The grounded Swift

A phone call from home brought news of a grounded Swift. ‘Catch it and pop it in a bird bag somewhere cool and quiet’ I said, pondering that it was too early in the season for this to be a young bird that had left the nest prematurely. Sure enough, upon arrival home, an inspection of the bird revealed an adult, seemingly bright, alert and lacking any obvious injury. It could have collided with something, perhaps even another Swift (something which has been documented upon occasion) but a grounded adult would normally be able to take to the air. The notion that a grounded Swift cannot take to the air on its own comes from the fact that the majority of grounded Swifts are young birds, not sufficiently developed or too weak to fly.

I gave the bird the opportunity to fly, releasing it from an upstairs window but all it could do was glide down to the ground below in a soft arc, catching itself on a bush where it clung with its sharp claws to a leaf stalk. Further investigation revealed that the bird was significantly underweight and I wondered if it had got caught in a sudden downpour earlier in the week. If so, bedraggled it may have been grounded for sometime, becoming weaker to the extent that it was now unable to fly.

I do not make a habit of rehabilitating birds but this one seemed sufficiently bright enough to suggest that it might have a fighting chance. So now I find myself catching flies in the garden. Dutifully dispatched, the flies are fed to the Swift, which sits wrapped in cloth, and bill firmly shut. By gentle manipulation I can open the bill and present each fly to an eager palette. Each one is taken down with a visible gulp, the pale throat undulating and the eyes blinking, but the Swift seemingly otherwise nonplussed by my tender efforts. It is going to take a good number of flies, and many patient feeds if this bird is to recover to a point where it can take to the air again.

I suspect that this is a first year bird, the shape of the primary (main flight) feathers suggesting as much. This means that it is unlikely to be part of a breeding pair, since Swifts do not normally breed in their first year. This is something of a relief, since it will not have chicks dependent upon it for food. It’s too early to say how it will fare, even if I can get it back to a decent weight it might not be able to fly, but it is certainly worth a go.

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.

Nightjar

Regular readers of this column may have seen my piece earlier in the week which mentioned our efforts to catch Nightjars in Thetford Forest. This work, which involves fitting tiny radio-transmitters to some of the birds we catch, is being carried out to support a PhD student who is looking at Nightjar movements and breeding ecology. Small teams of trained and licensed bird ringers are out several times a week at the moment (depending upon the weather) and we have been fortunate in being able to target and capture a small number of birds within our defined study areas. Our success rate has been such that there have been very few nights when we have failed to catch a bird.

I have been particularly fortunate to have seen a number of birds in the hand and to have caught a breeding female in one of my nets. This particular bird was tracked a few days later, through her radio transmitter, to a nest site just a few hundred metres from where I had caught her. She was found to be incubating two eggs (Nightjars typically lay from one to three eggs) and we will be able to follow her progress as the season continues. She might go on to make a second breeding attempt before departing south towards her African wintering grounds. Nightjar nests are placed on the ground, within an area that is bare or sparsely vegetated, and the female relies on her cryptic colouration (a mottle of greys and browns) to remain hidden from potential predators.

The Nightjar is found across much of lowland Britain but only where suitable breeding and feeding habitats exist, so it tends to be localised within this wider range. Thetford Forest is something of a stronghold for the species, with the birds making use of areas of clearfell within the larger blocks of coniferous woodland. The reliance on areas of clearfell and young plantation woodland means that the size of the breeding population within Thetford Forest is very much dependent upon the area of suitable habitat available. As the newly established plantations mature they become increasingly unsuitable for the Nightjars. Fortunately, the Forestry Commission now manages the Forest with this in mind, maintaining a good area of sufficiently young blocks to provide enough breeding habitat for the Nightjars and other early successional birds like Woodlark. Work already carried out within Thetford Forest suggests that the Nightjars forage for food within a kilometre or so of the nest, so suitable nesting and feeding habitats need to be located close together. The mosaic of forest blocks provides the mix of habitats required by the Nightjars and, with careful management, their future within the forest looks secure.

A glow in the forest

A soft green glow caught my eye the other night, a tiny patch of light in the grass which I recognised as that produced by a female Glow-worm. Soon after, I found other tiny lights, just a handful but a welcome sight nonetheless. Shinning my head torch toward each glow in turn revealed the plump outline of a female Glow-worm, sometimes alone, sometimes accompanied by one or more males. I encounter these insect beacons less frequently these days, a reflection of their general decline within the wider countryside. Gone, too, are the large gatherings of dozens of females grouped together in the vegetation, collectively giving the appearance of a miniature townscape by night.

The light is used by the wingless females to attract a mate, the males being fully winged and sporting the hard wing cases so typical of beetles.  For these are beetles and not, as the name might suggest, worms. In North America they are often referred to as ‘fire-flies’, another misnomer. Globally, there are many different species of glow-worm, although we only have two (possibly three) species in Britain and only one of these is widespread in its distribution and likely to be encountered.

