Surprising life of the Coal Tit
There are some surprising differences between the black-capped Coal Tit and its more colourful relatives
Dominic Couzens highlights key differences between Coal Tit and its more colourful relatives
BLUE TIT, GREAT Tit, Coal Tit – the triplet of names trips neatly off the tongue. It’s easy to consider these three species as close associates. After all, they flock together, bicker at bird feeders together and compete for garden nestboxes. But are they really so similar? Well, Blue and Great Tits parallels are striking.
The Coal Tit, though, is another story. It is separate in a multitude of ways, some of which will surprise you. There is a strong argument to separate the three amigos into two plus one. Get a call from the Coal Tit’s agent, and you will be persuaded by numerous facts. One difference that every birder would quote is that the Coal Tit is more readily found in conifer trees. On the one hand this is profoundly true, yet on the other hand the link is nuanced. Coal Tits are certainly physically adapted to foraging in conifers. They have longer feet and longer back claws and toes than the other tits. Coal Tits also have longer and narrower bills. It is easy to see how a narrow bill is ideal for probing in the narrow fissures between needles and for handling small prey. However, a long bill isn’t so good for spotting prey close to the bill tip, leading to the delightful thought that Coal Tits might keep missing nearby targets as they forage, saying “Darn it, missed!” repeatedly up in the treetops. Coal Tits, with their light bodies and relatively long, narrow wings, are also superior in their hovering and hawking ability, which presumably also helps in foraging around dense branches of needles. Having said that, Coal Tits do, of course, occur – indeed, flourish – in other types of woodland. They positively thrive in the Sessile Oak woods of the north and west, sometimes outnumbering Blue and Great Tits in this deciduous habitat. They are not, of course, one-trick ponies, and neither are their colourful cousins. Great Tits, in particular, do breed in coniferous stands, albeit with little enthusiasm, no more perhaps than a fallen millionaire swapping Fortnum and Mason for Aldi. Their breeding success is impaired, too, something that also evidently happens to exmillionaires. Interestingly, British Coal Tits, which are of a different race from continental Coal Tits, have a greater inclination to use broad-leaved woodlands for breeding and foraging, and have a fractionally thicker bill to match. It has been suggested that our birds may have had to adapt to the near-loss of hardwoods from Britain which occurred after the last glaciation, becoming better adapted to oaks and other trees in order to prevent dying out. They are, or were, in the process of evolving into a separate species, perhaps. Meanwhile, the continental Coal Tit is more slavishly ‘conifer-iferous’, but that’s hardly a hindrance; it has a much wider world distribution than the Blue Tit. While the Coal Tit is physically distinct from the more colourful tits, it exhibits many behavioural quirks that set it apart, too. Surprisingly, throughout the non-breeding season, Coal Tits regularly place food supplies into storage. During successful foraging sessions, they take items back into their winter territory and hide them away for later consumption, usually recovering them within a day or so. That doesn’t stop each individual squirrelling thousands of items away each year. Perhaps the most remarkable aspect of this hoarding behaviour is not that the Coal Tit does it, but that Blue Tits and Great Tits never, or hardly ever do it. They seem not to have the need or aptitude. Once breeding begins, several subtle and intriguing trends emerge that squeeze the Coal Tit further away from its relatives’ template. For one thing, Coal Tits begin laying their eggs a few days ahead of Blue Tits, in mid-april. They also frequently nest in different locations, albeit probably forced there by competition. They use mouse holes on the ground, and holes among tree roots, far lower down than the other tits do,
and far lower than they would prefer, as measured in conditions where competitors were removed. Coal Tits also choose slit-holes in preference to round holes, for the same reasons. Another, perhaps more fundamental difference lies in the Coal Tit’s regular habit of nesting twice in a season. Blue Tits and Great Tits time their breeding season acutely to coincide with maximum caterpillar production, and they raise very large broods all at once, often with more than 10 chicks. Once this short bloom is over, they only rarely nest again. Coal Tits, on the other hand, which raise just as many chicks, quite commonly attempt this feat twice in a season. Their young have a habit of staying put as a brood, hiding in thick vegetation once they have left the nest, rather than following the adults as Blue and Great Tit broods do. Their losses are fewer and, combined with the fact that Coal Tits may be double-brooded, one wonders why we aren’t knee-deep in them. Having two broods in a season has a significant social impact on Coal Tits, and exacerbates yet another big distinction between this species and ‘the rest’. Surprising as it might seem, Coal Tits are close to world leaders among birds in what is often referred to, a little coyly, as EPP. These letters stand for Extra-pair Paternity, and the phrase refers to the incidence of chicks in broods that have been fathered by a male other than the one that attends the nest and feeds them. It is thus a measure of cuckoldry, which is remarkably high. In the majority of bird species so far studied, including Blue and Great Tits, EPP levels rarely rise far above 10%. In the Coal Tit, however, they can reach the dizzy heights of 25%. In one study, for example, 40 out of 158 nestlings were found to have been sired by a male other than the practising father (the ‘social mate’). Why this might happen, nobody knows – what could be the evolutionary advantage to the Coal Tit of having such high levels of EPP? This is a full-blooded area of research, causing scientists’ minds to buzz. Everybody appreciates that males benefit by performing extra-pair copulations simply to increase their overall paternity, but why do the females indulge? Studies so far have not proven that extra-pair fledglings are any more successful in life than within-pair offspring. It is an intriguing question. What is clear is that the opportunity for a second brood encourages high levels of EPP. In some Coal Tit broods, the incidence rises close to 50% during the second, midsummer egg-laying. The same studies have shown that older, more experienced males produce more extra-pair young than first-years. Older is better. As mentioned above, nobody has any idea yet why Coal Tits, of all birds, should be so inclined towards high EPP rates, when other tits are not. However, it is a significant behavioural difference. All these studies on Extra-pair Paternity have made the Coal Tit’s reproductive processes an open book for all kinds of incidental discoveries. One fascinating, and highly unexpected one, was that different populations of Coal Tits have slightly different sperm. This could count as too much information, especially since the birds of a Norwegian population were found to have longer sperm than their German counterparts, and when this was announced at an international conference there was a bar-room brawl between the two nationalities. Actually, there wasn’t really a brawl, I’m teasing. But the sperm differences are real enough. You cannot make that up. And it goes to show that, not only is the Coal Tit very different from its colourful relatives, but Coal Tits here and there are different from each other. Birds are gloriously complicated.