The cuckoo's calling

I am not sure which is the more unpleasant: that I am stood in four feet of muddy water and have a leak in my chest waders, or that an unscheduled thunderstorm has soaked me to the skin and put an end to my reed-bed nest monitoring for the evening. It’s not even as if I get paid for this monitoring work! I am a volunteer, like dozens of others around the country who give up their time to monitor nests in support of conservation and research. What I do get, however, is the opportunity to see our birdlife from a privileged viewpoint, plus the knowledge that what I am doing is making a difference. And that, as the television adverts insist on telling you, is what life should be about.

That I am wet from top to toe does not matter; it has been a beautiful evening. A female cuckoo was calling upon my arrival; various damselflies could be seen dancing just above the water’s surface and the reed-bed echoed to the chattering songs of reed warblers. So far, none of the dozen nests in this particular reed-bed have been parasitized by the cuckoo, but many are yet to contain eggs and there’s a good chance that the cuckoo will pick her moment and lay her deception in the nest of an unsuspecting reed warbler. When she does, I will have the solemn task of completing two nest record cards: one charting the demise of the warblers’ own nesting attempt (failed due to being parasitized) and one charting the fortunes of the single cuckoo now demanding the full attentions of its foster parents.

Anecdotal reports suggest that it has been a good year for the cuckoos, with many different being reported from across the county. A fellow volunteer, monitoring a site not far up the road and just on the edge of the Brecks, has found 17 cuckoo eggs so far, the output of two different female cuckoos working his site. Given that reed warbler numbers are on the increase and that cuckoo numbers are in decline, one hopes that we will see a few more successful cuckoos this year.

The changing of the season

There is a sense of anticipation on these early spring mornings, a feeling that each new day holds the prospect of seeing and hearing returning summer migrants. It is a sense shared by more than just birdwatchers, for many of us look forward to the sight of our first swallow or the call of our first cuckoo. The absence of these species for so much of the year seems to make us treasure them all the more, transforming them into totems for the changing of the seasons and heralds of renewal.

Already the first of our cuckoos is back and the growing dawn chorus reverberates with the calls of sedge warbler, whitethroat, chiffchaff and blackcap. On the nearby fallow, a pair of stone-curlew has been in residence for several weeks; these birds are less obvious than the many pairs of breeding lapwing, whose shrill calls and unruly aggression are being directed at passing pheasants, partridges and brown hares, all of whom have strayed too close to active nests.

Many shrubs and trees are coming into leaf and the whole landscape is greening up. Walks taken along regular paths reveal the speed with which the ground cover is emerging, as the fresh greens of nettle and bramble replace the more earthy tones of winter. The gorse, always early into bloom, casts a scent on the warm breeze and the air begins to hum with flies, bees and other winged insects.

It is the kind of weather to tempt me out at dawn and to keep me out after work. With so much to take in it feels as if the lengthening days are still too short and that there is insufficient time to truly appreciate the surge in new life that is taking place around me. These are the days to be outside, to be in touch with the natural world and to revel in its diversity. To feel the strengthening warmth of the sun’s rays and to breathe in scents that mix on the gentle breeze stimulates senses that have been subdued by the weight of winter. It is this time of the year that brings me closest to the landscape, delivering the feeling that I am part of it.

Nesting season nears its end for most

It feels as if we are on the home straight as far as our nest monitoring is concerned and, as is usual by this time of the year, we are all starting to flag a bit. It has been something of a broken season for me, with less time in the field because of work pulling together a field guide to monitoring nests. Mind you, my involvement with the book has also enabled me to see nests that have been new to me. It has also been a fantastic opportunity to learn from the real experts in this field, discovering the tips and tricks that can help you pin down the location of a nest so that you can then monitor its progress and complete a nest record card. The information contained on these cards is used by the BTO (www.bto.org/nrs) to inform conservation practitioners and Government as to the changing fortunes of our breeding birds.

As such, it was good to be back out in the reed beds over the weekend, to see the progress of our Reed Warblers and to marvel at just how lush and tall the beds had become. Many of the warbler nests now contained bright-eyed chicks, soon to fledge ahead of their journey south to Africa. Others contained newly laid eggs, with a handful of birds laying a second clutch in the same nest as their first but with others building anew. A couple of pairs had combined the two approaches, building their new nest on the top of the old to make a bizarre looking tower, woven between the reed stems.

