Calcium shortage as a constraint on reproduction in great tits Parus major: a field experiment

Published source details Tilgar V., Mänd R. & Mägi M. (2002) Calcium shortage as a constraint on reproduction in great tits Parus major: a field experiment. Journal of Avian Biology (formerly Ornis Scandinavica 1970-1993), 33, 407-413.
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This study is summarised as evidence for the following.

	Action	Category
	Provide calcium supplements to increase survival or reproductive success Action Link

Provide calcium supplements to increase survival or reproductive success

A replicated, controlled trial in pine and deciduous forests in Estonia in 1995-7 and 1999-2000 (Tilgar er al. 2002) continued the study from Tilgar et al. 1999 and found that great tits Parus major supplied with supplementary calcium had significantly larger first broods, containing significantly larger chicks and hatching more eggs, than control (unsupplemented) birds (supplemented nests: 10.1-11.6 eggs/clutch, n = 172 nests, 9.2-10.4 hatchlings/nest, n = 101 nests and lower leg lengths of 19.1-19.8 mm, n = 73 chicks vs. 10.2-11.4 eggs/clutch, n = 254 nests, 8.2-10.1 hatchlings/nest, n = 110 nests and 18.8-19.7 mm, n = 97 chicks, for control nests). Supplemented nests also fledged more chicks in 1997, 1999 and 2000 (the only years tested), but this difference was not significant, unless unsuccessful nests were excluded (7.2-9.0 fledglings/nest for 81 supplemented nests vs. 4.2-8.7 fledglings/nest for 115 controls). In 1999 (the only year investigated), second broods were significantly larger and fledged significantly more chicks in supplemented than control nests (8.4-9.1 eggs/clutch, and 5.5-6.6 fledglings/nest for 13 supplemented nests vs. 9.8-9.6 eggs/clutch and 7.9-8.1 fledglings/nest for 15 controls). The authors note that when clutch size was included in models, fledging rates did not differ between treatments, suggesting that calcium provisioning acts mainly to increase clutch size, rather than nest survival. There were no differences between pine and deciduous forests. Calcium was provided in the form of small snail shell and eggshell fragments in a feeder within the nest boxes. Control nest boxes were given empty feeders in.