Sunday, 26 June 2011

Black-head squeeze, terns sore?

Tern rafts - 1) posh with street lamps and added BHG's, 2) double-decker accommodation at Walthamstow.

In the Lee Valley and elsewhere, a remarkable colonisation by breeding Black-headed Gulls has taken place in recent years, the birds squeezing onto rafts put out for Common Terns. This year at least 63 pairs are nesting at four sites in the valley. With the totals for the previous three years being 23, 9 and 1, it is clear Black-heads are moving in. Rafts at Walthamstow Reservoirs and Amwell have been colonised this year, adding to existing colonies at Rye Meads and Cheshunt.

Although the number of terns nesting in the valley has remained roughly steady at 70-90 pairs (with at least 81 pairs this year), there have been changes at individual sites. This is most notable at Rye Meads, formerly the largest colony in the valley, where a rise to around 40 pairs of Black-heads has roughly coincided with a drop in tern numbers, down to 11 pairs this year.

This has prompted much concern. Will the terns be ousted as the gulls increase? Are the two trends even related? And why have the gulls colonised now, after the terns have enjoyed up to 40 years of successful raft nesting?

Interactions between Black-headed Gulls and terns are well known; they may compete for space, the gulls may directly predate tern eggs and small chicks, and the gulls are also known as kleptoparasites – stealing food as the terns bring it in for the chicks. However, despite this, the two species regularly nest together; the terns may gain added security from predators from being within the gull colony. Watching the valley colonies, there are clearly a lot of noisy goings-on and squabbles. Terns with fish are chased, but mainly get through. Overall, both species appear to be having a very productive year with plenty of chicks on the rafts and the first young terns taking to the air. In excess of 100 young Black-heads are on the rafts.

What can be done, if anything, and does it even need doing? Black-headed Gulls themselves are declining at a number of coastal sites where even larger gulls predate them.  The techniques employed so far have been to delay the positioning, or to cover, some rafts until just before the terns settle (as Black-heads nest earlier). However, where terns frequently like to nest en-masse, the gulls seem to like to spread out amongst the rafts. Another solution may be to simply provide extra rafts to accommodate both species.

At Cheshunt recently, I spotted another ecological difference between the species, although it may not be statistically significant. I watched the terns bringing in a succession of small fish for the chicks. The only gull I saw regularly coming in with food appeared to have......(I joke not).....chips!


David, Scotland said...

I think the missing link in this reserve is Canis lupus but the reserve is too small. If there were wolves the behaviour of the horses, deer and cattle would change. The Durch ecologist I spoke to about this agreed, he also said there should be Elk in there as well.

Grumpy Ecologist said...

(I think this comment refers to 'what the Heck' - the Oostvaardersplassen.)

I certainly agree with your first sentence but as you say, even at 60 sq km, the reseve is too small. So they have to fiddle with it and reduce the numbers of herbivores - but there are strong differences in opinion on this.