Ibis (2006), 148, 564–567 © 2006 The Authors Journal compilation © 2006 British Ornithologists’ Union Blackwell Publishing Ltd Oxford, UK IBI Ibis 0019-1019 British Ornithologists' Union, 2006 ? 2006 148 ? Short communication Laying date and clutch size in the Canary Island Blue Tits E. Garcia-del-Rey et al. Variable effects of laying date on clutch size in the Canary Island Blue Tits (Cyanistes teneriffae) EDUARDO GARCIA-DEL-REY, 1 * WILL CRESSWELL, 2 CHRISTOPHER M. PERRINS 1 & ANDREW G. GOSLER 1 1 Edward Grey Institute of Field Ornithology, Department of Zoology, South Parks Road, Oxford OX1 3PS, UK 2 School of Biology, University of St Andrews, KY16 9TS, UK The relationships between clutch size and laying date have been widely studied in many species (Daan et al. 1988, Ludvig et al. 1995) as a system to understand how animals optimize life-history decisions (e.g. Tinbergen & Sanz 2004). Although the Blue Tit Cyanistes caeruleus (Perrins 1979) is one of the most extensively studied species in the world (Cramp & Perrins 1993), traits affecting its life history have been little studied in the Canary Islands (Garcia-del-Rey 2003), where the species is unusual in two major respects. First, it shows considerable systematic variation (Snow & Perrins 1998). On this oceanic archipelago at least four (possibly five, Christian Dietzen in litt.) endemic subspecies occur (C. c. degener, C. c. teneriffae, C. c. palmensis, C. c. ombriosus) (Vaurie 1957). Indeed, they have recently been proposed as a distinct species: Cyanistes teneriffae (Kvist et al. 2005; Salzburger et al. 2002). However, for our present purposes we will consider them ecologically (if not necessarily in terms of nomenclature) to be subspecies of C. caeruleus. Secondly, in most areas of its range the Blue Tit occurs almost exclusively in broad-leaved woodlands (Snow 1954), whereas in the Canaries it mainly inhabits pine woods and laurel forest of the higher mountainous islands (e.g. Tenerife, La Palma, El Hierro, La Gomera and Gran Canaria) (Bannerman 1963) and well-vegetated areas on the arid eastern islands (e.g. Phoenix/Tamarix community on Fuerteventura and palm groves on Lanzarote) (Garcia-del-Rey & Cresswell 2006). Because of their apparently long isolation and unique ecological conditions, the Canary Islands therefore provide a good opportunity to investigate how variation in ecological conditions (e.g. habitat) affects the evolution of life-history traits (Bennett & Owens 2002). In this paper we investigate, in the Canary Island Blue Tit, variation in clutch size, a life- history trait which, on the mainland, is frequently related to date of laying, with earlier clutches typically being larger (e.g. Perrins 1979). The present study tests whether clutch size varies with laying date on the Canary Islands, as else- where, and examines whether the relationship is affected by the atypical habitats used by these populations. We also compare the occurrence of second clutches between the habitats, and the relative success of first and second broods, to provide further baseline information on the species within the Canary Islands. STUDY AREA AND METHODS This study was carried out during the breeding seasons of 2000–02 inclusive, in the Canary Islands (27°37′-29°25′N, 13°20′-18°10′ W), on the islands of Fuerteventura, Tenerife, El Hierro and La Palma (hereafter sometimes referred as F, T, H and P, respectively). Data were also collected from Tenerife during 2003. Study areas were chosen in pine (1000–1673 m asl) and laurel forests (920–1088 m asl) on three islands (i.e. T, H, P) and in the main Fuerteventura Blue Tit habitat (i.e. Phoenix–Tamarix) (337 m asl) (Garcia-del-Rey 2004). In total, 496 wooden nestboxes were erected to monitor breeding around the islands and in different habitats (see Table 1). Compared with that of continental populations studied, the breeding season was extremely long. For the whole archipelago it started in mid-January and ended in mid-July. Data were gathered on the date of laying (i.e. day of first egg, assuming that females lay one egg per day; Kluijver 1951, Lack 1955, Perrins & McCleery 1989), clutch size (number of eggs laid in a clutch) and breeding success (hatched young - dead in the nest/eggs laid × 100%) of first and second clutches. Second clutches are those laid in a nestbox from which a brood of young had already fledged successfully. As most boxes were unused (Table 1), we assumed that a later clutch within the same box was laid by the same female. Each nestbox was visited at least once a week, more often when necessary. All boxes were cleaned at the end of each field season and tits were not disturbed during incubation. Statistical analyses were performed using SPSS 11.0 (Zar 1984). RESULTS Considering first clutches, a simple ANOVA shows that the relationship between clutch size and laying date varied according to habitat and year (Table 2). When we added the interaction term lay date × year × habitat to the model in Table 2, it was found not to be statistically significant (i.e. F 4,303 = 2.03, P = 0.09), thus indicating that the rela- tionship between the effects of lay date and habitat on clutch size did not differ between years. Furthermore, there was no significant effect of year (F 3,303 = 2.20, P = 0.09) *Corresponding author. Present address: Departamento de Ecología, Facultad de Biología, Universidad de La Laguna, 38260 La Laguna, Tenerife, Canary Islands, Spain. Email: avesecot@mail.teide.net