Tongue spots

 

How old is this bird?

Recently I was catching birds to fit them with rings (bands) as part of my activities as a British Trust For Ornithology (BTO) ringer. Specifically I was catching birds at Tophill Low Nature Reserve as past of the BTO Constant Effort Site  scheme (CES - more about that another day). As part of the activity we need to know the age of the bird we catch so that we can find out something about annual breeding success. So how old is the Reed Warbler in the picture?

 

Note the two black spots at the rear of the tongue

 

 

Well I know for a fact that on the day we caught it (back in August) it was only a few weeks old. And I know that because when it opened its beak I could see that it had two dark spots on its tongue. Lots of passerine chicks have tongue spots when they hatch that gradually fade as they age. I've always understood that the function of the spots is to make the gape of the chick more obvious to parent birds at feeding time - sort of like landing lights on a runway!