Saturday, 24 July 2010

White admirals seek out shade

It seems as if the White Admiral butterflies have had a good season across their British range. The hot June weather appears to benefit this delicate woodland species, shortening the time that it spends as a mature caterpillar and chrysalis. This is important, because both of these life cycle stages are prone to predation by hungry birds and the less time they spend going through these phases the more likely they are to reach adulthood and the all-important chance to reproduce.

White Admiral, by Mike Toms

Although easily overlooked, this is one of my favourite butterflies; a truly elegant species when seen in flight and a denizen of shaded mature woodland intersected by warm, sunlit rides. Adult White Admirals spend a lot of time in the canopy, where they bask in the sun and dine on honeydew produced by aphids. Early in the day good numbers may descend to the woodland floor, stopping to nectar on bramble flowers or to sip at water from puddles or the dissolved salts available in animal dung. Dark brown in colour, they are a good size and sport white wing markings that seem to dance through the air as the butterfly moves quickly on short glides and fluttering flight. Many writers have commented on their manoeuvrability and this is certainly evident when you watch the female White Admirals laying their eggs.

Egg-laying takes place on spindly honeysuckle plants, non-flowering and typically located within shade. Each female lays her eggs quickly, curving her abdomen down to deposit a single egg onto the larval food plant. The eggs, which are tiny and best viewed through a hand lens, resemble miniature sea urchins, with their erect bristles and honeycomb exterior. Although the eggs can be tricky to find, the caterpillars are less of a challenge. Each caterpillar will strip back a leaf of the foodplant from each edge, leaving the central rib in place and projecting forward. It is on this rib that the young caterpillar will rest, settling on a mat of faecal pellets and held together with silk. The caterpillars feed up throughout the summer before constructing a shelter for the winter. Again, this uses the leaf of the food plant, trimmed to size and then sewn in place with silk. Here the caterpillar will remain through until the following spring, at which point it will emerge to continue feeding. It is then that the chrysalis is formed; a green, black and silver-jewelled construction that reminds me of a bat, complete with long dark ‘ears’.

Locally, in those woods that support them, the White Admirals are on the wing from mid-June, typically peaking in the third week of July. They are well worth a visit if you can track them down.

Friday, 23 July 2010

Woodpigeons start nesting

Over the last few days I have noticed a growing and untidy pile of sticks on the roof of the chicken run. These are arranged in such a way as to suggest they have not come to rest here naturally. In fact, they are evidence that the Woodpigeons have initiated another nesting attempt in the Holly tree under which the chicken run is placed. Looking up into the tree’s branches, the scruffy excuse for a nest can be plainly seen, complete with a broad, square ended tail poking out behind.

I always feel a little disappointed by the Woodpigeon’s clear deficiencies in the building department. Not only does the nest seem woefully inadequate, full of gaps through which both sky and egg may be seen, but there is a sense of laziness that comes from the pile of wasted sticks below the nest. I cannot comprehend why the pigeon fails to make use of these. If a stick does not hold its position during building, but falls to the ground below, why does the pigeon not pick it up and try again? Surely, it must be less demanding to pick up the stick instead of going elsewhere to find a replacement.

While other birds build their nests and rear a family with industrious commitment, the pigeon’s approach seems lackadaisical, half-hearted and, to be brutally honest, amateurish. They lay just two eggs and it is not unusual to find one or both smashed on the ground beneath the nest, especially on days when there is a bit of a breeze. The females do, however, sit fairly tightly once they have begun incubation, patiently passing the 16­ or so days before the eggs come to hatch.

Young pigeons (which are known as squabs) are decidedly ugly, especially when newly hatched, but they grow rapidly and soon acquire a covering of body feathers. The squabs are reared on a diet of ‘milk’ (more properly known as ‘crop milk’), which the parent regurgitates from special cells in its crop. A hungry chick will force its head into its parent’s open bill to receive a mixture of water, fats and proteins not dissimilar to mammalian milk. For the first few days this is all that the young chicks receive but as they get older other foods are introduced into their diet. The production of crop milk by birds appears restricted to all pigeons and doves, and at least some penguins and flamingos.

Woodpigeons will have been nesting since very early in the year but late summer is very much the peak in their nesting activity, as pairs attempt first or second broods in the thick deciduous cover that is now much in evidence.

Thursday, 22 July 2010

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.

Wednesday, 21 July 2010

Cricket anyone?

There is a steady and penetrating buzzing sound that provides the soundscape to these lazy days of high summer, a sound that is not so very different from that which I have sometimes heard from electricity pylons during damp weather. This, however, is not a man-made sound but the song of an insect. The insect in question is Roesel's Bush-cricket, a large and distinctive species that was first recorded in Norfolk in 1997. These magnificent ‘wee beasties’ have done well in the Brecks now that they have become established here, part of a range expansion that has seen them spread across much of southern Britain in a short space of time.

Experts believe that this bush-cricket is a relatively late post-glacial colonist, with a long-standing population established within the Thames Basin, and with outlier populations in Wales and Ireland thought to be relicts of an earlier invasion that took place before the establishment of the 'wild wood' cover that dominated these islands for many hundreds of years. Even as recently as 1988, the range of this species within Britain & Ireland was still very much restricted to these geographically separated sites. Then it all changed, with a run of ideal summer conditions providing the impetus for a movement north and west.

