Arctic tern (Sterna paradisaea) and Common tern (Sterna hirundo)

Arctic tern (Sterna paradisaea) (fig. 1) and common tern (Sterna hirundo) are very similar species in appearance. Arctic tern adult breeding plumage consists of pale grey upperparts and white underparts, with a black cap, short red legs, and a dark red bill. Common tern adult breeding plumage is similar, but they tend to have a more orange-red bill with a dark tip (Svensson et al, 2020). 

Arctic and common tern usually breed in mixed colonies along the coast, although common tern will breed at inland sites also. Their diet largely consists of small fish such as sandeel. Both species breed at around 3 years, with the lifespan of common tern typically 12 years and the lifespan of arctic tern typically 13 years (Harris et al, 2024).

Population & Distribution:
Arctic and common tern are common summer visitors to the East Grampian Coast. Around 3% of the global arctic tern breeding population and 1-2% of the global common tern breeding population is hosted by Britain and Ireland, with 59% and 24% of these populations found in Scotland respectively (fig. 2) (Burnell et al, 2023; Harris et al, 2024). Arctic and common terns often nest together, with breeding colonies recorded at sites across the East Grampian Coast (fig. 3 & 4) at Forvie NNR, St Cyrus, St Fergus Gas Terminal and RSPB Loch of Strathbeg (fig. 5&6) (SOC North East Scotland Branch, n.d.). However in recent years the majority of the arctic and common tern population on the East Grampian Coast breeds at Forvie NNR (fig. 7). 

Conservation Status:
Arctic tern are a Red listed species due to severe breeding population declines over at least 25 years in the UK alongside a decline in breeding range. Common tern are an Amber listed species due to moderate breeding population declines over at least 25 years (Stanbury et al, 2024).

Factors Impacting Arctic and Common Tern:
As arctic and common tern are a ground nesting species, their chicks and eggs are vulnerable to predation (Harris et al, 2024). Over the years the breeding success of the arctic and common terns at their various breeding sites has been highly variable and this has largely been attributed to predation pressures from species such as stoat (Mustela erminea), otter (Lutra lutra), fox (Vulpes vulpes), and gulls (SOC North East Scotland Branch, n.d.). Predator proof fencing at Loch of Strathbeg and Forvie has proven successful at protecting breeding tern populations.

High mortality has been recorded in both arctic and common tern from Highly Pathogenic Avian Influenza (HPAI). Across the UK a 2% decline was recorded in arctic tern populations and a 42% decline was recorded in common tern populations when compared to pre-HPAI baselines (Tremlett et al, 2024).

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