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.

Woven wonders

For me, much of the early summer is spent looking at nests. Regardless of how many nests I see I am still amazed by the technical ability that goes into their construction. Admittedly there are some nests that are rather basic affairs, with little more than a few twigs placed together in a manner that, more often than not, leaves the nest contents in real danger of falling to the ground beneath. Most nests, however, are more solid in their construction and these provide a secure environment for the eggs and resulting young.

Many nests are made of several distinct layers of material, typically with bulkier material in the base and around the outside, finer material within this and a lining of feathers or other soft material. Some species reinforce their nests with mud, something which song thrushes take to an extreme with their lining of mud, rotten wood and saliva, which dries to a form of ‘woodchip’, rigid in structure and thought to reduce the attractiveness of the nest to nest parasites.

Some of the most delicate nests are those made by our warblers. Blackcaps and garden warblers, for example, construct their nests from fine grasses, the nest itself placed in thick bramble or nettle cover, while reed warblers use grasses, material from reed flowers and plant down in their nests, which are tied into to the vertical reed stems that support them. On occasion the amount of white plant down in a reed warbler nest can make it appear as if the nest is constructed entirely from wool.

Nests may incorporate all sorts of other materials. Red kites, for example, are known to add clothing taken from washing lines to their bulky nests, while many birds add feathers or animal fur, scavenged from the local area. Dog and cat fur, left out in the garden after a pet has been groomed, is often well used by nesting tits. Longer hairs can sometimes be a problem, however, the bird or its chicks getting caught in the material if it forms a loop. The same can be said for discarded wire or fishing line. Birds are certainly resourceful when it comes to sourcing nest material, so it is important for us to be aware of the consequences of leaving unsuitable material in our gardens or the countryside.

The wind

It is not so much the rain of June and early July that has caused the problems for our reed warbler project but the wind. The seemingly relentless run of windy days that hit during the second half of June tipped the contents from nests as the reeds bent and shifted. Weekly visits to the reed bed revealed an all too familiar picture; nests that had eggs or young chicks were now empty or gone altogether.

Reed warblers are, however, robust birds and quick to replace a nest with eggs that has been lost. In fact, some of the nests that disappear (leaving behind the tiny marker tape placed beneath the nest) are dismantled by the birds themselves, the material reused for the new nest placed just a few feet away. It was during our first field season that we first noticed this tendency to dismantle a failed nest and on occasion it caught us out. During that first season we only marked the location of a nest with a piece of tape placed on the track that we used to move through the beds. The nest itself would be unmarked but located within a few feet and easy enough to spot. Sometimes, however, you would arrive at the tape, recall the location of the nest and then adopt a puzzled expression at either your failure to refind it or to find that it had moved by a few feet. The addition of that second piece of tape leaves no doubt that a nest has been dismantled and moved.

Those nests placed in the most sheltered parts of the reed beds seem to have done alright, the birds rearing their young to fledging before probably making a second breeding attempt. Others may have lost their clutch to a cuckoo, the cuckoo chick typically hatching first and quickly jettisoning its hosts’ eggs over the side of the nest. The cuckoo chicks have not had it all their own way though, the windy conditions have taken their toll on them as well. So far, just one of the cuckoo chicks has fledged, others have only just hatched and others were lost to the weather.

Working so closely alongside these birds generates a degree of affection and you feel for the reed warbler pair in whose nest you spot a cuckoo egg. You know that this breeding attempt will be a failure for the warblers but the sadness is countered by the hope that a young cuckoo will fledge. After all, it is the cuckoo that is in decline while the reed warbler remains one of the few African migrants that seems to be flourishing. Lets hope the weather gives them both a fair crack over what is left of the season. 

Strong winds cause problems

The combination of chest waders and lifejacket is not the most flattering of outdoor looks but it has become my regular attire this summer. Once or twice a week I am out monitoring a reedbed population of Reed Warblers as part of what we hope will become an important long-term study. Thanks to the generosity of the landowner we are able to monitor the nesting behaviour and breeding success of a sizeable Reed Warbler population using a series of old gravel workings.

While many of our long-distance migrants are undergoing substantial declines, the Reed Warbler is currently showing an increase in abundance and so it has received rather less attention than is, perhaps, desirable. The Reed Warbler is an important host for the Cuckoo, a species which is in decline, and it is important for us to learn more about Reed Warbler populations and the extent to which Cuckoos use them. So far, this season has not been that good for the Cuckoos parasitizing our reedbed warblers. Some of the reedbeds on the site are rather exposed and the strong winds of late May resulted in many nests losing their contents, as the reed stems to which the nests were attached bent over in the wind. This resulted in the loss of some Cuckoo eggs and chicks. The Reed Warblers will try again though, as will the Cuckoos over the short time they remain in the country.

