SPOT THE DIFFERENCE GREAT AND BLUE TIT EGGS
Great tits usually lay in the region of 5–11 eggs per clutch, and blue tits lay 6–12, though the latter are known to produce up to 16. The female lays one egg each morning, and on the final day starts incubating her clutch – this process typically takes 12–14 days.
Tit eggs are cream with brown speckles. Historically, it was thought that the speckles helped with camouflage. But Andy Gosler realised that speckling wasn’t uniform: the first egg in a clutch tends to be less marked than the last, and some females had far more speckled eggs than others.
In fact, in great tits the speckles seem to have a structural function, as they occur where the shell is thinner. Females nesting on low-calcium soils laid thinner-shelled, more speckled eggs than females on high-calcium soils, and females may become calciumdeficient by the time they lay their last eggs.