A songbird that’s full of surprises

Richard Collins is fascinated by the colourful habits of dunnocks.

MOST birds are silent at this time of the year. The robin is the only regular singer, but close to where I live a dunnock performs regularly. This is rather odd, but then dunnocks often have surprises up their sleeves. For instance, there are about 1.6 million of these self-effacing little birds in Ireland, according to the Atlas of Breeding Birds. They are six times as numerous as willie-wagtails and yet few people even recognise the name dunnock.

People and birds, who lead exciting lives, drawing attention to themselves, are usually glamorously attired, while plain individuals, with bland lifestyles, go unnoticed. The dunnock, however, bucks the trend; although it’s a plain Jane, its behaviour is often scandalous.

The bird’s old name, hedge sparrow, is misleading; this is not a sparrow. The bill, a pincers for poking out insect larvae, is thin and pointed, whereas the sparrow’s is a thick conical wedge, used to crush seeds. Sparrows are noisy and gregarious, but dunnocks are solitary; you seldom see even two together. The call is a hoarse squeak, and the song a short shrill cadence, a bit like a wren’s, but not as energetic. A loner, the dunnock’s nearest relative lives in the Alps.

The name comes from donn, the Irish for brown; the plumage being brown and grey. Adult birds also have brown eyes, but the eggs are bright blue. Just why they are so colourful is a mystery. Laid in an open cup nest, their colour must be a liability, attracting unwanted attention. Indeed, cuckoos often lay in dunnocks’ nests. The deceived parents seldom notice that anything is amiss and raise the young intruder. A cuckoo’s egg generally resembles those of its host, but the egg never resembles a dunnock’s, suggesting the choice of this particular foster parent may be new. On the other hand, Chaucer mentions cuckoos laying in the nests of hedge sparrows, so cuckoos were already targeting dunnocks 600 years ago. Host species, generally, learn to spot an alien egg and cuckoos, through natural selection, gradually develop ones which resemble those of their victims. However, there is no sign, as yet, of blue eggs being laid by cuckoos in Ireland.

It used to be thought that songbirds led blameless monogamous lives. Birds would mate in the spring and remain faithful to their partner until autumn. Then, the Cambridge-based ornithologist, N B Davies, began a celebrated study of dunnocks, using DNA profiling to determine the parentage of eggs and young. What he found was startling. Although many females had only one partner, some had two. Males, likewise, might associate with a second, or even a third, female. There were cases where two, sometimes three, males had conjugal arrangements with up to four females. Deception was common; a female would try to lose her mate so as to enjoy the attentions of another male. Birds would slip quietly into the territories of neighbours for illicit unions. Conflict between adulterous territory-holding males sometimes declined over time, allowing the territories to coalesce.

A male would feed the young of any female with whom he had consorted. In doing so, he would probably be supporting his own offspring. There is anecdotal evidence of males destroying the eggs and young of females with whom they had not copulated, thus clearing the way for replacement broods, fathered by them.

That dunnocks behave so badly it comes as no surprise to geneticists. Males must maximise their paternity, and to do so they will avail of every conjugal opportunity. Females try to maximise their breeding success by linking their genes to those of several males, a sort of genetic insurance policy. Some of their lovers are likely to be of good stock, offering better prospects to their offspring. Limiting oneself to a single partner is unwise; there needs to be a second string to any bow.

Dunnock behaviour should be the norm as far as genetics is concerned. Just why other species don’t behave similarly is the real mystery. But Davies’s outing of the plain unassuming dunnock, that model of dullness and sobriety, was a bombshell for ornithologists. Other songbirds species, generally, appear to be seasonally monogamous and relatively faithful to their spouses. But are they?