Seasonal Update (June 2024)


May had some exceptionally sunny weather, with temperatures widely in the mid-20s Celsius, particularly during the early May Bank Holiday weekend. Some deep lows did bring heavy and persistent rain to the mid- and northern UK during the penultimate week, and May ended on and unsettled but warm note for most of England.

Website update

Not much has change on the site in the last month owing to other commitments taking my time, but a new section of the water deer article went live earlier this month. This section covers the species eaten by water deer across their range.

News & discoveries

Reprovable raptors. Many of us enjoy seeing birds of prey soaring in the skies, but they're not welcome everywhere. The latest RSPB Birdcrime report states that there were 61 confirmed cases of raptor persecution in the UK in 2022, with many more thought to have gone unreported. Red kites, peregrines, hen harriers and goshawks were among the species found poisoned, shot or trapped. According to the report, at least 64% were found on land associated with gamebird rearing. There were two convictions of gamekeepers for raptor persecution in 2022. The Countryside Alliance point out that the figure of 61 is actually 47 fewer than in 2021, and considerably lower than the previous 12-year average of 104 confirmed incidents, although this might reflect a continued decline in abundance and distribution of these raptors in recent years.

A male house sparrow (Passer domesticus). - Credit: Marc Baldwin

Star sparrows. For the 21st year running, house sparrows took the top spot of the RSPB's Big Garden Birdwatch 2024, being the most commonly recorded species, followed by the blue tit and starlings in second and third place. Great tits climbed one position to seven, taking the spot from goldfinches, which dropped to eighth. Despite retaining the top spot, sightings have still declined by 60% since the survey started in 1979, along with starlings (down 84%) and chaffinches (down 74%), while wood pigeon sightings increased by just over 1,000% and magpies by 210% in the same period.

Plastic prospects. According to APChemi, a plastics recycler in Mumbai, we generate about 350 million tonnes of plastic globally each year, and since the 1950s have produced an estimated 10 billion tonnes of the stuff, 80% of which has ended up as waste. Of this waste, just over half goes to landfill and only around 6% is recycled. There are many barriers to recycling, including the complexity of sorting the waste (which many people both struggle with and lack enthusiasm for), that plastics have a recycle lifetime (i.e., can only be recycled a handful of times before they become too brittle, so any recycling only postpones landfill/incineration), and that virgin plastic is so cheap it is generally much more expensive to use recycled materials given the laborious nature of the recycling process and the resulting poorer product. Recently, however, there has been a boost in funding and expansion of so-called “Advanced Recycling” in Europe and North America, using processes such as pyrolysis, gasification and solvolysis that can break down the plastics into their core components which can be built up from scratch into new plastics or other useful products. The suggestion is that 90% of plastics that go unrecycled today can be channelled back into the plastic-making process. This will work in tandem with new legislation around plastic production and distribution due to arrive in the next few years.

Seasonal highlight – The European cuckoo (Cuculus canorus)

When asked what marks the arrival of spring, many people say that it's hearing their first cuckoo. That familiar “cuck-oo…cuck-oo” call echoing around our woods, heaths, meadows, and fens heralds the arrival of a summer migrant: the Common cuckoo. The repeated 'cuckoo'-ing is, incidentally, the territorial call of the male; females make a loud 'bubbling churr'. Cuckoos have long fascinated birdwatchers and behaviourists alike. Not only do they represent an exotic arrival from the tropics, they're also one of the few parasitic bird species that trick other species into raising their offspring. This behaviour has long been recognised, with the Greek philosopher Aristotle being the first to formally describe the bird and its fascinating reproductive behaviour. In the 4th Century BC, Aristotle noted how cuckoos: 'do not sit, nor hatch, nor bring up their young, but when the young bird is born it casts out of the nest those with whom it has so far lived.'

A male European cuckoo (Cuculus canorus) - Credit: Sue Cro (CC BY-NC 2.0)

The common cuckoo is one of about 140 species of cuckoo that make up the family Cuculidae. The taxonomy within this family is still contentious, but there are currently thought to be six subfamilies and 28 genera. Along with those species we call cuckoos, the Cuculidae also contains the roadrunners, koels, malkohas, couas, coucals and anis, although some taxonomists argue that the latter two should be placed in their own families. It's the subfamily Cuculinae that we're interested in here; these are the brood-parasitic cuckoos of the Old World, and this subfamily holds the common cuckoo. There are four widely recognised subspecies of Cuculus canorus, with C. canorus canorus found in Britain.