One of the nests I checked on Saturday contained a young Cuckoo that we’d ringed earlier in the month. This Cuckoo was close to fledging, and sat on the rim of the nest, dwarfing the now flimsy construction that had once housed it. Alert, the chick turned to face me and opened its bill to reveal a vivid orange gape, a super signal to its foster parents to feed it with insect prey. I suspect that the brightness of the gape served another function, perhaps to warn off potential predators.

The one downside of working the reed beds this late in the season is the presence of a growing army of horse flies. They linger along the margins of the pools, most often where there is some damp and dappled shade; they soon discover any exposed flesh or, for that matter, any flesh protected only by a thin piece of material. My interest in insects means that I can still marvel at their jewel-coloured eyes, even when some of them manage to bite me. It has been a long season but a good one.

A cuckoo in the nest

Each year we find a few Cuckoos parasitizing the Reed Warbler nests that we monitor at sites in the Brecks. These are invariably found at the egg stage, the Cuckoo egg similar in background colour and pattern to the Reed Warbler eggs but somewhat larger. The degree to which the Cuckoo achieves successful mimicry of the Reed Warbler eggs is important, since it needs to trick the host into thinking the egg is one of its own and not that of another species. If the Reed Warblers are suspicious that they might have been parasitized by a Cuckoo then they will abandon the nesting attempt and start again.

Cuckoo eggs have been recorded from some fifty different host species in Britain but those most often targeted are Reed Warbler, Meadow Pipit, Dunnock, Pied Wagtail and Robin (roughly in that order). I have also seen a Cuckoo egg in the nest of a Wren, though how successful Cuckoos are in using Wrens is debatable. Wrens, like other small birds that build an enclosed nest, seem particularly sensitive to any change in the size of the nest entrance and any damage caused by the Cuckoo during laying is likely to cause the Wrens to desert. Additionally, there is the size difference between the eggs of the two species and the enclosed nature of a nest which seems unlikely to cope with the volume of a growing Cuckoo chick.

Just last week I saw a Cuckoo egg in one of the Dunnock nests that we are monitoring. As you probably know, the eggs of a Dunnock are bright blue and the Cuckoo egg in this nest was brown with splodges of darker colour – the female presumably a Reed Warbler mimic. You would think that a Dunnock would recognise the difference between the two eggs as well, but it seems that this is not the case. It is thought that this is because the Dunnock is a relatively recent host and that it has not yet developed the ability to recognise and respond to the presence of a Cuckoo. That Reed Warblers can and do respond, suggests that they have been Cuckoo hosts for a much longer period of time. However, even the Reed Warblers are fooled if they don’t suspect that a Cuckoo has visited the nest and once the egg hatches the resultant chick is reared as their own.

Finding a nest with a Cuckoo egg in it delivers mixed emotions. The presence of the egg spells the end of the host’s nesting attempt but, with Cuckoo numbers in decline, it gives hope that another Cuckoo will be recruited into a flagging population.

The Great Arrival

More than one author has conjured up the image of a Great Arrival, the 16 million or so migrant birds all heading north to summer on these northern breeding grounds. Most recently, in his excellent book Say Goodbye to the Cuckoo, Michael McCarthy speculated on how we would react were all these birds to arrive in a single flock. Such a flock would fill the sky; people would stop what they were doing in order to take in the spectacle and it would become an important signal that spring had reached our shores, celebrated with street parties and a public holiday.

Of course, our summer migrants do not arrive all at once in some grand flock; instead they turn up in small numbers or individually, with many arriving at night when the energetics of migration are better balanced. These overnight arrivals do provide one thing however; they deliver a sense of the unexpected to us birdwatchers, the thrill that the night just gone might have delivered some exciting find to an early morning trip to a patch of coastal scrub or waterside reedbed. Even those who would not describe themselves as a birdwatcher derive a thrill from seeing the summer’s first Swallow or hearing May’s first Cuckoo.

While these individual encounters act in some small way as a talisman for a wider arrival, the movement of this mass of birds, streaming out of Africa and up across Europe, goes largely unnoticed. Taken for granted, we find ourselves unaware of the problems faced by these many and varied travellers, problems which have resulted in dramatic population decline for some species. We only know of the changing fortunes of our summer visitors because of the work of organisations like the British Trust for Ornithology (BTO), whose partnership between researchers and birdwatchers provides regular monitoring of the birds breeding across Britain and Ireland. Yet even with this information we only know that something is wrong, we don’t fully know why. Are the numbers of Wheatears, Cuckoos, Turtle Doves and flycatchers falling because of difficulties here or is it because of problems on their wintering grounds or along their migration routes?

