Conservation Evidence strives to be as useful to conservationists as possible. Please take our survey to help the team improve our resource.

Providing evidence to improve practice

Individual study: Calcium supplementation does not affect reproductive success in blue tits

Published source details

Ramsay S.L. & Houston D.C. (1999) Do acid rain and calcium supply limit eggshell formation for blue tits (Parus caeruleus) in the UK? Journal of Zoology, 247, 121-125


The normal diet of most small passerines contains insufficient calcium for effective eggshell formation. Increased soil acidity can result in enhanced leaching of calcium and affected areas often contain reduced numbers of snails and other calcium-rich food items. This study investigated the effect of calcium supplementation on blue tit eggshell formation.


From mid-April to June in 1994, twelve blue tit pairs nesting in nestboxes in mixed oak woodlands were provided with calcium provided by hanging a dry, weighed cuttlefish bone close to the nestbox and placing two plastic trays, containing 200 g of oyster grit and 200 g of crushed eggshell, on the ground near the base of the tree. All eggs were numbered using permanent ink pens; 1 egg / nest was chosen at random and weighed with a 5 g pesola spring balance and the length and width measured with vernier callipers. Seven weeks after the end of laying all nests were checked for unhatched eggs in order to determine hatching success. Fifteen control nests were also analysed. Snail density was determined by monitoring randomly placed 1 x 1 m quadrats in the leaf litter.


Supplementary calcium did not significantly increase egg weight or size (1.12 and 1.14 g / egg; 1075 and 1082 mm3 / egg for experimental and control nests respectively). Similarly, there were no differences in shell weight or thickness (both 0.07 g and 0.038 mm / egg). Clutch size was also similar between pairs (9.3 and 9.5 eggs / nest for experimental and control nests respectively). Hatching success, expressed as the proportion of all eggs in a clutch that hatched, was also not significantly different between treatment groups (both median = 1). Of 22 quadrats, only 8 snail shells were found. Examination of a control female blue tit’s gizzard revealed a large number of small snails (Discus rotundatus and Carychium minimum corresponding to a dry mass of 62.3 and 5.2 mg respectively) and other calcareous fragments, including a small bone and a tooth.

Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at:;jsessionid=F845CA8F27F1C84D7505A22DF41281DE.d01t03