Ibis (2003), 145, 67–77 © 2003 British Ornithologists’ Union Blackwell Science, Ltd Does supplementary calcium reduce the cost of reproduction in the Pied Flycatcher Ficedula hypoleuca? RAIVO MÄND* & VALLO TILGAR Department of Zoology and Hydrobiology, University of Tartu, Vanemuise 46, Tartu 51014, Estonia Studies in acidified as well as in naturally base-poor areas have recently revealed that availability of extra calcium-rich food items is an important component of habitat quality affecting breeding performance in several bird species. However, these mostly short-term studies have provided equivocal results concerning the exact consequences of calcium shortage on different species in different regions. We studied the effect of calcium availability on reproduction of the Pied Flycatcher Ficedula hypoleuca breeding in pine forests in Estonia, NE Europe, over a period of 4 years. Experimental pairs were provided with supplementary calcium-rich material when breeding, while control pairs were left unsupplemented. Experi- mental females laid larger eggs and their nestlings had longer tarsi than those of controls. Moreover, the mass and condition of females tending larger than average clutches were increased by calcium-supplementation. Our results provide the first experimental evidence that calcium availability may affect the overall cost of reproduction in free-living passerines. We compared these results with similar data for the Great Tit Parus major, collected from the same area during the same study period. Great Tits responded to low calcium availability mainly by restrained reproductive behaviour and reduced breeding success, while Pied Flycatchers invested significantly more in current reproductive effort despite the increased cost of reproduction. Thus, the effects of calcium deficiency on birds seem to be species- specific or population-specific. This partly explains discrepancies between the results of earlier studies. Several experimental and comparative studies in acidified as well as in naturally base-poor areas have recently shown that availability of calcium-rich food items is an important component of habitat quality affecting breeding performance in several bird species (Glooschenko et al. 1986, Ormerod et al. 1988, 1991, Graveland et al. 1994, Eeva 1996, Graveland & van der Wal 1996, Graveland 1996a, 1996b, Beintema et al. 1997, Graveland & Drent 1997, Pahl et al. 1997, Scheuhammer et al. 1997, Weimer & Schmidt 1998, Zang 1998, Tilgar et al. 1999a, 1999b, Mänd et al. 2000a, 2000b, Bures & Weidinger 2000). According to Carey (1996), this issue deserves attention because energy may prove to be less limiting for some species in the wild than availability of certain micronutrients such as calcium. Breeding birds need calcium for eggshell production and for the skeletal growth of nestlings (Simkiss 1967, Graveland & van Gijzen 1994, Krementz & Ankney 1995, Graveland 1996a, Graveland & Berends 1997, Reynolds 1997, 2001, Bures & Weidinger 2000, Blum et al. 2001). Since the capacity for using endogenous reserves is quite limited in small passerines laying relatively large clutches (Houston et al. 1995, Krementz & Ankney 1995, Pahl et al. 1997, Reynolds 1997, 2001), and the calcium content of many foods is low (Graveland & van Gijzen 1994), many species have to rely on additional calcium-rich material, like mollusc shells, bones, eggshells and calcareous grit, during egg-laying and when feeding young (see, e.g., Graveland 1996a, Perrins 1996, Tilgar et al. 1999a, Mänd et al. 2000a, Bures & Weidinger 2000, for references). However, there are inconsistencies in the results from different studies. Although most studies report a positive effect of extra calcium on certain repro- ductive parameters of birds, some experimental *Corresponding author Email: raivo.mand@ut.ee