Common cuckoos are roughly dove-sized birds (although I have come across reports suggesting they can reach about the size of a mallard duck!), measuring about 33 cm (12 in.) in length and with a wingspan of 58 cm (just under 2ft). Males are, on average, slightly heavier than females, weighing in at around 130 and 110 grams, respectively. These birds have a slate blue/grey head, chest and back with dark grey and white barred under parts. Although the slate-grey plumage is probably the most often observed, females can be divided into one of two different plumage colours: the slate-grey and a dappled brown morph. One thing that is striking about these birds in flight is their sleek body, long tail, and pointed/sickle-shaped wings, which makes them similar in appearance to small birds of prey, such as kestrels or sparrowhawks. Indeed, one flew over me in the New Forest recently and, had it not called overhead, I'd have chalked it up as a kestrel. It has been suggested that this superficial resemblance to a hawk helps drive potential host species off their nest, allowing the female to deposit her eggs.

Males are usually the first to arrive in the spring and can be heard calling from high perches to attract females. - Credit: Andy Morffew (CC BY 2.0)

Most cuckoo species are sedentary, but some undertake long-distance seasonal migrations. In the case of common cuckoos, this involves leaving their wintering grounds in sub-Saharan Africa and southeast Asia to fly the 3,500 or so kilometres (2,200 miles) to the UK to breed. Cuckoos are summer visitors to whole of UK and Ireland, although they're most abundant in central and southern England. Adults typically arrive in late March or April, with males generally heard before females, and they mate and lay eggs during early summer months before returning to Africa/Asia during late summer. Young birds leave about a month after their parents, although there are records of juveniles from Hampshire as late as late October.

The RSPB estimate there to be between 10,000 and 20,000 breeding pairs in the UK but, while this may sound like a lot, cuckoos appear to be in serious global decline, to the extent that they are now classed as a Red List species in Britain. Globally, common cuckoo populations have suffered an estimated decline of 65% in the last three decades - the reason(s) for this decline is unknown, but habitat loss, declines in host species populations, and fluctuations in prey or prey habits have all been suggested.

The common cuckoo is a widely distributed bird that can be found in a variety of habitats, although it's primarily a bird of open land, favouring areas of mixed woodland/grassland, woodland/heathland, and fenland. Intriguingly, for such an iconic bird, we know very little about the diet of the adults - the diet of the chicks is understandably determined by the host parents, but there are very few dietary studies on adult cuckoos. We do know that cuckoos are insectivorous (feed predominantly on insects) and appear to particularly favour the hairy caterpillars that most other bird species avoid because they are either poisonous, look poisonous (i.e., are brightly coloured), or are covered in bristles. Cuckoos have a digestive system that's well adapted for handling tough bristles and dealing with this toxic prey. David Swan, as part of his zoology degree at Aberdeen University, studied the dietary preferences of cuckoos and found that the hairier and more brightly coloured the caterpillars were, the more attractive they were to hungry cuckoos. Hairy caterpillars were taken significantly more often than non-hairy ones (i.e., 66% compared to 33% of the diet). Overall, about 15 species of caterpillar have been recorded as cuckoo prey, including those of the March moth (Alsophila aescularia), garden tiger (Arctia caja), winter moth (Operophtera brumata), drinker moth (Philudoria potatoria) and cinnabar (Tyria jacobaeae). There are also reports that cuckoos will supplement their caterpillar diet with beetles, and there are records of them occasionally taking the eggs or chicks of their host species.

Perhaps the feature for which cuckoos are best known is that of their reproductive modus operandi of brood parasitism, which means they lay their eggs in the nests of other birds (host species). When the chick hatches it pushes the hosts eggs/chicks out of the nest, and it's then raised by the host birds as one of their own. Cuckoos are known to parasitize more than 100 host species, including common redstart, brambling, robin and pied wagtails. Meadow pipits, dunnocks and reed warblers are, however, the most common victims in Britain.

