Tuesday, 5 November 2013

Tern around

Previous blogs have discussed the remarkable increase of breeding Black-headed Gulls on rafts in the Lee Valley and their interactions with the Common Terns the rafts were intended for.  There is a widespread feeling that the terns are being squeezed out by the Black-heads.  Last years total of 56 pairs of Common Terns was the lowest in the valley for some 30 years and productivity was poor.  However, this summer has seen a more encouraging picture with an increase to 72 pairs spread across four sites and with a minimum of 77 young birds fledged.   The Black-heads continued their rapid increase; from 107 to 180 pairs. The graph below shows Lee Valley nesting Common Terns in blue, Black-heads in red.


This year, a few new c-rafty ideas were tried out.  One theory has it that the gulls prefer nesting against an edge, whereas terns prefer the open spaces in the middle of the raft.   At Rye Meads, 4 normal sized 3 x 3m rafts with wire-framed side panels were locked together and put out with a new giant 6 x 6m raft with open edges sloping into the water (see pics above).  We sat back and waited.  The gulls arrived and occupied all the corners and edges of the smaller rafts, with only the last few birds settling on the large raft, and most of these nesting against the chick shelters provided for the terns. Somewhat surprisingly, the first terns settled in the middle of the gulls on the smaller rafts, with a few on the new raft.  On a group of rafts on another site, a similar pattern was noted, with the terns settling amongst gulls despite considerable space being available.  Are the terns nesting amongst gulls for protection? And balancing this against any hassle they may get from the gulls?

So the jury is still out on whether Black-heads are squeezing out terns, as long as there is enough space for both.   Try larger rafts, with Black-heads around the edges and terns in the central space.  Put at least one raft out later, just as the terns are arriving, so at least some space is available for them. Then sit back and enjoy both gulls and terns.

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