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 . 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 Park, Isle of . 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 Park, Isle of . 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 , 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 , 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). The development of mixed singing could also be expected from Short-toed Treecreepers when isolated from conspecifics. For the Goldcrest Regulus regulus and the Firecrest R. ignicapillus, Becker (1977) proposed a mechanism by which the individual learns by choosing as tutor a conspecific if available, and the sibling species if the former is unavailable. This leads to the assumption that the mixed-singing species in an area of sympatry should be the less numerous of the sibling pair. This seems to work also for the treecreepers. For example, Common is the mixed singer in West Germany, where Short-toed is by far the more numerous and where Common has its westernmost range limits on the Continent (apart from scattered populations in France and the Pyrenees: Yeatman 1976). In Denmark, the situation is reversed: Short-toed is the mixed singer, here at its northernmost limits, where Common is the abundant species. According to this theory, a treecreeper giving mixed singing in Britain is more likely to be a dispersing Short-toed than a Common, particularly as the latter species is resident in Britain (Flegg 1973) and thus has few, if any, chances to learn song elements of Short-toed. How mixed-singing Short-toed Treecreepers have evolved in the vicinity of Paris (Chappuis 1976) remains unclear. As yet, there are few proven records of Common Treecreeper around Paris, whereas Short-toed is abundant (Yeatman 1976).

Imitation singing Juveniles of both treecreeper species, when hand-raised, develop songs highly different from those of their respective species in the wild (Thielcke 1970, 1984a, b). If song behaviour during hand-raising can be taken to reflect that in natural situations, this implies little likelihood that a Short- toed Treecreeper, isolated from learning from conspecifics, would develop 502 Mixed singers among Short-toed Treecreepers the normal song phrase of 6-7 figures that is found in its wild populations. Imitation singers among Short-toed Treecreepers have a fully developed Short-toed phrase, and we therefore interpret these as individuals which have had a conspecific tutor, but have also learned from Common Treecreeper. We believe imitations to have a functional explanation. In both cases of imitation singing by Short-toeds, the individuals involved had male Common Treecreepers in neighbouring territories. The two species are interspecifically territorial in Denmark (J. Madsen, unpublished; personal observations), a phenomenon reported also from Lower Saxony (Schnebel 1972). Further, the 1984 imitation-singing male was polygynous, having both a Short-toed female and a Common female nesting in his territory (unfortunately, neither nest was successful). Thus, imitations may be used 'consciously' to communicate with members of the other species. In support of this interpretation, we can also report a case of induced imitation by a Common Treecreeper, observed by J. Madsen and S. Toft at Grasten, southern Jutland, in June 1983. This bird responded eagerly to play-back of Short-toed song phrases: at first it was singing its own species' normal song, but after a short time it started to modify this and gradually, over about five minutes, the song was changed into a seemingly perfect imitation of Short-toed. Clearly, in this case, the imitation was directed at an apparent intruding rival, by 'speaking the same language' as that rival. Acknowledgments Thanks are due to P. Hansen, J. Madsen and J. G. Mather for critical comments on the manuscript. The former also provided much technical assistance, and allowed us to use the Bio-acoustical Laboratory of the Natural History Museum, Aarhus University. J. G. Mather made improvements to our English. Summary A survey in Denmark during 1979-84 showed Short-toed Treecreeper Certhia brackydactyla to be more common than previously realised, although occupying a more restricted habitat than the abundant Common Treecreeper C. famiHaris. Tape-recordings were made of individuals of both species. "Mixed singing' by Short-toed (song containing elements of both species' normal songs, but lacking any fully developed phrase of the species' own song) was noted from five individuals, and 'imitation singing' (imitation of Common Treecreeper, but including a fully developed phrase of its own species' song) from two. Both types of singing are described. It would appear that mixed singing evolves in an area of sympatry, whereby the less numerous of two sibling species learns from the more abundant species. It is suggested that imitation singing may be used 'consciously' to communicate with members of the other species (the two treecreeper species are interspecifically territorial in Denmark). References BECKER, P. 1977. Verhalten auf Lautausserungen der Zwillingsart, interspezifische Territor- ialitat und Habitatanspruche von Winter- und Sommergoldhanehen (Regulus regulus, R. ignicapittus).J. Om. 118: 233-260. BONDRSKN, P. 1979. The hierarchy of bio-acoustic units expressed by a 'phrase formula'. Biophon 6 (3): 2-6. , & DAVIS, L. I. 1966. Sound analysis within biological acoustics. Terms and definitions, Naturajuthndka 12: 235-239. CHAPPUIS, C. 1976. Quelques donnees acoustiques sur les grimpereaux du bassin Parisien et de l'ouest de la France. L'Oiseau et R.F.O. 46: 195-199. 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Gesangslernen bei Gartenbaumlaufer (Certhia brachydactyla). Die Vogelwarte 32: 282-297. —— 1984b. Lebenslange Stabilitat des Gesangs handaufgezogener Gartenbaumlaufer (Certhia brachyaactyla).J. Orn. 125: 447-454. 1986. Constant proportions of mixed singers in tree creeper populations (Certhia familiaris). Ethology 72: 154-164. YEATMAN, L. 1976. Atlas des Oiseaux Nicheurs de France de 1970 a 1975. Paris.

P. Clausen and S. Toft, Institute qfZoobgy and Zoophysiology, Zoological Laboratory, University ofAarhus, DK-8000 Aarhus C, Denmark