Managed a look into the blue tits nest box today, having spotted the parents leaving (do they qualify as parents before the chicks hatch?) This is a good sized clutch of eggs – it amazes me that such a small bird can produce so many. More data for the Nest Record Scheme. An [article on the BTO](https://www.bto.org/understanding-birds/species-focus/blue-tit) website explains that:> An average clutch of Blue Tit eggs contains roughly 0.5 g of calcium, which might not sound like a lot; but given that there is only about 0.6 g of calcium in a Blue Tit skeleton, this represents a significant investment for the female and it is likely that she will need to source this from the environment, presumably in the form of fragments of snail shell and other calcium-rich material.