One aspect of our work that is of particular interest is the pattern of breeding attempts. Some authors suggest that Reed Warblers have three broods in a season, while others think that the three broods are actually three waves of breeding: the first and last being attempts by older birds (in effect first and second broods), while the second wave is made up of young birds (in their first year of life) who return to the breeding grounds somewhat later in the summer. To test this, we are attempting to fit rings to all of the chicks reared on the site to see if and when we recapture them in future years. In order to be systematic about the recapturing component of our work, we operate a series of nets in set positions, once every ten days.

Monitoring the nests also provides information on breeding success. Last year, for example, the number of eggs laid (3.67 per nest) was lower than the national average but the survival of the nests through to fledging was higher, so that the number of fledglings produced per breeding attempt was similar to that at other UK sites. Large-scale, long-term studies are uncommon, which makes our work particularly important. It is going to be fascinating to see what we discover over the coming years.

Summer heat

July's heat drives me to seek out the shade offered by our young stand of Hazel. These adolescent trees, still untidy in habit, are one of the greenest things left in the garden; everything else looks faded and listless, the bone-dry soil holding little relief for thirsty plants.

It is easy to overlook the impact of the summer heat on our wildlife. While we can retreat to the cool of the thick-walled cottage or the deep shade offered by a patch of woodland, other creatures remain exposed to the full intensity of the sun. This is certainly the case for the Reed Warblers whose reed bed nests often lack covering shade. The females will actively shade their helpless young, perching above them in the nest during the hottest part of the day. The females obviously feel the heat themselves, panting with bill wide open in an attempt to keep cool. Quite suddenly one of the females will 'collapse', tipping from her sitting position onto one side as if in a faint. The bird will then raise a wing high to the vertical. This seems to do the trick, perhaps exposing veins close to the surface under the wing and reducing her body temperature through heat exchange. Just as suddenly she will recover. It will be the youngest chicks - those still naked and helpless - who will suffer most in this incessant heat.

Some creatures clearly revel in the sun's rays; a female Blackbird in the garden basks on the warm slate path, her wing feathers spread and her body feathers fluffed up. Such basking helps to drive out feather parasites but comes at a price, the bird panting heavily in the heat and only able to remain in the open for short bouts before retreating to the relative cool of the summer borders. I have seen other birds indulge in this means of feather maintenance, including one of the local Robins, but others prefer the bird bath or make use of the now dusty soil to bathe. House Sparrows, in particular, seem rather fond of a dust bath.

Many of the insects seem to be doing well and Norfolk's moth traps positively bulge with hundreds of specimens caught on these sultry nights. This bodes well for the local bats, providing an abundance of prey, and presumably for the Nightjars nesting just a few miles away in the forest. Of course, such tinderbox conditions place all of the forest creatures at risk should a careless cigarette butt or piece of discarded glass bring about a forest fire. It is a time for us to be considerate, to keep our bird baths full (and clean) and to think of the consequences of lazy behaviour.

Lack of reeds bad news for warbler

A pair of chest waders affords me access to the reedbed and, entering into its rustling embrace, I feel that I am in another world. The crowded stems, mostly of last year’s growth, remove horizons and play tricks with the limited perception of distance. This is a world dominated by sound, not least the soft drone of summer insects and the harsh staccato songs of Reed and Sedge Warblers. While most of the smaller reedbeds which fringe these old gravel pits are lush with new growth, this older bed is rather sparse. No doubt the emergent willow growth has had something to do with this, shading the bed and reducing the amount of standing water. Management work to remove the willow will not bear fruit until next year.

Each step forward is careful and considered. Not only do I have to be aware of underwater obstacles but also there are delicate nests in here. Little Grebe, Water Rail and Moorhen will use these beds for nesting, hiding their eggs within covered mounds of vegetation. Reed Buntings will tuck a nest away within the rough tumble of last year’s stems, compacted down by the winter snowfall. Then there are the low nests of Sedge Warblers and the more refined, woven nests of Reed Warblers that I am here to find and monitor.

Some of the nests had only been partially constructed on my last visit but are now finished and contain eggs. Some of the earlier pairs already have young; small, black-skinned and naked, they huddle helplessly in the nest, raising heads and bright gapes upon my approach, thinking I might be an adult returning with food. Reed Warblers tether their nests to reed stems and rely on this year’s growth for a solid support. As these stems grow, so the nest is transported upwards. Sadly, the lack of new growth has led one pair to attach its nest to a mixture of live and dead stems, the nest tipping precariously as the new growth pushes up beyond the single dead stem that has been incorporated. The chicks might just make it though, so short is their time in the nest (between a week and a fortnight from hatching).

Of course, the Reed Warblers face many other dangers. One of the most evident of these over recent days has been the Cuckoo, a female of which has been seen and heard around the reedbed. In fact, one of the nests in a nearby reedbed already contains a Cuckoo egg, its fate firmly sealed. Over the coming days I hope to see these blind and helpless chicks transform into bright-eyed, well-feathered youngsters ready to leave the nest and prepare for the journey south.

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.