A slow start to spring

Despite the tantalizing period of exceptionally early spring warmth, it has been something of a slow start to the season. There have been some early nests, with young Robins, Song Thrushes and Blackbirds evident in many gardens across the county, but other individuals were held up by the returning cold conditions. Now, however, it feels as if the pace is quickening.

We have seen a succession of arrivals, as returning summer migrants have been quick to settle on favoured breeding territories and deliver song. The Chiffchaffs were the first of these to be noticed here, around the brecks, followed by Woodlark, Blackcap, Willow Warbler and, most recently, Whitethroat. Early Reed and Sedge Warblers appeared late-month, with numbers of these reed- and sedge-bed songsters now increasing. The hirundines (swallows and martins) were slow to return in numbers and my first sightings of these summer visitors were behind those of recent years.

We have also had a smattering of passage migrants, in particular a good arrival of Ring Ouzels. These striking thrushes resemble a large Blackbird but with beautiful sculptured plumage markings and, in adult males, a white crescent around the throat. We have also seen some less common visitors in the shape of at least two Hoopoes, continental breeders that have overshot their breeding grounds off the back of southerly winds. They won’t linger, however, and will soon reorientate to return south. One of the most fascinating aspects of the unfolding migration has been the return of the Cuckoos, fitted with satellite tags by the BTO. Regular updates on their progress continue to appear on the BTO website (www.bto.org/cuckoos). The five tagged last summer, were all captured in East Anglia and it will be interesting to see if they return to the same sites.

The pattern of activity seen in our birds has been mirrored in the county’s butterflies, with a good number of early reports of species that overwinter in the adult form, followed by several lean weeks in which few sightings were forthcoming. Other species are now being reported on the wing, including Orange Tip and Holly Blue, so it does feel as if the butterfly season is now properly underway. Other insects have been taking advantage of early season nectar, with good numbers of bumblebees foraging in my garden and quite a number of hoverflies as well. The changeable weather that we see at this time of the year does deliver something of a stop-start feel and while it can appear that spring is here to stay one morning, the next can suggest a return to winter. It is a great time of the year to be out and about though as each new sighting delivers a thrill.

Something of a mixed year for butterflies

The results generated by the various recording schemes for butterflies have started to appear over the last few weeks. Looking across these it is possible to establish an idea of how 2010 shaped up and it seems that many species benefited from the hot June and July weather, which followed on from a cold winter.

As I noted in one of last summer’s columns, the summer heat helped species like White Admiral, which were able to shorten the time spent as a mature caterpillar and chrysalis and so reduce the levels of predation, a key factor to determining the size of the summer generation. Other butterflies, notably some of our less common species, also did well, with the Silver-washed Fritillary delivering the big success story for East Anglia. It appeared across the region in good numbers, re-establishing itself at some sites after a long gap.

Many of our familiar species did less well, especially immigrants like Red Admiral and Painted Lady, numbers of which were both very much down on the 2009 influx that grabbed the headlines. Such short-term fluctuations are not cause for concern. Some species exhibit regular changes in abundance that are shaped by parasite populations. The Holly Blue, for example, has populations that cycle depending on how common a particular small parasitic wasp happens to be. When the Holly Blue enjoys a good year, this allows the parasite to do well, meaning that the levels of parasitism will be greater the following year; this, in turn, brings the Holly Blue population back down.

Being able to see longer-term trends is a key component for any attempts to conserve our butterfly populations. This is where systematic recording schemes, like the Wider Countryside Butterfly Survey, carried out by volunteers from Butterfly Conservation and the BTO (many of the BTO’s bird recorders revisit their survey squares to record butterflies for this survey). Less common species are monitored by coordinated counts at key sites, as is the case with the Silver-studded Blue monitoring that takes place on some of the Norfolk heaths. Volunteers provide a key role in this work, working in collaboration with researchers to keep tabs on these changing populations.

Most butterflies are fairly straightforward to identify, meaning that (with a little training) you can make a valuable contribution to the work of organisations like Butterfly Conservation. If you would like to get involved in some of the butterfly recording work taking place across the county, or simply want to find out more about Norfolk’s butterflies, then why not contact the Norfolk branch of Butterfly Conservation. Visit www.norfolk-butterflies.org.uk or contact Chris Dawson on 01603 545092 if you would be interested in helping with their Wider Countryside Butterfly Survey.

Wasp regulates butterfly population

One of the joys delivered by the arrival of spring is the opportunity to sit out in the garden and watch wildlife closer to home. One of the creatures to make an early appearance in the garden this year has been the holly blue butterfly. Over recent days several individuals have been seen together flitting around the holly tree and over the old boundary wall, covered in a mass of ancient ivy. This small butterfly is the only blue on the wing at this time of the year and it is also the one most likely to be encountered in gardens. Pastel blue in colour, it can be separated from the common blue (which may visit more rural gardens later in the year) by the lack of any orange markings on the underside of its wings. The holly blue also differs in its behaviour; while other blues fly low over the ground, the holly blue flies high around bushes and shrubs.

In spring, the butterfly lays its eggs on holly, the caterpillars timing their emergence to coincide with the availability of tender new buds. Later in the year a second generation (and in some years also a third) will be reared on ivy. The number of generations each year is determined by temperature; long, warm summers will allow a third brood in the south of the country but only a second brood in the north of England, where a single brood is more typical.

The larvae, feeding on flower buds, berries and terminal leaves can be found with a little bit of effort, as can signs of the damage caused by their feeding behaviour. At this stage, they are vulnerable to a parasitic wasp called Listrodomus nycthemerus. This wasp almost exclusively attacks the holly blue and, in some years, can inflict substantial damage on the population. The female wasp deposits an egg into the holly blue caterpillar, without killing it. The egg remains within the body, unhatched, until the butterfly enters its pupal stage. At this point, the wasp egg hatches and the emerging larva proceeds to eat the entire contents of the pupal case and, by doing so, kills the butterfly, before completing its own life cycle. The occurrence of such parasitization is thought to be behind the strong cyclical fluctuations seen in holly blue populations. As the butterfly becomes more abundant, so wasp numbers build up to the extent that they eventually cause the butterfly to decline in abundance; a wasp decline follows and the cycle starts over again. These fluctuations, and the long-term increase in this species, are monitored by Butterfly Conservation. For more information on their work, contact the Norfolk Branch through Hilary Beynon 01493-740499.