Look closely at a coal tit in Ireland and you might notice something unusual.
The little bird has the familiar black cap, white nape and soft grey-green back of a coal tit, but there is a splash of creamy yellow on its cheeks. It is a subtle difference, easy to miss when the bird is darting between branches, but it tells us something interesting about where this particular coal tit comes from.
Across much of Europe, coal tits have uniformly white cheek patches. The…