Where will they tern up?

Rachel Taylor, Steve Dodd and Katharine Bowgen write:

Many of you will already have heard the news that the Skerries tern colonies have been almost completely abandoned this year; an event thought to have been triggered by predators colonising the islands; which in a normal year would likely have been managed by RSPB colony wardens. Sad though an event like this appears, it isn’t all that uncommon for colonial seabirds to move around; and entire colonies have been known to disappear and recover over time. What’s different about the Skerries is that recent colour-marking work provides an opportunity to understand the demographic processes behind this huge event – and birdwatchers can be a key part of the work.

 

The Skerries helipad. Photo by Rachel Taylor

The Skerries are a group of rocky islands in the Irish Sea, near Holyhead on Anglesey, owned by Trinity House. The significant seabird colonies are managed, protected and monitored by RSPB.

The Arctic Tern colony is the largest in the UK, with 3,816 pairs in 2016 - along with 290 pairs of Common Terns and an increasing probability of breeding attempts by Roseate Tern – in 2018 fledging two chicks for the first time in decades. In a normal year the terns arrive early in May, with hatching starting at the beginning of June and the first chicks fledging in early July.

Leg-flagged Arctic Tern on The Skerries image by Rachel Taylor

Since 2013, RSPB has supported local ringers Steve Dodd and Rachel Taylor in running a set of small demographic projects in the colonies on Skerries. A sample of 500 Arctic Tern chicks are ringed each year, and Retrapping Adults for Survival (RAS) studies run on both Arctic and Common Terns. The RAS studies use individually coded leg-flags to identify individuals. There are several good reasons why RSPB supported our work, but one major justification was an interest in understanding colony dynamics in terns.

Colonial seabirds in general have something of a dilemma in choosing a nest site. Colonies have advantages: for example in recognising and exploiting local resources like safe sites for nests, and good feeding sites. Individual nests run a lower risk of loss to predators, because there are many nests to choose from, and many eyes watching for those predators. But they also create problems – such as risk of disease transmission, increased competition for food resources, and vulnerability to local resource changes, random disturbance events and becoming a target for predators. Major colony collapses may be a result of several pressures operating at once. But terns are long-lived; several studies have identified the linked nature of adjacent colonies, with large colonies exporting chicks to smaller sites, and local populations remaining broadly stable despite rises and falls in breeding numbers at individual sites. Site abandonment tends to be more-or-less temporary, and if the pressure that caused the birds to leave is managed or removed entirely, the birds return.

Leg-flagged Common Tern 'CJ' in The Skerries. Photo by Rachel Taylor

When we set up the adult survival projects on the Skerries, we knew that most large colonies north  of Anglesey had at some time in the past, experienced significant population declines or abandonment – one of the most dramatic being the abandonment of the Shotton Steelworks Common Tern colony on the Dee in 2009. The reasons for that collapse, and the work underpinning the colony’s subsequent recovery, provide a fascinating example of the complex issues that long-lived seabirds experience. But we wanted to know some very specific things about the birds’ experience of a ‘colony collapse’ event. Of course, at the time the Skerries colony was going from strength to strength and we expected that to continue; but the national pattern was clear. We could learn from adult survival data in a healthy colony, but would also be prepared with a good  baseline in the event of something changing for the Skerries.

We had a specific set of ‘in-the-event’ questions. Firstly, since Arctic Terns are more site-faithful than Common or Sandwich terns, would a colony-breaking event cause them to simply take a year off and then return the following year, or would they immediately try somewhere else? Secondly, having attempted (and possibly been successful) elsewhere, would they return to the Skerries in subsequent years, or emigrate for life? Thirdly, terns are reluctant to change their breeding site for very good reasons; we wanted to understand the survival implications of the birds experiencing such a major upheaval as a colony collapse event. Survival studies take time, and like dispersal studies, depend on birds being individually recognisable without causing data-skewing and inappropriate disturbance – hence the common use of colour marking in RAS.

Lots of terns. Photo by Rachel Taylor

The collapse of the Skerries tern colonies this year is very sad, but certainly isn’t the end of the story. There is good evidence that such a collapse will be temporary if – as already seems likely - the cause is identified and can be managed. But more than that, the Skerries is a huge opportunity to learn about collapse events, and about how individual terns respond and recover from them. Birders are already beginning to look for birds from the Skerries, marked with coded orange (Arctic Tern) or yellow (Common Tern) leg-flags. In particular, the nearest colony on Anglesey (Caemlyn) has already seen an influx of Arctic Tern, and the wardens are on the lookout for marked adults. If you live or bird around the coasts of NW England, North and Mid Wales or the Isle of Man, or the Irish Sea coasts of the Republic of Ireland and Northern Ireland and if Government restrictions allow, please keep your eyes peeled and your bins poised – and help us find the scientific silver lining inside the dark cloud of this year’s Skerries colony abandonment.