The Glow-worm’s light is the result of a string of controlled chemical reactions which take place within special light organs located in the female’s abdomen. The two most important compounds involved are luciferin and luciferase, the latter acting as a catalyst by bringing together other molecules and holding them in place until a reaction takes place. The reactive process is extremely efficient, wasting less than two per cent of the energy used; compare this with one of the standard light bulbs, now being replaced by low-energy equivalents, which waste 96 per cent of the energy used. A similar process occurs in certain jellyfish, bacteria and even fungi. The light that is produced is a yellow-green colour, which is handy for the human observer as it falls within that part of the light spectrum to which the human eye is most sensitive.

Each species of glow-worm emits its own distinctly-coloured light, effectively a ‘call sign’, which prevents males from approaching a female of a different species. However, a North American glow-worm called Photuris versicolor has been discovered to impersonate the ‘call signs’ of several related species. Males of these other species, fooled into visiting a female Photuris versicolor, may find themselves on the menu!

The almost magical light produced by these insects has attracted our attention for many hundreds of years. In fact, references to glow-worms appear in literature extending back over two thousand years, with, for example, a mention in an ancient Chinese encyclopaedia produced by a pupil of Confucius.

The return of the Buzzard

I feel especially privileged today, having recently visited a Buzzard’s nest on the edge of the forest. Although the Buzzard is a common and widespread bird nationally, it is one that has, until relatively recently, been decidedly uncommon within Norfolk. Buzzard populations have changed dramatically over the last two hundred years. The species was common throughout most of mainland Britain during the early 1800s but from the second half of that century there was a pronounced decline in numbers. The decline was linked to the levels of persecution associated with game rearing interests, the Buzzard all but exterminated from large parts of its former breeding range.

Relief for the Buzzard came following the outbreak of the First World War, as keepers were called to fight for their country. Although the level of keepering continued to fall after the war had ended, new threats appeared. During the 1950s and 1960s Buzzards, like certain other top predators, suffered from the effects of organochlorine pesticides that had found their way into the food chain. Productivity declined, as nesting attempts failed through eggshell thinning and subsequent breakage, and the population again slipped away. Another problem was the arrival of myxomatosis and the rapid and pronounced decline in Rabbit numbers, the Rabbit being a favoured prey species for the Buzzard.

It is only over the last couple of decades that things have improved and we have seen a tremendous eastwards expansion of Buzzard populations, with birds moving out from strongholds in the west of the country. The first modern breeding record for Norfolk did not come until 1992, when a pair in central Norfolk raised two young. The year that I arrived in Norfolk saw just two confirmed breeding attempts and over the time that I have lived here I have seen more and more Buzzards within the county. Over the last few years, they have been regular around Thetford and this spring, for the first time, I have seen one regularly where I walk my dogs in the forest. The most recent Norfolk Bird Report (that for 2007) suggests a population of 30-40 breeding pairs within the county and this will almost certainly increase over time.

Work carried out on populations colonising other parts of the country would suggest that we could achieve a much larger breeding population, particularly as we appear to have a mixture of habitats suitable for hunting and breeding. The light, sandy soils of the Brecks, for instance, support good numbers of Rabbits and there are many small woodlots and blocks of plantation woodland to provide nesting opportunities. So, while I will always feel privileged to see nestling Buzzards at close quarters, I feel especially privileged today to have seen my first ‘local’ Buzzard chick.

Transition

It is still light when we arrive, the brightness of a summer’s day slowly slipping into the softer tones of evening. We are in the forest to catch and radio-tag Nightjars for a PhD student studying their ecology. Having chosen suitable spots, we quickly set our fine mesh nets and retire to a nearby forest ride to await the arrival of night and the stirring of the Nightjars. There is time now for patient watching and hushed conversation.

The background drone of flies dissipates and the final bird song slips towards silence, a solitary Song Thrush a last bastion of the daytime brigade. The time for the ‘night watch’ approaches and the first of the evening’s bats appears; a Noctule, an early riser that can sometimes be seen feeding alongside swifts, hawking the air for flying insects. The bat makes several passes up and down the ride and I curse myself for having left the bat detector at home; it would have been good to ‘listen’ to the Noctule as well as watch it pass.

The colour begins to leach out of the vegetation as the daylight fades to replace greens and browns with silvers and greys. A short squeaking note, much like a child’s toy, catches our attention and we glance up to see the dumpy form of a Woodcock silhouetted overhead. The forest seems to support fair numbers of these rather unusual waders – a woodland bird that probes damp soil for earthworms and other invertebrates. Then we hear it, a soft mechanical churring call – a Nightjar, distant but welcome. Soon it is joined by other birds, two close by and in the area of our nets – this bodes well. The closest of the birds is in flight, the churring interspersed with wing claps and calls, moving around off to our right. Then, it is caught, silhouetted against the sky, its effortless buoyant flight taking it in an arc around us and up into the top of a nearby oak. The bird can be seen perched on its song-post, advertising its presence to other birds.

These first excursions provide breathtaking views and we can stand and enjoy them, knowing that it will be another hour before we can turn on the tape lures that should attract the birds into our nets. Being nocturnal, the Nightjar has excellent visual acuity and would be able to avoid the nets if we tried to operate them in this half-light. A sheet of cloud has formed and by 10.30 it is dark enough to try our luck. Each of us manning a single net, tape lure running, we crouch nearby watching and listening, hopeful that we might catch one of these truly amazing birds.