The William Girling Reservoir holds the largest wintering group of Black-necked Grebes in the UK. Yet this is not a bird-rich site. It is not especially favoured by diving duck, it is shunned by Shoveler, poo-pooed by Pochard and used principally as a dormitory by dabblers. It is a concrete doughnut holding water. But presumably the B-n G’s have some competitive edge here. Although there are occasional records from other sites in the Lee Valley, no other local site regularly holds wintering birds. The peak count on the Girling over the last ten years was of 32 birds in December 2007. Peak winter numbers have been around the mid-20s for the last three winters.
With 24 birds arriving on the reservoir in the first week of August last year, around this number of birds have remained all winter. Numbers are just beginning to drop as the first birds move off, 18 were still present today. Seven months on the Girling! What is there left to eat?
Black-necked Grebes are a specialised invertebrate feeder, principally seeking aquatic insects and their larvae by ‘gleaning’, although they may also take molluscs, crustaceans and small fish. They tend to favour shallower waters in the summer, moving to deeper, mainly coastal sites in winter. This concrete-edged basin with water depth of around 14 metres would seem to be at their edge of tolerance, with BWP stating diving depths of up to 5.5 metres for up to 50 seconds. Birds diving around the margins (where there are concrete ‘roads’ a metre or so down) stay under for 30-40 seconds, whilst those well away from the reservoir margins re-surfaced after 50-60 seconds. It would be fascinating to find out what they are feeding on. The other big question is where are they between February and August?
2 comments:
My impression is that the species here (Friuli Venezia Giulia, NE Italy) is in steep decline in winter and the large numbers ('000s) observed in the 1990s are a thing of the past with wintering numbers perhaps now only in the hundreds. The birds winter in shallow muddy brackish coastal waters. That said it does seem to be increasing inland as a breeding bird in Italy here and there and I wonder if the spread of alien species of Crayfish is anything to do with this? They certainly represent a major change in the fauna of lakes and wetlands in Europe. The juvenile forms of Procambarus clarkii and Pacifastacus leniusculus would seem to represent ideal food and are often present at huge densities. Are there ay Signal Crayfish (Pacifastacus leniusculus) in the Girling? Are they eaten by the birds at the Herts. breeding colony?
The Herts site was formerly riddled with Turkish Crayfish, and there was some debate about whether they formed a food source. However, a recent survey found no sign of them - they appear to have died out. At the same time B-n Gs have become more productive. Perhaps the crayfish were actually a food competitor.
Having said that, it is interesting how many species in some areas are now eating large numbers of these non-native crayfish eg Otters, Bitterns.
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