Today's blog is by Linda Wilson, Senior Conservation Scientist.

One of my first tasks when I joined RSPB’s Centre for Conservation Science in 2016 was to visit some beautiful sand and shingle beaches along Britain’s stunning coastline. Not a bad way to spend a few summer days. But why was this important to RSPB? It just so happens that the same places that we flock to during the great British summer are also home to one of Britain’s rarest seabirds, the little tern.

With an estimated population of fewer than 2,000 pairs that is suffering ongoing declines, conservation of this stunning seabird is challenging as the issues they face are diverse. These range from disturbance (beach goers for example), a host of skilled predators that kill not just chicks but adults too, and tidal flooding of nest sites, a frequent occurrence given the birds’ penchant for nesting at the edge of the high-water mark.

A little tern with two chicks in its beach nest scrape © Kevin Simmonds

But somewhat paradoxically, my summer beach visits were less to do with their experiences during the breeding season and more to do with what happens to them over the winter. Little terns only visit us in summer, arriving in April/May for the breeding season before heading off again in August to spend the winter off West Africa. But what we don’t know is how many of these birds survive the migration and over-wintering period to be able to return to their breeding colonies the following season.

It was against this backdrop that an RSPB-led partnership launched the first coordinated national programme of little tern colour-ringing in 2014, as part of the EU-funded Little Tern Project. By using colour rings with unique, field-readable codes, the idea was to generate a data-rich set of re-sighting histories that would allow us (through some complex modelling) to estimate survival.

Since the project began, over 1,000 little terns have been colour-ringed by a team of dedicated BTO-licensed bird ringers. This is a feat in itself, but we also had the task of re-sighting as many birds as possible in subsequent years. Easier said than done. Little terns are…well…little (about the same length as a blackbird but half the weight), which means that the colour rings are only 8mm high. Given there are two to three digits squeezed into this space, reading these on a mobile bird that’s notoriously ‘flighty’ takes a lot of patience, some very high quality optical equipment, and a lot of time spent looking for birds in the places they like to be.

Hence my summer beach visits. But I was not alone in squinting through a telescope at the seaside, as we also had a small army of wardens, rangers, researchers and volunteers on the look-out for these colour-ringed individuals. Thanks to their efforts, our overall re-sighting rate so far is 22% (and reached 57% for an individual colony population) and we estimate that within a few years we should have gathered enough data to be able to calculate the first ever estimate of annual survival for little terns in Britain and Ireland.

Little tern colour rings are extremely small and normally require a good quality telescope or digital camera to read them in the field © RSPB

In the meantime, our new paper shows how our re-sightings are already giving us unique insights into the movements of these birds, revealing for the first time the extensive exchanges that occur between colonies. These were particularly apparent for colonies bordering the Irish Sea, with several birds recorded switching between breeding colonies on the Isle of Man, Ireland and North Wales.

One intrepid fledgling even crossed the Irish Sea twice in one season, recorded in North Wales less than a month after it had been ringed as a chick in County Wicklow, only to be seen again just five days later in County Dublin (and presumably crossing the Irish Sea once more as it embarked on its southward migration). Such rapid, wide ranging, post-fledging movements are presumably exploratory, allowing young birds to learn their surrounding geography and the location of other, potential alternative breeding sites should their natal site prove unsuitable in the future.

Little tern with colour-ring Yellow XV0 next to another carrying a larval fish. © Angela Thomas

Little terns are traditionally assumed to remain in their wintering quarters during their first summer after fledgling, so it was a surprise to record 18 birds returning to us at one-year-old; previously there was only a single record of this, back in 2000. It is often tricky to distinguish first-summer little terns from adults in the field, so our data raises the interesting prospect that returning first-summer birds may be more common than previously thought.

At the other end of the age spectrum, one of the metal-ringed little terns that was re-caught and colour-ringed during the project broke the little tern longevity record, being 27 years old when it was re-sighted in 2020, exceeding the pre-project record by 8 years! In common with many other seabirds, a long lifespan is a demographic trait that helps buffer against short term periods of poor breeding success. But this can only help up to a point and the ongoing little tern population decline shows that it has not been enough to offset their repeated widespread breeding failures over recent decades.

Which brings me back to why techniques like colour-ringing are so valuable. To understand how and why populations are changing, we need to do more than just track their rates of reproduction. By following marked individuals over time and space, this can help us fill in the other key factors underpinning population dynamics - the interplay between births, deaths, immigration and emigration – helping us to identify the full suite of causes of decline, and therefore inform conservation efforts going forward.

You can help! Please keep an eye out for colour-ringed little terns from April – August and report any sightings to See here for information on where to see them around our coastline. Please note that Little terns are protected under Schedule 1 of the Wildlife and Countryside Act 1981 and it is illegal to disturb them during breeding; local site staff should always be contacted before attempting any re-sighting at a breeding colony.

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