Bird Study ISSN: 0006-3657 (Print) 1944-6705 (Online) Journal homepage: http://www.tandfonline.com/loi/tbis20 Why do curlews Numenius have decurved bills? N. C. Davidson , D. J. Townsend , M. W. Pienkowski & J. R. Speakman To cite this article: N. C. Davidson , D. J. Townsend , M. W. Pienkowski & J. R. Speakman (1986) Why do curlews Numenius have decurved bills?, Bird Study, 33:2, 61-69, DOI: 10.1080/00063658609476896 To link to this article: http://dx.doi.org/10.1080/00063658609476896 Published online: 24 Jun 2009. Submit your article to this journal Article views: 194 View related articles Citing articles: 10 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tbis20 Download by: [University of Aberdeen] Date: 20 November 2015, At: 08:22 Bird Study (1986) 33, 61-69 Why do curlews Numenius have decurved bills? N.C. DAVIDSON 1* , D.J. TOWNSHEND 11- , M.W. PIENKOWSKI 2 and J.R. SPEAKMAN 3 ' Department of Zoology, University of Durham, South Road, Durham DM 3LE, UK 2 Chief Scientist Directorate, Nature Conservancy Council, Northminster House, Peterborough PE11UA, UK "Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen AB9 2TN, UK The functions of the long, decurved bill of the Common Curlew are compared with those of the straight bill of the Bar-tailed Godwit. Comparison is also made with the dimensions of other species of curlews and godwits. We argue that decurvature is adapted to a technique of prey capture in which the bill tip follows a complex three-dimensional search path, and that the long bill of the Common Curlew is adapted to the intact removal of long prey (e.g. worms) from mudflats. The evidence indicates that strong selection for bill form operates on the wintering grounds. he bill morphology of a bird is closely cor- foraging behaviour of several species of shore- T related with its feeding habits. However in birds in north-east Britain (e.g. Pienkowski species that occupy several habitats or take dif- 1973; Evans et al. 1979; Davidson 1980; Tow ns- ferent foods at different times of year, it can be hend 1981, 1982, 1985; Pienkowski et al. 1984; difficult to identify to which, if any, of several Speakman 1984a). In particular, we compare purposes the bill form is chiefly adapted. Thus the foraging behaviour of the Common Curlew Owens (1984) suggested that the decurved bill and Bar-tailed Godwit Limosa lapponica, since of the Common Curlew Numenius arquata had both of these species probe deeply into arisen by selection favouring either a greater sediments to capture polychaete worms, search-arc for detection of prey beneath the especially ragworms Nereis divers icolor and Downloaded by [University of Aberdeen] at 08:22 20 November 2015 surface of mudflats in winter (an idea earlier lugworms Arenicola marina. We then compare proposed by Burton 1974) or easier capture of the dimensions of the Common Curlew with insects amongst long vegetation on the breed- those of other curlews Numenius spp. and of ing grounds. godwits Limosa spp. and consider briefly the Owens (1984) suggested that careful obser- incidence of bill decurvature amongst other vation of foraging behaviour may yield clearer groups of birds. We argue that decurvature is evidence than he could provide for the func- an adaptation for probing along complex tion of the Common Curlew's decurved bill. pathways and to aid extraction of worms with- We present such evidence here, drawn from out breaking them, and that the very long our observations during the last 12 years of the decurved bill of the Common Curlew is chiefly Present addresses: *England Headquarters, Nature an adaptation for probing along such pathways Conservancy Council, Northminster House, Peter- deep into mudflats. This differs from Owen's borough PE1 lUA, UK; +Nature Conservancy view. We also comment on several points of Council, Archbold House, Archbold Terrace, Owens' arguments which are not supported by Newcastle NE2 1EG, UK. the available evidence. 62 N.C. Davidson et al. RESULTS AND DISCUSSION Burton (1974) pointed out that decurvature aids vertical insertion of the bill tip towards prey Prey capture techniques of Common Curlews detected whilst walking across the mudflats, and Bar-tailed Godwits on mudflats and that when the birds are foraging visually the bill tip may have to be moved a shorter Common Curlews and Bar-tailed Godwits distance to the point of insertion into the foraging on mudflats can detect prey both by sediment. When foraging by touch, Common touch and by sight. In tactile foraging, both Curlews can probe the surface some distance in species walk over the flats frequently inserting advance of their feet (see Fig. 1(ii)) whereas the tip of the bill into the sediment, and some- Bar-tailed Godwits, which likewise insert