As with all small birds, the chances of a baby wren surviving to its first birthday are very low. Many birds do not even get as far as fledging, especially if cold or wet weather – or, ironically, a prolonged drought – reduces the availability of food when they are in the nest. Others are seized by a predator soon after leaving or, even if they do make it past the fledging stage, die of starvation during harsh winter weather. Whatever the reasons, according to the BTO a juvenile wren has only a one-in-four chance of surviving to the age of a year old.
Adults do not fare much better than their offspring: in a typical year, fewer than one in three adult wrens survive. Typically, if they get through the dangers in the early part of their life, wrens will reach a lifespan of two years. But the oldest known bird – which was ringed as a chick in the nest in 1997 and then recovered in 2004 – was seven years, three months and six days old.
So what kills them? The number-one factor, as for most small birds, is a shortage of food – especially at crucial times of the year such as midwinter. A wren’s energy consumption is phenomenal: it must eat between one-third and half its body weight every single day – the equivalent of an adult human consuming between 100 and 200 Big Macs daily.
The next most likely cause of death, again as with other small birds, is being killed by a predator. This is especially true of wrens, which make their home in our gardens. That’s because by providing such a concentrated tapestry of habitats, with food, water, shelter, and places to roost and nest all within a small space, we also create a permanent rolling buffet for predators.
The one advantage the wren has over other garden birds like robins, blackbirds and thrushes is its small size and agility. Its short wings and stubby shape make it very manoeuvrable, and because it spends so much of its time hidden away in the undergrowth, it does not present such an easy target to any potential attacker.
Adult wrens have two very different strategies to avoid predators. Usually, and especially if they have youngsters nearby, they will sound a loud alarm, warning their offspring of danger. But on occasions they will ‘freeze’ into an immobile pose, remaining completely still for the whole time any predator is in the neighbourhood. For a bird that rarely stays still even for a few seconds, to remain unmoving for so long is very unusual indeed. One adult wren was recorded as maintaining this static pose for almost three minutes – a stupendous feat of self-control for such a little ball of energy.
Predators also take wren eggs, chicks and occasionally adults straight from the nest. Stoats and weasels are good climbers, and weasels are certainly small enough to get inside the entrance hole, as are rats. Snakes, too, occasionally take eggs from birds’ nests, especially when they are built close to the ground. And those perennial nest robbers – magpies and jays – have been observed tearing off the dome of a wren’s nest with their powerful bills, after which they rapidly make a meal of the unfortunate chicks within.
Parasites, both large and small, can also be a problem. Mites – tiny arthropods distantly related to spiders – can infest a wren’s nest so badly that the chicks sicken and die, while wrens are also hosts to three species of feather-lice and two kinds of flea. At the other end of the scale is nest-parasitism by the cuckoo, a problem shared by many other small songbirds.
Given that even a day-old cuckoo chick may weigh 15 grams – about one-and-a-half times the weight of an adult wren – it is perhaps not surprising that the wren is the smallest regular host of the cuckoo in Britain. But compared with the three main victims of the cuckoo – dunnock, reed warbler and meadow pipit, which between them account for the vast majority of records – the wren is rarely chosen.
As they do with their other host species, female cuckoos will carefully watch a pair of wrens, observing where they have made their nest, before dashing in when the parents are away to lay their single egg. Once the baby cuckoo hatches out, it instinctively ejects any of the wrens’ own eggs or chicks from the nest, so that it gets all the food brought back by the parents.
Despite the massive size difference – a fully grown cuckoo chick can weigh 100 grams, which is between ten and twelve times the weight of an adult wren – the foster parents do somehow manage to bring back enough food to sustain this monster in their nest. And we know that wrens can raise a baby cuckoo successfully: one ringed in a wren’s nest was later recovered at Cap Gris Nez, on the other side of the English Channel, in France.
It is often asked how the single cuckoo chick, which looks so different from the usual brood, can fool its host parents so effectively. One reason is that all parent songbirds are genetically programmed to respond to the sight of any chick’s open gape, and fetch food in response. Being so large, the cuckoo’s gape acts as a ‘super-stimulus’ to the host parents to bring back even more food.
But, according to the doyen of cuckoo scientists, Nick Davies, this visual stimulus is not always enough on its own to sustain feeding. He and his colleagues discovered that the main stimulus to provoke the hosts to bring back a constant supply of food is, in fact, sound.
In experiments in the field, they found that the cuckoo chick produces a loud and persistent series of begging calls – ‘si-si-si-si-si’ – which mimics an entire brood of the host’s chicks. ‘We suggest that the cuckoo needs vocal trickery,’ they concluded, ‘to stimulate adequate care to compensate for the fact that it presents a visual stimulus of just one gape.’ So sound may be just as important, perhaps even more so, than the visual stimulus.
Edward Armstrong reported several unusual examples of wrens interacting with cuckoos. In one case, a cuckoo laid its egg in a dunnock’s nest, which had itself been built on top of a wren’s nest containing its young. In an even more unusual instance, a newly fledged brood of young wrens sheltered for the night in a dunnock’s nest in which there was also a young cuckoo. When the observer visited one evening to inspect the dunnock’s nest, he was astonished to find four baby wrens sheltering under the wing of the cuckoo chick, which he only noticed when their heads popped out, presumably to investigate this human intruder in their midst. Because the young cuckoo’s instinct to evict any chicks from its nest disappears after about six days, Armstrong pointed out, it was happy to tolerate these temporary housemates.
