The remarkable increase of breeding
Black-headed Gulls in the Lee Valley has continued this year, nearly all of
them nesting on rafts provided for Common Terns. This year’s total amounted to 107 pairs
spread across four sites, with good numbers of young birds fledged. By
contrast, the Common Terns have had a very poor year. The total of 52 pairs is the lowest in the
valley for some 30 years. Not only that,
but productivity has been equally poor.
But are these two stories linked?
Terns in general appear to have had a
very mixed year, both between and within species. The poor summer, with a high proportion of
windy days preventing fishing and the downpours chilling eggs and chicks, has
been a major problem. In the valley,
many terns failed to settle until very late this year, with limited success. In general, productivity at inland colonies
is frequently better than the average. Common
Terns nesting on freshwater sites have been shown to make shorter feeding
forays than coastal birds, allowing for better colony protection as birds are away
from the nest less.
Although the Black-headed Gull is over
ten times as numerous as the Common Tern as a breeding bird, both species have shown
an interesting similarity in trend in distribution and abundance in recent
decades. Although there have been declines
in the north and west of the UK, increases have taken place in inland areas of
southern England, much linked to the spread of flooded gravel pits in river
valleys. In recent years, Black-headed
Gulls have spread rapidly along the Thames valleys and its tributaries, in many
cases nesting on rafts provided for Common Terns. The reasons for the declines in the north are
unclear but an increase in predation, particularly by Mink, and unfavourable
land-use changes have been suggested as the cause.
With the Black-heads increasing
rapidly in recent years, the general feeling is that the terns are being
squeezed out. Despite this, all the
local colonies had available space on the rafts. In recent years, the delaying of the
positioning of rafts has shown some limited benefits for the later nesting
terns. Perhaps the time has come to vary
the design of the standard tern raft to better accommodate both species as they
seem to have subtle differences in nest location. Around the world you can find examples of a
very different approach. Look at the 22,000
m2 floating island designed for Caspian Terns in Oregon, USA for
example (below). Some trials next summer at a
number of sites may begin to show the way.
