Common Terns at Rye Meads from graham white on Vimeo.
As regular readers will recall, the colonization of rafts designed
for terns by Black-headed Gulls is good news for gull fans but can be less
positive for the terns. After the first
colonization of rafts in the Lee Valley on the Herts/Essex border in 2008, Black-headed
Gulls had risen to a total of 306 pairs at 3 sites by 2015. Meanwhile, Common Terns had declined to a low
point of 58 pairs in the Valley in 2012 with a slight recovery since. Further analysis of individual colony total and
productivity in the Valley has helped clarify the situation.
There are four main colonies in the Valley: at Rye Meads, Amwell, 70
Acres lake in the Cheshunt gravel pit complex and at Walthamstow Reservoirs. The
combined populations peaked at 117 pairs in 2007 but then began a steady
decline to a low point of 58 pairs in 2012 (a very poor year generally due to
bad weather) before rising slightly to 67 pairs in 2015. In general terms, the populations at Rye
Meads and Walthamstow have declined, whilst those at Amwell and Cheshunt have
remained stable or increased. The
decline at Walthamstow has been particularly severe, with just four pairs in
2015 in contrast to 42 pairs in 2007.
This decline is likely to be due primarily to the poor condition of the
rafts. With no maintenance being
undertaken in recent years, the rafts are now covered in vegetation. In addition, increasing populations of
breeding Herring and Lesser Black-backed Gulls at Walthamstow are known to
predate the terns. There are no Black-headed Gulls nesting at Walthamstow. If Walthamstow is excluded from the total
counts, the total at the other three sites has shown a more moderate decline
from a peak of 75 pairs in 2007 to 63 in 2015. The majority of the population decline in the Lee Valley can be
attributed to the situation at Walthamstow Reservoirs.
So has the colonization of the Lee Valley rafts by
Black-headed Gulls had any impact on the Common Terns? Firstly, analysis of colony
data shows that the population and trend of Common Terns in the Lee Valley
overall is not dissimilar from the national situation. Also, there is no significant impact on the
terns productivity after the arrival of the gulls, and furthermore the
productivity of terns in the Valley remains higher than the national average.
There is some evidence that the terns actually
prefer to nest amongst the gulls, and certainly don’t avoid them, probably
gaining some protection from predators. However, the major impact appears to be
that with increasing numbers of gulls, there is simply less space on the rafts
for the terns, and this pressure is increasing each year. With Black-headed Gull nesting earlier,
much of the available space may be taken by the time the terns settle. Common
Terns appear to be nesting later in recent years with unfledged chicks
frequently remaining on the rafts long after the gulls have departed. The reasons for later nesting are unclear but
food supply may be suspected.
As the 2016 breeding season approached, we suggested that new rafts
were constructed at all the colonies.
Unfortunately this happened only at Rye Meads, where a new 6 x 6m raft was
constructed to create more space for breeding Common Terns. The existing rafts
were cleaned and positioned in March for the Black-headed Gulls to colonise. By contrast, the new raft was positioned very
late, in the last few days of May as Common Terns were beginning to settle.
The results? Black-headed
gulls increased in numbers at all 3 sites to reach a total of at least 466
pairs (from 306 in 2015). Common Terns slumped to their lowest Lee Valley count
for many years, with a total of just 57 pairs. However, at Rye Meads, numbers
increased to 28 pairs, the highest since 2008, with the new raft holding the
majority of pairs and fledging good numbers of young. Let’s get building some more rafts next year.
Below: rafts at Walthamstow are overgrown and falling apart, with just a few pairs of terns nesting.
