Blue tit songbird a marvel in miniature

Another Life: The nest-box was more for our benefit than the tits': there's no shortage of natural holes in our ancient hawthorns and weather-beaten sheds.

But we wanted blue tits nesting outside the kitchen window, like everybody else, and now that BirdWatch Ireland has gone in for mass-production, it's even easier to get a box with the crucial size of aperture. We nailed it up in January to get the new smell off it, although, given a choice, tits seem to choose a new box for preference, knowing they won't be jumped on by the former occupants' fleas.

Within weeks of installation, a pair of tits began to take breaks from the nuts to inspect it. A territorial male led the way, but it's the female who decides and spends time inside, prodding the walls like any first-time buyer. As March passed, she was in and out like a rocket: blink and you'd missed her. Soup has gone cold in our spoons as we've squinted to see what she's carrying.

As I write, it's little twigs, for the base of the nest cup, and strands of dead grass for weaving, all pressed down by her belly, round and round, to give it the proper shape. Then she'll line it with moss, or sheep's wool from the fence, or even hair from the dog's spring moult.

We'll get tired of watching, no doubt. After she's laid her eggs (one a day, up to 10 or so) there'll be a couple of quiet weeks while she's incubating. Then the real work starts for both parents, feeding each chick with little caterpillars or other insects perhaps 100 times a day, and taking out their droppings in tiny plastic bags. Ten thousand trips, or thereabouts, are what it takes to rear a whole blue tit brood.

What goes on inside the box is well exposed by now, as built-in video cameras share their images on the internet: just Google "blue tit" and search among the 1.25 million entries for sites displaying all those gaping little beaks and scrambling bodies. It's also a good way to catch up on the latest science of what is certainly the most-studied songbird in the world. The tit's willingness to raise its chicks under lift-up lids, ready for ringing, blood-sampling or whatever, has made it the favourite of ornithologists, both in lab and field.

In 2001, behavioural ecologists from half a dozen countries set up the European Blue Tit Network at the famous Max Planck Research Centre in Germany. They have chosen Parus caeruleus as their model species "to address some of the most exciting questions in evolutionary ecology". Some of these deal with how and why female blue tits select their mates.

For humans, even telling male tit from female is next to impossible, but researchers in Sweden and Britain found that using ultraviolet light, which birds can see but we can't, showed up an essential difference, an iridescence on the male bird's crown even brighter than that on the female.

In early spring, when the male erects a "crest" of neck feathers in sexual display, this can help to clinch a female's mating choice.

It also links in to the discovery that, while songbirds such as blue tits can be the very picture of loyal and monogamous pairings, the females are often quite promiscuous during the mating period, sneaking off to solicit attention from the breeding tit next door or even much further afield: the young they rear in the same brood often have different fathers.

These "extra-pair copulations", which have shown up in DNA paternity tests on the offspring, have a different evolutionary purpose for males and females. The cock birds just want to pass on their genes in as many directions as possible. The females need to pass individual genetic diversity to their young, to help in survival and further reproduction. This "heterozygosity" happens to be most marked in male tits whose crowns glow most strongly in reflecting ultra-violet wavelengths.

When the tit fledglings burst out of the box into the nearest bush on a warm, dry day early in June (7am seems a favourite time) they will all look much the same in their development: not until winter will differences in fitness and alertness mark the inheritors of superior genes.

But any blue tit, seen close up, is a miraculous 11 grams of life. In Norman McCaig's poem,

"A cubic inch of some stars/

weighs a hundred tons - Blue tit,/

who could measure the power/

of your tiny spark of energy? Your hair- thin legs/

(one north-east, one due west) support/

a scrap of volcano, four inches/

of hurricane: and, seeing me, you make the sound/

of a grain of sawdust being sawn/

by the minutest of saws."