a trade-off between the availability of food in a particular habitat and the predation risk its structure presents. Although Woodcock may comprise only a small proportion of the prey items in the diet of Sparrowhawks Accipiter nisus, Goshawks Accipiter gentilis and Tawny Owls Strix aluco (Zomerdijk 1983, Newton 1986, Toyne 1998), predation by these avian predators may be an important cause of mortality for Woodcock. Of ten ringed or radiotagged Woodcock killed by predators at Whitwell Wood, three were taken by Sparrowhawks and two by Tawny Owls, with the others killed by mammals. Sparrowhawks were observed chasing roding Woodcock at Millden and Woodcock feathers were found at plucking posts.
At Whitwell, Sycamore–bramble and Beech–Ramsons areas supported earthworm densities at least as high as those in Sycamore–Dog's Mercury stands, suggesting that habitat structure also influenced the choice of feeding locations. Dog's Mercury provides a good combination of overhead cover and ease of passage for Woodcock compared with bramble and Ramsons. Similarly, young birch areas did not support the highest earthworm densities at Millden, which were found under mature birch with grass ground cover. However, the latter areas were grazed by sheep and deer and, certainly early in the breeding season, would have provided little cover from avian predators. Areas of sapling-stage birch typically had sparse ground vegetation but the density of stems made them impenetrable to Sparrowhawks and Tawny Owls. Provided that overhead cover is sufficiently dense to reduce predation risk significantly, sparse ground cover and leaf litter presumably facilitate probing for food.
Early studies of the American Woodcock found similar habitat use. Areas highly used by American Woodcock were characterized by a high density of shrubs and reduced herb cover (Rabe 1977), but in woodland with a poorly developed shrub layer, birds selected for herb stem density and ground cover (Morgenweck 1977). Radiotracking studies of American Woodcock have also shown that individuals utilize intensively only small areas of even preferred habitat types (Dunford & Owen 1973, Morgenweck 1977). Our results at Whitwell Wood are consistent with this, with feeding locations having more ground cover, less leaf litter and smaller trees than random locations; this effect was not simply the product of the habitat chosen. Hirons and Johnson (1987) demonstrated that these patches contain higher earthworm densities than random sites at Whitwell Wood. In the American Woodcock, an experimental trial showed that the number of probes per capture decreased when earthworm density increased (22 probes at 26 earthworms/m2, i.e. a probing success of 0.05, decreasing to nine probes at 105 worms/m2, i.e. probing success of 0.11, Rabe et al. 1983). The density of earthworms above which Woodcock intake rate is not limited is not known, but this density is unlikely to be reached in woodland soils.
We were unable to detect structural differences between feeding sites and random locations after controlling for habitat at Millden, suggesting that structure was more uniform within habitats than at Whitwell Wood or that further selection was of no advantage to the Woodcock. In the United States, Alder Alnus glutinosa and Aspen Populus tremula between 10 and 20 years old constitutes the preferred feeding habitat of American Woodcock (Gregg 1984, Sepik et al. 1992). Throughout the eastern United States Woodcock populations have been declining as a result of the loss of early-successional forest habitats (Dessecker & McAuley 2001).
Could habitat change be responsible for a decline in breeding Woodcock?
This study has concentrated largely on habitats used by Woodcock for feeding. Clearly habitat suitable for displaying and nesting is also important if Woodcock are to breed successfully. Hirons and Johnson (1987) have shown that nesting areas at Whitwell are characterized by less dense ground vegetation than areas used for feeding and brood rearing. Based on just a small sample of nests (n = 10), the same seemed to be true at Millden, with half of nests found in bracken and heather outside the woodland (Hoodless 1994). Again, studies of American Woodcock in naturally regenerating Alder and Aspen woodland have shown that birds nest in relatively open areas whereas broods and solitary birds use heavier cover (Rabe 1977). Nevertheless, Hirons (1988) found that early season nests in Beech were near to the edges of stands, and hence close to Sycamore areas that provided better feeding.