Flag sightings to sg.dodd@yahoo.com

Namibian Common and Spanish Roseate 'tern' up in the UK

We often blog about the photographers capturing images of ringed birds (such as the Spanish Black-necked Grebe, wandering Bearded Tits, Sandwich Tern in the USA, a lost Snow Bunting and a returning Great Grey Shrike), but we do also get plenty of people reading rings with good optics.

We recently received a run of reports of ringed Common Terns read with a 'scope at Preston Dock. Most of these were birds ringed at Shotton Steel Works or Banks Marsh on the Ribble estuary; all relatively local if you're a long-distance migrant. Interestingly, these birds were all at least five years old (and up to 11 years old), but then the recent decline/crash at Shotton means there are fewer birds to resight, so the story is never that simple! It's not all bad news at Shotton though, as the colony has recovered somewhat this year, with over 350 chicks ringed recently.

But two birds also really demonstrate what these birds get up to when not on our shores. 4H58786 was a bird ringed in Namibia in March 2011 and currently paired up with an egg at Preston Docks: there are fewer than 10 records of Namibian-ringed Common Terns in the UK. The other ring was a French ring, M26822, possibly ringed in Senegal in or around 2000, but details aren't clear yet. More details on Common Tern movements can be found in the latest version of the BTO's Online Ringing Report.

4H58786 at Preston Dock (Paul Slade)

This sequence of photos shows the effort needed to read a ring
from photos, but the effort was certainly worth it (Chris Batty)

Interestingly, Preston Dock also produced another fascinating recovery report recently. A ringed Great Skua found there on 15th June proved to be from Norway (ringed on in Herøy in August 2013), but had to be taken into care by RSPCA. Extra details and photos of this bird are on the Bamber Bridge Birder blog.

NOS 3033460 before being taken into care by RSPCA (c) Bamber Bridge Birder

As if the run of records from Preston Dock wasn't enough, the northeast hit back last week with an equally notable ring read. Coquet Island is well known as the largest British (and only English) breeding colony of Roseate Terns and birds from here are regularly ringed by the Northumbria Ringing Group (experience the thrill of a ringing visit to the island on this BBC Radio4 Living World programme). But when the RSPB warden Wez Davies recently read ring 1V015109 it certainly wasn't a BTO ring! This turned out to be a bird ringed at Salinas de la Tapa, Cadiz, Spain in August 2010.

Northumbria Ringing Group hard at work on Coquet Island

What makes this ring read so exceptional is that it's the first EVER recovery of a foreign-ringed Roseate Tern in the UK. Ringing of birds as chicks is commonplace at the few extant colonies, and we have over 500 foreign recoveries of these birds (all detailed in the Online Ringing Report - it really is a great source of information!), including over 260 in Ghana, 32 in Ivory Coast and 18 in Togo.

***STOP PRESS*** We've also just heard from Coquet Island that after reading the Spanish Roseate Tern ring, warden Wez has also found a dead Common Tern with a 'Pretoria' ring on it: 4H51221. This could be a bird ringed in South Africa but, as for the above bird from Preston Dock, could also be from Namibia. Watch this space for further details...

Thanks to Paul Ellis for the reports, from the Fylde Bird Club website, of the Preston Dock birds and to Tom Cadwallender and Chris Redfern for chasing up the Coquet records.

Terns around the World

The Common Tern is a seabird which can be found breeding in most of Europe, Asia and North America. After breeding in the northern hemisphere they migrate south along the coasts to South America, southern Africa, Falkland Islands, India and Australasia except New Zealand.

We don't receive very many reports of Common Terns ringed in other countries. In fact, until last year, we only had 144 recoveries of Common Terns ringed abroad. Of these birds 50% came from Scandinavia.


Last month, ringers from Tees Ringing Group had some very good mist netting catches of terns. Their efforts paid off in the form of several exciting controls: one Common Tern from Finland, one from Sweden, one from Norway and one from Russia!

Common Tern MOSCOW TA64035, captured in Seal Sands in Teesmouth on the 3rd Sep 2010, is the first ever Common Tern from Russia and we can't wait to hear about the ringing details. We'll keep you posted.

Thank you to Allan Snape and Eric Wood for sharing this story and supplying the photographs.