SUMMER is here. I have started my annual survey of woodland breeding birds for the Yorkshire Wildlife Trust, so it's official.

Actually, my first visit to Chafer Wood, near Ebberston, should have been sometime during the last week of April, but the weather then was so awful that I delayed it by a couple of weeks.

Those very late frosts and hail squalls went very much against the trend for what has been an incredibly mild winter just gone. Let's just hope that early flowering fruit trees haven't been damaged – we will only know in autumn, of course.

One group of animals that should have particularly benefited from our recent warm winter are the small, warm-blooded ones – the birds and mammals.

Endotherms, to give them their correct scientific label, maintain a steady body temperature regardless of the outside conditions, the human figure famously being 37°C.

Ectotherms, like amphibians and reptiles, have a body temperature which follows that of the outside environment; consequently most of them cannot operate during a British winter so shut down and hibernate.

Endotherms need to keep their body temperature up by burning food, but when it's cold small animals lose this heat at a much faster rate than bigger individuals. During harsh winters many just can't find enough food to replace this heat and mortality rates among mice, shrews and tiny birds can be enormous.

Some small birds try to cope with very chilly conditions by cuddling up to friends in an enclosed space like a hollow tree or nest-box. Sparrows and long-tailed tits commonly indulge in communal roosting (not with each other though) but the cuddler par-excellence is the humble wren. Roosts of 10 to 15 individuals are standard, but the record is an amazing 60 wrens together in one nest-box.

This liking for holes and hollows inspired the Latin name for the wren, Troglodytes troglodytes (meaning hole or cave dweller) – a very big name for a small bird.

As I started the first Chafer Wood survey it was in the back of my mind that there may be a lot of small winter survivors around, and sure enough I wasn't disappointed. Blue tits seemed to be everywhere, bouncing, flitting, and dangling from twigs like Christmas tree decorations, and during my two hour circumnavigation of the wood I counted 22 singing male wrens.

Last year Chafer Wood had 19 wren territories, which is probably its capacity, so at least three wren families will lose out in the battle for breeding space and have to move elsewhere, maybe into neighbouring Ebberston gardens.

Now, I deliberately referred to wren "families" rather than "pairs" because these birds have an unusual breeding system in that they are polygamous.

Each male builds numerous nests within his territory and then sings his heart out to entice in as many females as possible. The ones that take him up on his offer (usually two or three) will choose a nest, allow him to mate and then lay a clutch of up to eight eggs. I can’t believe that he then has the energy to help his concubines feed their 24 chicks but who knows.

The overflow of surplus small woodland birds is likely to be happening right across Ryedale, so keep an eye on the resident birds in your back garden. If some unexpected wrens turn up or previously unused nest boxes are taken over by blue tits, then you'll know where they have probably come from.