22 June 2020

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

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