Tuesday, 16 August 2016

A tern around at Rye Meads

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.