Wednesday, 31 January 2018

Tented colonies

The homes of the Winter Webworm Ocnogyna beatica (Martin Kelsey)

I am standing beside pasture in late winter, facing the low sun, and watching the light refracting from silvery patches that freckle the field. They glow like medallions on the green baize of the meadow. I approach one and bend down to take a closer look. The structure is a canopy, closely woven, holding tiny globules of dew which collectively provide the silver sheen on the web. But I am more curious of its inhabitants, for below this tent squirm several hundred tiny caterpillars. They are the Winter Webworm, the larva of a tiger moth Ocnogyna beatica, a species of the western Mediterranean basin.

The Winter Webworm Ocnogyna beatica colony (Martin Kelsey)

Hatching in the middle of winter, they spend the first few weeks of their lives in these tented colonies. Their diet is catholic, consuming the winter greens of fresh foliage of clovers, mallows, vetches and mustards, and probably pretty much anything else growing around them. It strikes me that these veiled tents under which they are domiciled not only protect them from hoar frosts, but must also act like cloches for the food plants around the larvae. In a dry winter like this one, dew formation is a crucial source of moisture and perhaps the structures that the caterpillars have made create a favourable micro climate for the plants they feed on.

Winter Webworm larva on a sand crocus in early March (Martin Kelsey)

Once they are two or three centimetres long, come late February, the caterpillers evacuate their tents and roam widely. In places they become super-abundant, masses crossing tracks and roads, all heading in one direction, synchronised around a mysterious cue. By late spring they enter their pupa stage, a chrysalis that lies deep in the ground (10 to 20 centimetres) far from the severe dessicating drought of summer. This is the longest stage of  their annual cycle.

With the autumnal rains the adults emerge, the male with attractive variegated black and white wings and the wingless female, looking like a miniscule yak - plump with shaggy auburn hair. She will lay clusters of eggs and her progeny just three months further on will dapple the pastures with their minature greenhouses.

Sunday, 21 January 2018

Returning north

Migrating Greylag Geese at dawn on 20th January (Martin Kelsey)

I have seen a couple of Barn Swallows already this month, and they are most likely to be early returning migrants rather than overwintering birds, but it is still mid-January and, as the proverb says one (or even two) swallows do not make a summer. However, one clue that happened yesterday as I was hanging out the washing was an incontrovertible signal that the wheels of seasonal change are starting to roll - and it summoned a tinge of sadness within me.

The sun had yet to break the horizon of the hills to our east, but the House Sparrows were stirring with their chirruping waking conversations. Above this sound came another. It too was conversational but more strident and urgent. It was the honking of geese. Then from the south, and crossing my field of vision, was the skein. This was no short-distance foraging foray. These birds were flying high and with a purpose. They were returning north, Sightings of geese over our garden are exclusively birds on migration: in October and November heading south and then late January and February heading north. There are wintering populations just 25 km away as the goose flies, but nowhere closer to home are there any favoured feeding areas.

These geese were Greylags. This is a species which arouses little interest for visiting birders from Britain, for whom this species is associated with large,boisterous feral flocks. But these Greylags are truly wild birds, starting a journey back to their breeding grounds in Scandinavia. For me, our wintering Greylag Geese provoke a sense of wonder and respect, as do the Cranes, in their highly visual and audible migration. When I see them like this, I can only marvel at the drive, part innate and part learnt, that triggers the timing and direction of their journey.

In the centre of Extremadura, the Greylags feed mainly on rice stubble fields, alongside the cranes. But they select just a few particular zones, perhaps because there they are close to water bodies where they will roost. One of my favourite winter pastimes is spending the last hour of daylight at a spot where I can look across one of these sites. The low sun behind me provides wonderful viewing conditions across an expanse of rice stubble. Marsh and Hen Harriers will be out, causing flurries of Spanish Sparrows and Corn Buntings to erupt from amongst the dead stems. Common Cranes in family groups daintly pick at the debris on the ground. But it is the geese I focus on, aware that with each minute that passes, I have less and less day light to work with. It is about patience, lots of it. I am searching the flock for any unusual geese that may have migrated south along with the Greylags. As anyone who has watched feeding geese will know, most of the flock will have their heads down feeding, others will be asleep, leaving just a few with their heads held high in vigilence.  The problem is that when searching for unusual species, it is the bill, head and neck that carry the easiest to spot differences. In the tall rice stubble, the situation is even more complicated. Sometimes just the top of the back of a goose will be visible, and sometimes it will disappear completely as it moves into a runnel. And as good as my vantage point is, some of the flock will simply be too far away to see well.
Wintering Greylag Geese in rice stubble (Martin Kelsey)

On almost every visit, therefore, I content myself in knowing that  I have at least tried and that, despite the frustrations, it was a truly wonderful way to spend a late winter's afternoon. Twice though this winter, the effort did pay off. Once with finding two White-fronted Geese and then a month later, discovering a much rarer Pink-footed Goose in the same place.
White-fronted Goose (closest bird) with Greylag Geese in rice stubble (Martin Kelsey)

Seeing the migrating geese yesterday, prompted me to return to this viewpoint in the afternoon. I had passed by there just two days earlier and several hundred geese were present, but I could not pause for long there because  I was busy counting cranes, and besides the light was too bright to scan the geese carefully. Back there yesterday, the message conveyed by the migrating geese was all too clearly confirmed. Just three Greylag Geese remained. And so my afternoon goose watching season had ended for another year.