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.

Holl-utopia

A new wetland makes its debut this week. Back in July, work started on a new coastal lagoon in Suffolk, in the style of a newly created site we had seen in The Netherlands.   Utopia on the island of Texel came to Hollesley in Suffolk and we now have Holl-utopia.  After the moving around of 16,000 m³ of soils and the installation of two sluices on the 18 ha site, water was allowed onto the site last weekend.  A day later, the first good bird, a Glossy Ibis, turned up.

Holl-utopia is a shallow lagoon with a high percentage of islands; some grassy, some bare, some covered in sand or gravel.  The objective is to attract breeding waders, such as Avocet, Ringed Plover and Redshank, hopefully breeding terns, and certainly passage and roosting waders.  Water levels will drop during the spring and summer to expose extensive muddy areas.  An electric anti-predator fence around the margin will keep the local foxes as mere spectators. 

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.

Black-head squeeze continues

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.

 

Spoon-fed

Feeding Spoonbill pictures taken by John Bottomley at Titchwell. 

The Wadden Sea islands are remarkable for both migrant and breeding birds. Texel, at the southern end if the chain, is perhaps the best known to birders and has high levels of conservation management involving terns, waders, plovers and Spoonbill.

One key fact is that the island has virtually no mammalian predators, just a few Stoats. This was clear at our first stop; a Sandwich Tern colony of some 1200 pairs, on easily accessible spits and islands but full of chicks. The next stop was one of two Spoonbill colonies on the island, one amongst scrub, the other in saltmarsh, but both on the ground.

Spoonbills have been increasing in the Netherlands since the 1980s. There was an expansion into the Wadden Islands after the colonies at Oostvaardersplassen moved when the site was dried out for some years. The two colonies appear to feed principally on different food. The 'scrub' colony feeds on sticklebacks, with birds foraging principally on the Dutch mainland many miles away. The saltmarsh birds feed on prawns in the Waddensea. The Dutch have developed specialised sluices to allow spring-running Three-spined Sticklebacks to enter freshwater from the sea. This forms a key early food for Spoonbills, as discussed in a previous blog. Prawns increase in size and abundant later in the season and thus the saltmarsh colony breeds later.

So what do we need to get Spoonbills established as a breeding bird? A large wetland nesting site, secure from predation and disturbance, and large food-rich shallow wetlands. Birds may travel many miles between these sites.

Wandering on through dunes we saw numerous Bluethroats (60 pairs on Texel), Bittern (5 boomers in dune pools), abundant Marsh Helleborine and Fen Orchid of the subspecies ovata, with its more oval, hooded and erect leaves. Kentish Plover previously nested here but are now declining in The Netherlands, with the population down to c200 pairs, mainly in the delta region. The habitat looked good and was extensive but human disturbance is a problem.

A newly created scrape held many Little, Common and Arctic Terns, as well as 100+ pairs of Avocets. With increasing pressure on beaches, this is exactly the sort of conservation action that is required for our terns. Nearby we looked over a saltmarsh wader roost, holding 10,000+ each of Curlew, Oystercatcher and Knot, and 20,000+ Bar-tailed Godwits.

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!

Pilsey Island 29th July.

As we arrived on Pilsey Island an Osprey drifting over, carrying a
mullet back for lunch, was a good start to today's site visit. The
excellent vegetation included Lax-flowered Sea Lavender, Long-bracted
Sedge, Sea Holly and Sea Heath. Few seabirds had nested here this
year though, with regular disturbance in this busy part of Chichester
Harbour being a likely reason.

We continued round and out into Langstone Harbour, where there was a
different story. This year, the islands supported 400 pairs of Med
Gull, 205 pairs of Sandwich Tern, 81 pairs of Common Terns, and most
significantly, the 61 pairs of Little Tern had reared at least 47
young. 'Recharge' of the islands with gravel (to increase their
height above high tides), combined with some predator control, has
helped produce an excellent season. Disturbance is less of an issue
here. However, the Med Gulls also appear to be part of the problem
for the terns. Whilst the adults rearing chicks seem to forage inland
for invertebrates, the failed and non breeders hang around the
colonies predating eggs and chicks.

As high tide approached, the wader roosts gathered; 500+ Curlews, 200+
Oystercatchers, and small numbers of Whimbrel, Dunlin and Ringed
Plover were noted.

Terning the tide

Gadwall brood and Draper scrape with Garganey

Rye Meads is a highly productive site for breeding waterbirds, ducklings seemed to be everywhere during my visit today, with a minimum of 15 broods of Tufted Duck, 7 of Pochard and 11 of Gadwall scattered around the various lagoons. The Draper scrape also produced a juvenile Garganey, a Shelduck, 2 Green Sandpipers and a Common Sandpiper.

However, it was the Common Tern colony that was the main reason for the visit. They have been squeezed off their rafts by Black-headed Gulls in the last 2 or 3 years. This year the totals appear to be 19 pairs of terns to 21 pairs of gulls. The gulls have been very productive, the terns have not, only six tern chicks were visible today. How to provide for the terns in the face of the advancing gulls is a management dilemma at the moment, with a variety of ideas to be trialled over the next year or so. Elsewhere in the Lee Valley, terns are doing well, with productive colonies at Amwell and Cheshunt (although only 2 pairs of BHGs are nesting at the latter site). Competition between terns and gulls is commonplace around the coastal colonies, with terns having minimal productivity or being completely ousted at some sites. However, inland on tern rafts, this is a new and growing trend. These tern colonies have traditionally been very productive. Time will tell if they continue to be so.