Mixed singers and imitation singers among Short-toed Treecreepers P. Clausen and S. Toft he identification of treecreepers Certhia has been a popular theme for Tdiscussion for many decades. In connection with the first British record of Short-toed Treecreeper C. brachydactyla (Scott 1976), Mead & Wallace (1976) gave a comprehensive treatment of the distinctions between Short-toed and Common Treecreepers* C. familiaris and also reviewed much of the literature on treecreeper identification. Mead & Wallace (1976) considered differences in the song patterns of the two species to be useful in identification, although care needed to be taken owing to occasional 'mixed singing' by Common Treecreepers. The present paper offers some of our findings in relation to the song behaviour of the two species in Denmark. In particular, we wish to *For the purposes of unambiguous and easy reading, the Treecreeper Certhia familiaris is referred to throughout this text as Common Treecreeper. continued ... 49g [Brit. Birds 81: 496-503, October 1988] Mixed singers among Short-toed Treecreepers 497 propose a distinction between two types of mixed singers among treecreepers. Methods During 1979-84, a survey was conducted which showed the Short-toed Treecreeper to be more numerous and widespread in Denmark than previously believed (Clausen & Madsen 1986). Many Short-toed Tree- creepers were identified by song and by certain distinct calls (Thielcke 1964), as well as by the plumage details proposed by Osieck (1975), Mead & Wallace (1976) and Hirschfeld (1984), namely indistinct short supercilia; dull greyish upperparts; and dirty grey underparts (most Danish individuals having only the throat pure white). Some individuals were tape-recorded as documentation for the Danish Rarities Committee. The voices of Common Treecreepers were also recorded during 1984-86, for comparison. All recordings were filtered through A JVC S.E.A. 80 graphic equaliser, attenuating frequencies below 2 kHz by 24 dB, and analysed on a Voiceprint 700 sonagraph, using the wide (300 Hz) filter and high-shape setting. All sonagrams presented here (figs. 2 & 3) are tracings. The bio-acoustical terminology used follows the definitions given by Bondesen & Davis (1966) and Bondesen (1979). Results Habitat and distribution, present and past In Denmark, the Common Treecreeper is indeed common (10,000- 100,000 breeding pairs: Dybbro 1978), and is also widely distributed (fig. 1). It occurs in broadleaved as well as coniferous forests and parks, probably having done so for more than 8,000 years (L,0ppenthin 1967). The Short-toed Treecreeper is rarer than the Common Treecreeper, having an estimated population in Denmark of only 250-300 breeding pairs (Clausen & Madsen 1986). Although more limited in distribution than Common (fig. 1), it has expanded its range in Denmark considerably during the present century. Short-toed is also more restricted in habitat, preferring old and open broadleaved forests and parks, in particular those with plentiful oaks Quercus. Songs Most treecreepers showed song characters similar to those described by Mead & Wallace (1976). A few Common Treecreepers, however, had considerably shorter song phrases than average (fig. 2h): they omitted or abbreviated one of the two nearly identical trills of the normal phrase (cf. fig. 2g), this resulting in a time duration similar to that of Short-toed. Seven cases of unusual singing by Short-toed Treecreepers were recorded: in five of these, individuals showed characteristics of mixed singing, and in two other cases Short-toeds imitated the song of Common. MIXED SINGING For treecreepers, we define mixed singing as song containing a repertoire 498 Mixed singers among Short-toed Treecreepers Fig. 1. The present distribution of treecreepers Certhia in Denmark. Horizontal hatching = Common Treecreeper C.familiaris (after Dybbro 1976 and Moller 1978). Vertical hatching = Short-toed Treecreeper C. brachydactyla (after Clausen & Madsen 1986); east to the Store Baelt (dotted line) Short-toed is rare (only one proven case of breeding). Black stars indicate sites where mixed-singing Short-toed were recorded. White stars within black circles indicate sites where imitation singers were recorded: the northernmost the two Short-toed at Moesgaard; the southernmost Grasten, where a Common Treecreeper was induced to imitate Short-toed (see 'Discussion') of phrases that includes figures from the repertoires of both species, given either in a single phrase (e.g. fig. 2d) or separately (e.g. figs. 2e-f), but lacking a fully developed normal song phrase of the species itself that is singing. All the mixed singers encountered were identified as Short-toed by their 'rival-calls' (cf. fig. 2b) and plumage coloration. The first individual was recorded on 26th March 1983, in Manor Pederstrup Park, Isle of Lolland. It had a mixed phrase consisting of three initial figures of the normal Short-toed phrase, followed by a Common­ like trill as well as the final figure of the Common phrase (fig. 2d). It also gave a 'staccato-song' (sensu Schwerdtfeger & Thielcke 1986) consisting