In his autobiography An Eye for a Bird, the bird photographer Eric Hosking recalled an incident which serves as a warning to this infamous brood parasite. In the autumn of 1931, Hosking and the gamekeeper George Boast inspected more than 150 nest boxes they had erected around Staverton Park, near Daventry in Northamptonshire. In one box they discovered the grisly remains of a bird’s skeleton, which Boast immediately realised was that of a young cuckoo inside the nest of a wren. Hosking takes up the tale:
Presumably the hen cuckoo had projected her egg into the nesting box, the wren had hatched it, and the young cuckoo had been unable to throw out the wren’s eggs as these were found broken into tiny pieces. When the cuckoo was fully grown it was, of course, too large to get out of the entrance hole.
As in the case of the wrens roosting under a young cuckoo, occasionally the roles are reversed, with newly fledged wrens found roosting for the night in the nests of other small birds. One family occupied a willow warbler’s nest, in which there were four chicks. The parent wren visited the nest with food, but would only give it to her own offspring; whereas the willow warblers seemed happy to feed both their own chicks and the young wrens. Eventually the wrens moved on and, three days later, the willow warbler chicks successfully fledged, apparently none the worse for their shared experience.
Adult wrens have occasionally been seen feeding a range of other baby birds in the vicinity of their nest, including spotted flycatchers, great tits and linnets. Other species of wren in the Americas have also been observed as foster parents. All these cases demonstrate the innate drive found in all songbirds to provide food for any hungry youngster.
Once the youngsters from the first brood are safely fledged, and more or less independent of their parents for food, the adult wrens immediately begin work on their second brood. Often, though, they are in such a hurry to get started that the male will still be feeding his first family of offspring while his mate is busily bringing back feathers to line the new nest.
As we have seen, wrens in the south of their range are more likely to raise two broods than those in the north, or on outlying islands or harsh habitats such as the tundra north of the Arctic Circle – yet two broods have been recorded as far north as Finland.
Generally, a second brood involves more or less starting from scratch – old nests are very occasionally re-used, but only after the male has refurbished them, as they can get very tatty after a whole brood of chicks has lived there for several weeks. However, successive generations of wrens will re-use the same nest site – a particular nook or cranny that they feel is safe from predators – year after year. Whether these are the offspring from a previous nest, or newcomers attracted by the location, is not known.
During one year in the 1930s, the bird photographer George Marples made an intensive study of a single wren’s nest, in which the pair successfully raised a second brood of chicks. What astonished him was the sheer persistence of the adult birds throughout. In one period of just 90 minutes early one morning, when the nest was under construction, it was visited no fewer than twenty-seven times – almost once every three minutes. The male also sang 158 times in eighty minutes, a rate of one song every thirty seconds. Later on, food was taken to the growing chicks every three minutes.
Towards the end of his observations, disaster almost struck when unseasonal summer gales blew the whole nest out of the bush and onto the ground. Fortunately, the chicks had by then grown large enough to leave their wrecked home, and so they survived.
Whether raising their first or second broods, wrens do not always build their own nests. When the opportunity occurs, they will sometimes take over the homes of other birds. The range of species in whose nests wrens have been known to squat is very varied. They include the swallow, house martin, house sparrow and dipper, which you might expect given that these enclosed nests all resemble that of the wren, but also the grey heron – whose nest is a vast superstructure of twigs.
Wrens also often nest close to an eagle’s eyrie, perhaps because the much larger eagles lend them a degree of protection against any passing predators. The golden eagle expert Jeff Watson was one who noted that wrens sometimes nest as close as a few metres from the eyrie, and although he had never heard of one actually making its nest inside the huge construction, he speculated that this probably did occur. The scientist Derek Ratcliffe was another to note the proximity of many wrens’ nests to eagle eyries, drawing attention to the longstanding mythological association between these two species, which I shall explore in the next chapter.
The only definitive account of a wren actually building its nest inside a golden eagle’s eyrie comes from Dorothy Hosking, the wife of bird photographer Eric. True to form, Hosking had taken his wife to Scotland on their honeymoon – not, as she might have hoped, for a relaxing visit to a luxury hotel, but to photograph golden eagles. Dorothy, who fortunately seems to have shared her husband’s enthusiasms, takes up the story, in this extract from her diary, wryly entitled I Married a Naturalist:
To our delight we discovered a wren had built her small nest in the large structure of the eagle’s eyrie. This wee bird seemed much more concerned with her family’s welfare than the golden eagle was with hers. Back and forth she flew, and sometimes the cock would sing his sweet shrill song. I always marvel how so small a bird can sing so loudly.
By the end of June, though sometimes as late as August, many wrens – especially those in the south of Britain – will already have successfully raised two broods of young. From then on, both young and adult wrens become – like most other songbirds – more and more elusive. That’s because they are now undergoing the process of moult, re-growing their plumage to face the rigours of the autumn and winter to come.
Midsummer is the ideal time to moult: there is plenty of available food, and long daylight hours in which to forage for it. Also, the foliage of bushes, shrubs and trees is now at its thickest, which means that, as they lose their old feathers and grow new ones, wrens are able to hide away even more effectively than usual. The juvenile birds, which only left the nest a few weeks before, undergo what is known as a ‘post-juvenile moult’ into the adult plumage, after which they look exactly like their parents.
Wrens also fall silent at this time of year: although on my local patch or in my garden I occasionally hear a wren singing in early July, they usually don’t start again until September. They then continue singing regularly for the rest of the year, though far less frequently and less intensely than during the breeding season.
From now on, they have very different priorities. Summer may still have a long way to run, but it is not so long before autumn arrives, and with it another great challenge in the life of this small bird: to survive until the next breeding season.
