Sur­pris­ing life of the Coal Tit

There are some sur­pris­ing dif­fer­ences be­tween the black-capped Coal Tit and its more colour­ful rel­a­tives

Bird Watching (UK) - - Contents -

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.

LOW NESTS Coal Tits nest twice a year, in holes much nearer to the ground than their close rel­a­tives BIG FEET Coal Tits have rel­a­tively longer feet and bills than Blue and Great Tits, adap­ta­tions to feed­ing in conifers

LONG WINGS Longer, nar­row wings help in hov­er­ing to pick small in­ver­te­brates CONIFER-IF­ER­OUS

Coal Tits have a stronger pref­er­ence for conifers than Blue and Great Tits

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