Roseate tern (Sterna dougallii)

Figure 1. Number of roseate tern AON in the UK, 1986–2019.

The UK roseate tern population has undergone considerable long-term decline, decreasing from 950 AON in 1969-70 to 320 in 1985-88, and falling to 56 AON by the Seabird 2000 census (1998–2002). This is most likely to have been due to mortality of immature birds in their wintering grounds in west Africa¹, which reduced subsequent recruitment into the breeding population. On the wintering grounds, boys trapped and killed mainly immature birds for food, sport or profit, and while education programmes in the late 1980s and early 1990s reduced mortality rates, these need to be maintained or a resurgence in trapping is likely².

Food supply in the species’ wintering grounds is also likely to have affected immature survival rates. The above conservation measures (and providing shelter and protection from avian predators in the form of nest boxes at some colonies) have resulted in the UK population starting to recover. By 2019, the UK population had risen to 126 AON. However, recovery has mostly been confined to just Coquet Island and, although numbers are increasing there, full recovery of the UK population is likely to be a long way off.

Productivity

Figure 2. Trend in UK productivity (no. of chicks fledged per pair) of roseate tern 1987–2019. Based on SMP data; view the methods of analysis.

Although productivity was low in two years in the late 1980s when the population was declining, the number of chicks fledged in UK roseate tern colonies has generally been moderate to high throughout the reporting period. This is partly due to increased conservation effort. Predation of chicks was the likely cause of low productivity in 1987 and 1988, and poor weather affected west coast colonies in 1990, but the cause of low productivity in 2008 was not reported. Since 2010, roseate terns have not bred in Scotland successfully however, in 2019, a roseate tern paired with a common tern on the Forth Islands and produced a chick (Figure 3). In 2019 productivity was 0.8 chicks fledged per pair.

Figure 3. Number of roseate tern AON on the Forth Islands, 1986–2019.

The three national censuses show a large decline in roseate tern numbers in Scotland, from 134 AON in 1969-70 to 14 AON during Seabird 2000. In the Firth of Forth, the previous stronghold of the species, the decline recorded there since the late 1980s was fairly steady, albeit with some fluctuation (Figure 3). Three islands in the Forth formerly held colonies of roseate terns, although the largest colony had effectively disappeared by the early 1990s, largely due to increased competition for nesting habitat with herring gulls. Elsewhere in Scotland, single pairs occasionally frequent other tern colonies just maintaining its status as a breeding species in the country.

Productivity

An average of 0.25 chicks were fledged per pair per year at Scottish colonies between 1986 and 2019. Due to the small number of roseate tern colonies monitored frequently in Scotland, it is not possible to produce a meaningful productivity trend.

Figure 4. Number of roseate tern AON on Coquet Island, 1986–2019.

In contrast to Scotland, the one extant roseate tern colony in England, on Coquet Island, has fared better. National census results show that a large decline occurred in England between Operation Seafarer and the Seabird Colony Register, both in terms of the number of birds and number of colonies. Numbers have increased since then, but the species is now confined to Coquet. The rate of increase at this colony was slow at first (from 17 to 33 AON between 1987 and 2000) but then increased more rapidly to 94 AON in 2006. Since then, the population has fluctuated but has increased overall, reaching 122 AON in 2019. Active management on Coquet, via the provision of nest boxes for shelter and protection from avian predators, together with habitat management, has undoubtedly helped the species thrive there, perhaps to the detriment of other nearby colonies as birds abandon them in favour of Coquet.

Productivity

Productivity has averaged 0.47 chicks fledged per pair per year between 1986 and 2019. Due to the small number of roseate tern colonies monitored frequently in England, it is not possible to produce a meaningful productivity trend.

Figure 5. Number of roseate tern AON at colonies in Wales, 1986–2019.

The decline in roseate tern numbers in Wales was steep after the Seabird Colony Register, although prior to this numbers appeared stable in contrast to other populations in Britain and Ireland. By 1991, very few breeding pairs were left and, although there was a slight increase in 1993 and 1994, numbers soon decreased again and have never recovered. Ringing studies have shown the decline was probably due to terns deserting colonies in Wales and emigrating to those in the Republic of Ireland where active management has created sites of higher quality³. Roseate terns have started to nest in Wales again, with two AON being recorded in both 2018 and 2019.

Productivity

Roseate terns last bred in Wales in 2006, when a pair fledged one chick. In 2019 and 2019 two chicks have been raised each year. Roseate tern productivity in Wales averaged 0.46 chicks fledged per pair per year between 1987 and 2019.

Figure 6. Number of roseate tern AON at three colonies in Northern Ireland, 1986–2019.

In common with Scotland and England, the roseate tern population of Northern Ireland declined between Operation Seafarer and the Seabird Colony Register censuses, although the nearby Welsh population remained stable during this period. Then, in conjunction with Welsh colonies, a steep fall in numbers occurred so that few were left breeding by 1991. Emigration of birds to higher quality breeding sites in the Republic of Ireland was at least part of the reason for the decline. From the mid-1990s, the population fluctuated without showing any prolonged recovery and from 2003 declined again. Since 2009, only one AON has been recorded each year, except for 2014 when two AON were recorded. Single non-breeding birds have also been recorded at other sites in recent years.

Productivity

In Northern Ireland, roseate tern productivity averaged 0.68 chicks fledged per pair per year between 1991 and 2019. Due to the small number of roseate tern colonies monitored frequently in Northern Ireland, it is not possible to produce a meaningful productivity trend.

Figure 7. Number of roseate tern AON at Rockabill in the Republic of Ireland, 1986–2019.

After the near ubiquitous decline recorded throughout Britain and Ireland between Operation Seafarer and the Seabird Colony Register, roseate tern numbers in the Republic of Ireland have undergone a healthy increase which continues to the present. However, breeding is now confined to three colonies one of which, Rockabill (Figure 7), holds almost 90% of the Irish population⁴ and is the largest colony in Europe. As on Coquet Island (England), the provision of nest boxes, in conjunction with other management techniques (e.g. predator control and habitat creation), have benefited the species⁵. A recent seabird census in the Republic of Ireland (2015–2018)⁴ recorded 1,820 roseate tern AON, an increase of 148% since Seabird 2000 and 28% higher than recorded during Operation Seafarer.

Productivity

Figure 8. Trend in productivity (no. of chicks fledged per pair) of roseate tern at Lady’s Island Lake in the Republic of Ireland, 1989–2019.

One of the three colonies hosting roseate terns is located at Lady’s Island Lake. Here average productivity was 1.07 chicks fledged per pair between 1989 and 2019 which is higher than the national average of 0.98 during the same time period. In 2019, 1.17 chicks fledged per pair. Due to the small number of roseate tern colonies monitored frequently in the Republic of Ireland, it is not possible to produce a meaningful productivity trend for the country.