of only three figures, very much like the first three in fig. 2f. Staccato-songs are easily recognisable, and are apparent in the song repertoires of most Short-toed Treecreepers in Denmark (P. Clausen, unpublished). During our observations, this individual fought a Common Treecreeper, and so we were able to compare directly the plumages of the two as they climbed the same tree trunk, just 60 cm apart: both showed their respective Mixed singers among Short-toed Treecreepers 499 Fig. 2. Sonagrams of voices of Danish treecreepers Certhia. a-f = Short-toed C. brachydactyfa: a & c, two examples of normal song phrases; b, two each of the two distinct rival-calls, used in identification (see Thielcke 1964); d, the mixed phrase of one individual; e & f, the mixed song repertoire of another individual, g-i = Common Treecreeper C. familiaris: g, an example of normal song phrase; h, a very short phrase; i, the distinct rival-call, used in identification (see Thielcke 1964). Solid and outline traces on the sonagrams indicate figures derived from the normal repertoires of, respectively, Short-toed and Common Treecreepers. See text species' characters, and the Common Treecreeper sang a phrase typical of its species. The second mixed-singing Short-toed Treecreeper was tape-recorded on 27th March 1983, in Manor Corselitze Park, Isle of Falster. It had a very short phrase of six elements (fig. 2e), the whole of which appears to be derived from the central parts of the song phrase of Common Treecreeper. It also had a staccato-song of four elements (fig. 2f). Neither of the above two individuals uttered fully developed Short-toed 500 Mixed singers among Short-toed Treecreepers phrases of 6-7 figures; nor did the three Short-toeds that gave mixed singing, but were not tape-recorded. One of the latter, at Bregentved, Isle of Sjalland, on 1st April 1983, had a mixed song of approximately two seconds' duration; this sounded as if it was begun and terminated with normal Short-toed figures, but included a Common Treecreeper trill of 5- 7 figures. One individual, observed on 1st May 1983 at Varde, southwest Jutland, had a song pattern quite like that of the Pederstrup Short-toed. Finally, the third Short-toed, at Horsens, eastern Jutland, on 6th February 1985, had a song pattern like that of the Bregentved individual. IMITATION SINGING Two Short-toed Treecreepers had a quite normal species-specific phrase, as well as giving an imitation of Common Treecreeper (fig. 3). At no time did they mix the song figures and, moreover, they both had a fully developed phrase of their own species. We therefore distinguish these individuals from the mixed singers and call them 'imitation singers' (see discussion). Fig. 3. Repertoires of two Short-toed Treecreepers Certhia brachydactyla giving imitation song, recorded at Moesgaard, Denmark, a & b, the 1983 individual; c & d, the 1984 individual, a & c show the normal Short-toed phrases, and b & d the imitations of phrases of Common Treecreeper C. familiaris. For explanation of traces, see fig. 2. See also text Both imitation singers were recorded at Moesgaard, near Aarhus, eastern Jutland, the first in April-June 1983 and the second in April- September 1984. We believe them to be different individuals because of the differences between their normal (Short-toed) phrases (figs. 3a & 3c), as well as their different imitations (figs. 3b & 3d), the 1984 male singing an almost perfect imitation of Common Treecreeper. This second male was caught and measured on 26th April, its bill-to-skull measurement being 19.7 mm and the hindclaw 8.3 mm; using both Svensson's (1970) ratio and Mead & Wallace's (1976) discriminant, this bird can be identified as a Short-toed Treecreeper. Discussion Mixed singing Mixed singing by treecreepers was described as early as the end of the last Mixed singers among Short- toed Treecreepers 501 century (review by Schwerdtfeger & Thielcke 1986). It was not, however, until 1960 that a more detailed description, involving the use of tape- recording and sonagraphic techniques, was made (Thielcke 1960). Thielcke (1972) stated that only Common Treecreeper appeared to demonstrate this phenomenon and that those individuals previously considered to be mixed-singing Short-toed might in fact have been misidentified Common Treecreepers. Despite this, Schwerdtfeger & Thielcke (1986) published the 'first' record of mixed singing by a German Short-toed Treecreeper, from the Harz area. In West Germany, mixed singing is, however, still far more frequent among Common Treecreepers than among Short-toed. From Thielcke's extensive studies on treecreeper vocalisations, it is evident that (i) juvenile Short-toeds must at least learn part of their song from conspecifics, and they probably distinguish between conspecifics and heterospecifics by their rival-calls, which are assumed to be innate (Thielcke 1984a); and that (ii) a similar situation seems to exist with Common Treecreeper, which, under the influence of Short-toed and isolated from learning from a conspecific 'tutor', does, however, evolve a mixed song pattern (Thielcke 1960, 1972, 1986).