the times then probing deeply—probably only bill tip vertically, must do so much closer to after detection of the prey (Townshend 1982; their feet. Since some prey become less active, Cramp & Simmons 1983; pers. obs.). The exact and move deeper, when a shorebird walks way in which prey is detected is unknown: it across the mud (Goss-Custard 1970), detection may be by direct contact, by detection of of prey in advance of the feet may be of con- vibrations from the worm, or by detection of an siderable benefit. The ability of prey to detect occupied burrow by taste, as contributes to the passage of a wader implies that Owens' prey location in Dunlins Calidris alpina and (1984) suggestion, that the decurvature confers Sanderlings C. alba (van Heezik, Gerritsen & an element of surprise through a sideways Swennen 1983; Cramp & Simmons 1983). In attack on a worm in its burrow, is improbable, visual foraging, the deep probe is not preceded since the bill tip of the Common Curlew ends by a shallow peck at the same place (pers. up closer to the feet than does the bill tip of a obs.). Bar-tailed Godwit. A decurved bill is more The major difference between the species lies readily manipulated than a straight bill along a in their technique of deep probing for prey. complex capture path (following a burrow Bar-tailed Godwits plunge their straight bills system) within the sediment, since a change in rapidly deep into the sediment, almost in- direction can be achieved by a rotation of the variably vertically downwards. Thereafter, the head or the neck, rather than requiring a lateral bill may be vigorously worked up and down, movement of the entire head and neck. This and the bird may pivot round the hole with the rotation is illustrated in Owens (1984), albeit bill deep in the mud, perhaps to get a better when the Common Curlew has probed almost grip on its prey. This rapid plunge probably to its fullest extent with the bill totally helps to catch prey such as lugworms that can immersed. retreat beyond the reach of the bill (see Smith & Evans 1973). In contrast, after detecting a Removal of prey from the substrate worm a Common Curlew probes deep into the mud more slowly than does a Bar-tailed God- Common Curlews always, and Bar-tailed wit. The bill may initially follow a simple path Godwits usually, must remove a worm from Downloaded by [University of Aberdeen] at 08:22 20 November 2015 into the mud but more often during repeated the substrate before swallowing it (Burton probes at a site the bill tip follows a complex 1974, 1986). However, worms are fragile and route which involves movements both in line break easily. Breakages are disadvantageous with and perpendicular to the axis of the bill. because it takes longer to grasp and remove Unlike Bar-tailed Godwits, Common Curlews several small pieces of worm than a whole one seldom pivot round the probe site with the bill and because pieces of broken worm are some- deeply inserted. The probing of Common times left in the burrow after the bird moves Curlews is described further in Burton (1974) on. This is particularly so for lugworms, which and Townshend (1982). are pulled out 'tail first', since any breakage leaves behind the much thicker head end, which contains 65% of the energy content of Advantages of a decurved bill for foraging on mudflats the worm (Smith 1975). When pulling a worm from the sediment, A decurved bill has several advantages over a the risk of breakage may be higher if the worm straight bill for such foraging on mudflats. is not pulled vertically clear of the mud or if the Curlew bills 63 a) Bar-tailed Godwit )? Figure 1. Techniques used by (a) a Bar-tailed Godwit and (b) a Common Curlew to pull a worm of similar length (relative to the bill) from the mud. Drawings (i) to (iii) show the normal sequence. Drawing (a (iv)) illustrates the difficulty of pulling a long worm vertically from its burrow with a straight bill (see text). Points A and B indicates the points at which the risk of fracture is increased if the angle between worm and substrate, or worm and bill, changes during extraction. angle between worm and bill changes. It is thus the bill angle increasingly towards horizontal Downloaded by [University of Aberdeen] at 08:22 20 November 2015 best to pull worms vertically from the sedi- whilst the pull is made (Fig. 1(iii)), thus increas- ment. Bar-tailed Godwits almost invariably, ing the risk of breakage of the worm where it is and Common Curlews usually (except in very grasped by the bird. In contrast, the decurved soft mud), withdraw the bill from the insertion bill of a Common Curlew allows a vertical grip hole so that the part of the bill at the mud to be maintained throughout the vertical surface is vertical (Fig.