I recall their arrival in the Brecks and discussions about just how far north they had spread across the county. These days there are few bits of rank grassland around here that do not buzz to their song during these hot summer months. Not everyone can hear the song, as the ability to detect these higher frequency noises tends to be lost as we get older. A friend of mine lost his ability to hear their call a couple of years ago and now relies on either his bat detector or the ears of his two young daughters to pick them up. Even if you can hear the song, they can be difficult to pin-point and see well. The males quite often deliver their song from towards the top of a grass stem but the song has something of a ventriloquistic quality and it can be difficult to home in on the bush-cricket itself. I find that cupping my ears and then rotating my head slowly from side to side, while facing in the general direction of the sound, usually helps. Once you have spotted your bush-cricket you need to approach with care as the males have a habit of dropping down into the vegetation if disturbed. A good view will reveal the diagnostic prominent cream-coloured line that runs around the margin of the pronotum (effectively the middle section of the body) and a sturdy and surprisingly large insect.

Tuesday, 20 July 2010

Woodcock displays parenting skills

There are some sights in the natural world that really do have to be seen to be believed. The other day, a wildlife photographer friend of mine told me of a recent encounter with a Woodcock. He had been out in one of his local woods searching for butterflies when he disturbed the bird. The Woodcock took flight and he was immediately struck by the fact that it was carrying something between its legs. As the bird dropped towards cover and released what it had been carrying, he could clearly see that it was a young chick. This was a behaviour that he had read about but, like me, never quite believed possible. Searching the ground from where the bird had taken off he soon found another chick, the parent obviously only able to carry one of her precious young at a time.

The Woodcock is a curious bird; a wader that is solitary in habits and which nests in a most unwader-like manner, selecting open woodland with some ground cover. These are familiar birds locally, with many pairs making use of the plantation woodland which grows on the sandy Breckland soils. Here they may be encountered at dusk during the early part of the breeding season, as the males follow regular circuits through the wood along which they indulge in a slow and distinctive display flight. The flight, known as ‘roding’ appears somewhere between that of an owl and an oversized bat.

Nesting Woodcock are extremely well camouflaged and not often encountered, the nest placed on the ground and often within a few feet of a tree, which provides some degree of shelter. The young are not born naked and helpless like songbirds but are covered with down and are well-advanced when they emerge from the egg. This means that they can soon leave the nest, following their mother and learning the skills needed to survive on their own. The few reports of Woodcock carrying young suggest that these birds will pick up chicks between their legs to move them away from an area that the parent considers unsafe. There are also one or two reports to suggest that a parent might also use this approach to carry chicks over a substantial obstacle, perhaps one blocking their route at ground level.

This does go to show that there is always something new to see when you spend time outside watching wildlife. If you put in the hours and take the trouble to really immerse yourself in the natural world then you may occasionally be rewarded by something truly amazing. It might not be a Woodcock carrying a chick but it could easily be something that is witnessed only rarely.

Monday, 19 July 2010

Willow Warblers struggle for success

It has been a tough few weeks for some of the Willow Warblers nesting on the local nature reserve. These diminutive birds are summer visitors to our shores, familiar to most birdwatchers for their soft descending verse and rather plain plumage. Breeding across a range of scrubby and woodland habitats, they are one of the northern summer’s commonest breeding birds. The delicate appearance of this ‘leaf’ warbler (as this and other closely related species are collectively known) is matched by a delicately formed nest, placed at ground level and usually well hidden from prying eyes.

Finding Willow Warbler nests can be tricky, especially early in the season if you have missed them actively building their domed construction of moss, grass and bracken. We first locate territory holding males and then spend time watching to see if we can locate the female, who sometimes gives away her presence when off the nest by a characteristic call note. If this fails to reveal the female or her nest then we will very carefully search through those areas which seem to hold suitable ground cover for nest placement. Because the nest is so well camouflaged, every step has to be considered and checked before a footfall is placed; each tuft of grass or tangle of bracken has to be gently tapped and then inspected. It is slow and steady work but important nonetheless, as we strive to collect information on breeding success (such as clutch size, brood size and number of chicks fledged) for the BTO’s Nest Record Scheme. In a typical year fewer than 150 Willow Warbler nests are found and visited nationwide, yet still a sufficient sample for the BTO to produce national figures in support of important conservation work.

Some of our birds have struggled this year, though quite possibly no more so than in a typical year, with several falling victim to nest predators like crows, dogs and, we rather suspect, snakes. Some of the nests have been located close to old gravel pits, in areas where we often encounter basking Grass Snakes and these reptiles could easily polish off a nest of young warblers. When we find a predated nest we look to see if and how the nest itself has been damaged. Because Willow Warblers make a domed construction, with a small entrance hole in the side, most predators will inflict a certain amount of damage as they try to extract the nestlings or eggs from within. Nests that appear undamaged may have fallen victim to snakes or Weasels, both of which are small enough to use the entrance hole provided by the birds. It would certainly be interesting to deploy cameras to monitor some of these nests next year.