Pied Flycatchers, seen in Huw Jenkins' video above, fly in from Africa every spring and stay with us for just a short while to nest and rear their young.

Timing is of the essence to coincide with the tasty caterpillars that feed off the tender young leaves of oak trees - too soon or too late and there won't be enough food for the chicks.

There are 50 boxes for the Pied Flycatchers in the sessile oakwoods of Maentwrog Nature Reserve and each year since 1976 the wardens have been recording the date when the first egg was laid.

They do this by counting the number of eggs - the female lays one egg each morning so if there are two eggs in the box the first one was laid yesterday.

The birds are arriving about ten days earlier than they used to in Maentwrog - yet another indicator of climate change.

In Huw Jenkins' report for Country Focus on BBC Radio Wales Doug Oliver, the senior Countryside Council for Wales warden, counts eggs and talks about these intriguing birds, while Geoff Gibbs from the British Trust for Ornithology holds the ladder and records sightings for the bird atlas.

Huw Jenkins reports for Country Focus.