NOTES Bill length and bill shape of Semipalmated Sandpiper It is well known that Semipalmated Sandpipers Calidris pusilla and Western Sandpipers C. mauri are difficult to separate, particularly when in winter (basic) plumage, in part because of the great variability in bill length of Semipalmated. The range in bill length is, however, even greater than is generally reported. As examples, Cramp & Simmons (1983) gave a range of 16.6-22.8 mm for bill length of Semipalmated Sandpiper, and 21.7-27.8 mm for Western, while Hayman et al (1986) gave a range of 15-23 mm for Semipalmated and 21-30 mm for Western. During my three years of participation in Manomet Bird Observatory's shore- bird research programme, during 1971-73, we banded (ringed) thousands of Semipalmated Sandpipers at Plymouth Beach along the southeastern coast of

Fig. 1. Extremes of bill length of Semipalmated Sandpipers Calidris pusilla (top two: left, 15.5 mm; right 25 mm) compared with a Western Sandpiper C. mauri (25 mm). Traced from photographs of individuals mist-netted in Massachusetts, USA (see text).

Massachusetts. They occur there on passage, mostly during the months of May to October. The bill lengths (exposed oilmen) ranged from 15.5 to 25.0 mm. The average bill length was about 20.5 mm, with approximately 1:100 or 1:200 having a bill as long as 25 mm. The line-drawings (fig. 1), traced from photographs of mist-netted birds, illustrate the extremes of bill length that we encountered. In this same period, we netted only one Western Sandpiper, a juvenile, also shown in fig. 1. As can be seen, a long-billed Semipalmated Sandpiper can on occasion show considerably more decurvature of the bill than a Western Sandpiper with approximately the same bill length. PAUL K. DONAHUE PO Box 554, Machias, Maine 04654, USA

234 British Birds, vol. 89, no. 5, May 1996 235 References CRAMP, S., & SIMMONS, K. E. L. (eds.) 1983. The Birds of the Western Palearctic. vol. 3. Oxford. HAYMAN, P., MARCHANT, J., & PRATER, T. 1986. Shorebirds - an identification guide. London. EDITORIAL COMMENT Dr R. J. Chandler has commented: 'The bill shape of the Western Sandpiper shown in fig. 1 is not entirely typical of that species, being straighter than is usually the case. It is also worth noting that it has long been recognised that "eastern" Semipalmated Sandpipers can be rather long-billed (see, for examples, BWP), and the birds described here were doubtless from this population. This Note is of particular relevance to the rather long-billed Semipalmated Sandpiper that occurred at Felixstowe, Suffolk, in 1982 (Brit. Birds 79: 545, 609-621; 82: 375-380).'

Orientation of Sky Lark in field In April-August 1992, during a comparative study of the feeding ecology of the Sky Lark Alauda arvensis in arable land near Wimborne St Giles, Dorset, and near Fordingbridge, Hampshire, I located nests by observing the adults' activities, .for practical reasons, two or three sticks, each with a yellow tag at the top, were used as reference markers, these being moved several metres every 20-30 minutes in order gradually to encircle the site of a nest. When the sticks' positions were moved as frequently as this, the larks' time of approach before descending to the nest was not affected, nor was it when I approached the nest without moving the stick(s); the larks would usually fly directly to the site of the nest, hover above it for a few seconds, and then descend to deliver food. By contrast, moving the sticks after they had been in the same position for several hours (e.g. overnight) resulted in a much longer approach time: the larks could take 10-35 minutes to find their nest, all the while carrying the same food package in their bill (a similar effect was observed when a grass field was mowed). The observations showed that the larks used the stick(s) as reference points for the position of the nest in the field, thereby indicating that the Sky Lark is able to recognise features in the field and to use these as markers for orientation. If less-prominent features, such as the colour mosaic (different crop densities, pattern of weeds present) and topographical differences, can also be perceived, remembered and recognised later, this could explain reports that Sky Larks have returned to the same field, and in some cases established territory at the same site, year after year. JOHN GRYNDERUP POULSEN Game Conservancy Trust, Fordingbridge, Hampshire SP6 1EF

EDITORIAL COMMENT In contrast, the inability of some species to distinguish between near-identical reference points has been demonstrated by multiple- nest-building behaviour (e.g. by Blackbird Turdus merula in a stack of lobster creels, Brit. Birds 42: 183-184). Phylloscopus warbler persistently following Northern Wheatear On 30th August 1992, at Grune Point, Cumbria, my son Paul and I flushed a Northern Wheatear Oenanthe oenanthe from the beach. It flew to perch on a nearby bush, closely followed by a smaller bird which landed about 1 m from it. I was surprised to see through my telescope that the smaller bird was a Phylloscopus 236 Notes other bushes and, once, back to the beach; each time, the warbler followed and landed behind it (even on the beach), remaining constantly at a distance of about 1 m behind. When the wheatear landed in a bush, it always perched at the very top; the warbler would perch within the bush, and it always flew off immediately the wheatear did. On one occasion, the wheatear attempted, unsuccessfully, to drive off the smaller bird. The two finally disappeared near the Point, and on reaching this area we found several Northern Wheatears feeding on a patch of grass and a Chiffchaff P. collybita feeding in some bushes, but no sign of any association between them. I cannot be certain that the Chiffchaff was the same bird as we had earlier seen associating with the wheatear, but we found no other Phylbscopus warbler in the area. JOHN HEADON EDITORIAL COMMENT Derek Goodwin has commented: 'Could it be that, in places where cover is scarce (such as desert stopovers on migration), a warbler might be safer if it kept near the only other smallish bird, in this case the wheatear? Four eyes are better than two, and, if a Sparrowhawk Accipiter nisus or other predator came near, there would be a 50% chance that it would go for the other potential prey.' Previous notes have recorded Dartford Warbler Sylvia undata associating with Common Stonechat Saxicola torquata (Brit. Birds 68: 196-202; 71: 181-183; 86: 185-186), Spectacled Warbler Sylvia conspicillata with Finsch's Wheatear O. finschii (Brit. Birds 83: 72-73) and with Desert Wheatear O. deserti and Spotted Sandgrouse Pterocles senegallus (Sandgrouse 3: 57), and Northern Wheatear with Corn Crake Crex crex (Brit. Birds 88: 297). While at least some of these instances involved feeding associations, whereby one bird took advantage of food disturbed by the other's activities, P. A. Amies (Brit. Birds 83: 72-73) sug­ gested that in some cases a species may benefit from the greater vigilance of the other (which would give warning of any approaching predator), a view put for­ ward earlier by Dr C. J. Bibby (Brit. Birds 71: 183) and expanded upon (above) by Derek Goodwin.

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