Woodcock are likely to be sensitive to habitat change because they are specialist feeders and have specific habitat requirements. They need large woods containing a mosaic of clearings and rides, stands with relatively sparse, open ground cover, and areas with continuous ground cover or patches of dense shrubs and saplings. They are also dependent on fields near woodland for food for a large part of the year. Changes in woodland management, forestry policy and agricultural practice are therefore all likely to affect breeding Woodcock.
There has been a 76% increase in the total area of woodland in Britain since the 1940s (Warren & Key 1991). However, there has been a major change in the abundance of different types of woodland. There was a 199% increase in coniferous woodland during 1940–90, but managed coppice declined by 82% during the same period (Warren & Key 1991). Although some of the conifer areas at Whitwell Wood were highly utilized, these consisted of young Scots Pine (10 years old), which was still relatively open and supported dense ground vegetation. The mature Scots Pine areas at Millden ranked lowest for relative use. A recent national survey of breeding Woodcock showed that birds often rode over conifer forests, but that numbers of registrations of roding males were lower than in deciduous woods (Hoodless et al. 2006). Parslow (1967) supposed that Woodcock benefited from the large-scale planting of conifers in southern Scotland and north Wales in the 1960s, but the value of this habitat to Woodcock for breeding is probably now much reduced owing to closure of the forest canopy. Further work on Woodcock breeding ecology should focus on how birds use conifer forests, particularly in Scotland and Wales. In these countries Woodcock trends may be closely linked to the species, age and configuration of plantations. In many lowland Oak woods, Hazel coppice, particularly the younger growth stages, may provide habitat of broadly similar structure to young birch thickets and hence comprise important foraging habitat. However, active coppice management is likely to be important to ensure continued use by Woodcock.
There is now good evidence for changes in the structure of British broadleaved and mixed woods during the last 20–30 years. Kirby et al. (2005) found a reduction in small woody stems (< 10 cm dbh), an increase in the basal area of woody species and a decrease in open habitats, such as rides and glades, between 1971 and 2001, as well as a marked decline in ground flora richness. They concluded that without deliberate management intervention woods are, on average, likely to become older and darker in the next 20 years, leading to a decline in species associated with open space and young growth. Amar et al. (2006) recorded increases in Bracken, vegetation cover at 4–10 m and dead trees, along with a reduction in shrub diversity, between the 1980s and 2003/04, which they suggest are indicative of a reduction in woodland management and/or an increase in grazing/browsing. Both of these processes, and the resultant changes in woodland structure, are likely to lead to reductions in suitable habitat for breeding Woodcock, which require open habitats and young growth.
Declining management, such as the abandonment of coppicing, reduced stand thinning and general neglect resulting in the closure of rides and open spaces, may be widespread in lowland woods (Fuller et al. 2005) and could be a major factor in local Woodcock declines in southern and eastern England. There is currently widespread concern about increased deer numbers in lowland England and intensified grazing and browsing pressure on woodlands (Fuller & Gill 2001, Fuller et al. 2005). Circumstantial evidence indicates that grazing pressure can affect the abundance of many bird species (Donald et al. 1998, Perrins & Overall 2001), but a better understanding of the processes is required. A reduction in the density and height of shrubs and the removal of bramble is probably of most relevance to Woodcock.
Outside woodland, Woodcock may be susceptible to a number of the large-scale agricultural changes which have affected food availability for other declining birds such as the Starling Sturnus vulgaris, Song Thrush, Mistle Thrush Turdus viscivorus and Lapwing Vanellus vanellus. The conversion of permanent pasture to cereal production, the loss of livestock and organic fertilizers from arable farms and the installation of under-field drainage systems all result in lower densities of soil invertebrates like earthworms (Edwards 1984, O’Connor & Shrubb 1986, Peach et al. 2004).