Recently, the BTO and RSPB have launched projects in Africa to look at our migrants on their wintering grounds, seeking to understand what is happening to the habitats they use during our winter. This work is only the start of a wider process which, ultimately, will need to feed into conservation action targeted at the different species across their whole range. The scale of this work also serves to bring home the message that while we view these birds as being ‘ours’, they are only ours for part of the year.

The imposter

I feel sorry for this pair of Reed Warblers, working hard to raise a chick that is not their own. Completely filling the nest cup, the grey-brown, ever-hungry youngster is an impostor, a nestling Cuckoo that has dumped the warbler’s own chicks or eggs overboard so that it may dominate the attentions of its foster parents. The chick is already larger than the two birds that deliver food into its demanding gape and it will grow much larger still. However, I have a certain amount of respect for this bloated, duplicitous lodger. After all, it is our only parasitic bird and it is fascinating to ponder on the evolution of such curious behaviour.

Globally, our Cuckoo is not alone in engaging in brood parasitism (to give this behaviour its technical name); it has 49 parasitic relatives and the behaviour also occurs within four other bird families. Even so, such parasitism is rare among higher animals and it is easy to see why our Cuckoo has been so well studied.

Adult Cuckoos are renowned for favouring hairy, sometimes toxic, caterpillars that are usually avoided by other bird species. Early in the season Cuckoos devour large numbers of Drinker moth caterpillars, collecting them from damp open habitats, but they also take other species, including the Magpie Moth, whose caterpillars show warning colouration and are distasteful to most would-be predators. Nestling Cuckoos are fed on whatever food each particular host species would normally provide for its own chicks and, for this reason, favoured host species are invariably insectivorous. These Reed Warbers will be feeding this particular young Cuckoo on aphids, moths and even the occasional butterfly. It is only upon reaching independence, at roughly five weeks of age, that the young Cuckoo will switch to feeding upon the kind of caterpillars favoured by the adults.

The act of parasitizing a nest is not simply an opportunistic one, since each female Cuckoo will specialise in a particular host. Only by doing so can she produce eggs that are close enough in colouration and pattern to the host’s own eggs to sneak one in undetected. Female Cuckoos will therefore work an area, locating nests of the host species and checking on their status. If a particular nest is unsuitable, perhaps because it is at too advanced a stage, then the female Cuckoo may predate the nest. This forces the unfortunate hosts to re-lay and thus creates a new opportunity for the Cuckoo.

Our Cuckoo has recently been flagged as being of conservation concern because of a long-term decline in its numbers. As such, it is good to see this particular Cuckoo nestling doing so well, even if one does feel sorry for its hosts.

A Cuckoo in the nest

One of the pairs of reed warblers on my local nature reserve is playing host to an unwanted visitor, in the shape of a young cuckoo. Perhaps this should be unsurprising as cuckoos have been seen and heard locally since the end of April. The cuckoo is an unusual bird; not only does it lay its eggs in the nests of other species, but it also tackles the hairy caterpillars which are unpalatable (or even toxic) to other creatures. The first of our cuckoos to arrive are male; the females, on average, arriving a week or so later. Each bird (both male and female) will set up its own territory, the size of which appears to be dependent upon food availability, the number of vantage points and, importantly, the abundance of host species. A female cuckoo, for example, may have a territory that is as small as 30ha if the density of host species is sufficiently great. This circumstance may come about if she happens to specialise in parasitizing reed warblers, which often nest semi-colonially, and so occur at high density.

Although young cuckoos have been recorded in the nests of more than 40 different species of bird, it is the reed warbler and meadow pipit that are most commonly targeted in Britain (together making up more than 85% of the cuckoo nest record cards held by the British Trust for Ornithology). Each female specialises in a particular host species and will seek out unguarded nests into which she may deposit her eggs. Over the course of the short breeding season (adult cuckoos usually depart towards the end of June) she may have deposited 25 eggs. Such specialisation is the key to her being able to pass off one of her eggs as that of the host species.

The cuckoo chick instinctively ejects any young or unhatched eggs belonging to its adopted parents. In this way it will receive all of the food that the parents bring in to the nest. Such large quantities of food are essential if the chick is to grow rapidly, often gaining 10 times its hatching weight during the first week alone. The rapid develop means that it soon outgrows the nest and will leave in under 18 days, though it does remain dependent upon the hosts for a few more weeks. Although the presence of a cuckoo can mean that as many as 20% of the suitable nests in the area are parasitized, this appears to be the exception rather than the rule. What is clear, however, is that declines in favoured host species could be behind the current decline in cuckoo numbers. The presence of this cuckoo chick, then, should be welcome.