A fledgling common cuckoo waiting to be fed by the parent species. - Credit: Sergey Yeliseev (CC BY-NC-ND 2.0)

A female cuckoo will seek out a suitable nest and wait nearby for the parent to leave, at which point she lands in the nest, pushes one egg out, and lays one of her own before flying off - this process takes about 10 seconds and it's estimated that a female may visit up to 50 nests during a single breeding season. If the cuckoo is out of phase with a clutch of host eggs, she will eat them all so that the hosts are forced to start another brood. The eggs laid by the cuckoo may be significantly larger than those of the host, cuckoo eggs typically measuring 2.2 cm by 1.6 cm (0.9x0.6 in.) and weighing about three grams, but still relatively small for the size of the bird. The female can retain the egg within the uterus for 24 hours prior to laying, during which the egg is maintained at approximately 40C (104F), allowing the cuckoo chick to grow faster than it would during the same period in the nest. The result is that the cuckoo chick is further developed than those of the host and will hatch first.

As soon as the chick hatches it sets about pushing any other eggs out of the nest using its back and shoulders. The chicks are significantly larger than those of the host and so need to monopolise the food brought by the parents. The host parents will dutifully feed the chick until it fledges three or four weeks later. Once fledged, the juvenile cuckoos fend for themselves before returning to Africa/Asia in September. Cuckoos are reproductively active by their second year.

How the cuckoos have evolved to dupe other species into raising their young, and the counter adaptations these host species have developed to try foil the cuckoos, are really quite amazing. Indeed, theses have been written on the subject, and while I couldn't hope to cover it all here, let's take a very quick look.

Thanks to the keen observations of 18th century ornithologists, we have long known that cuckoos are capable of laying eggs that look very similar to those of their host (so-called mimetic eggs), they can produce relatively small eggs, only a single egg is laid in each nest, and the egg is laid there before the host has completed their clutch. Our understanding of cuckoo behaviour was greatly improved during the summers of 1985 and '86, when Nick Davies and Mike Brooke immersed themselves in the Cambridge fens to study the relationship between the common cuckoo and one of its main hosts, the reed warbler (Acrocephalus scirpaceus). Their results were published in a lengthy-but-fascinating paper to the journal Animal Behaviour in 1988.

Using dummy eggs, Davies and Brooke were able to establish that mimetic eggs, afternoon laying, small eggs and rapid visits helped the cuckoo dupe their host, while the act of removing one of the eggs before laying probably helped the cuckoo chick by improving incubation. In fact,, we now know that female cuckoos can be categorised according to the species of host they parasitize; these populations, called gentes, favour a particular host species and lay eggs that closely match them in colour and pattern. Perhaps even more fascinating than the cuckoo's behaviour is the observation that in response to cuckoo parasitism, hosts have evolved improved egg discrimination (i.e., are now better able to tell their own eggs apart from those of a cuckoo) and are more likely to reject eggs, even their own, if they've seen a cuckoo near the nest. Unsurprisingly, the longer a host species has been subjected to cuckoo parasitism, the more likely it is to have evolved to spot cuckoo eggs. That said, Davies and Brooke also observed that warblers were unable to tell the difference between their own chicks and those of another bird - when the researchers swapped out some of their eggs for dunnock chicks, the warblers fed the dunnock chicks as they did their own.

Chick of common cuckoo in the nest of a tree pipit (Anthus trivialis). - Credit: vladlen666 (WikiMedia Commons)

There are a few theories as to why warblers don't distinguish their own chicks from those of cuckoos, including simple evolutionary lag (i.e., this just hasn't evolved yet) and that cuckoo chicks are exceptional mimics themselves. Indeed, observations on cuckoo chicks have revealed that they persuade the host parents to feed them by making rapid begging calls that sound remarkably like a whole clutch of chicks. It has been suggested that the chicks need to be able to simulate the noise of a full nest because they need all that food, but need to compensate for the fact that the parents only see a single gape, rather than four or five. So, there is a continual evolutionary “arms race” going on, with host species becoming better at spotting cuckoo eggs and cuckoos evolving to produce eggs more similar to those of their hosts, making it more difficult to get caught out. How far this race will run, and where it will end, only time will tell.

For a round-up of Britain's seasonal wildlife highlights for early summer, check out my Wildlife Watching - June blog.

Related reading