My bird of the month for June is a bit of a cheat as I want to talk about a pair of species: Arctic and common terns. These very similar looking summer migrants can be difficult to tell apart without good views. The amalgamation of the two names, ‘commic tern’, can be useful in these situations!
Luckily, living in Shetland is giving me lots of opportunities to see both species, often side by side, although we have more Arctics here than commons. I feel like I am really getting to grips with the differences between these two beautiful seabirds.
But more than that, I just love watching them go about their business: fishing, pair-bonding, constantly chatting. I have even been lucky enough to hear them calling from the bedroom window! The Shetland dialect name for both species is ‘tirrick’, which sums up these calls wonderfully.
Arctic terns famously migrate further than any other bird each year. During our summer months, they breed in a continuous circle around the Arctic and sub-Arctic. They then head to the waters around the Antarctic for the southern summer. As well as travelling further than any other bird, they probably see more sunlight than any other creature.
In order to take advantage of the oceanic winds, their migration routes are not in straight lines. Combine this with the fact they are long-lived birds, reaching ages of up to 30 years, and scientists have calculated that the average bird will travel approximately 2.4 million kilometres in its lifetime. This is the same as going to the moon and back three times!
Arctic terns nest in colonies, usually on the ground. They often share these with common terns, although they tend to breed more exclusively on the coast than their cousins. Anyone who has been near a colony will know that the parents are extremely aggressive and will attack any predator approaching the nest. This can seem at odds with their reputations as the graceful swallows of the sea, but it is an important tactic for ground-nesting birds.
There are concerns about the impact of climate change on this species. Research is already showing that it is forcing them to forage further during their time in the Antarctic. They are also vulnerable to loss of habitat in the breeding grounds, increased predation from animals able to survive further north as polar areas warm and to nest loss as extreme weather events become more frequent.
Common terns have a shorter migration than Arctic terns. They don’t breed as far north as their relatives, preferring sub-Arctic and temperate zones, and have a much larger breeding range north to south. The species has colonies from northern Norway as far south as north Africa. Winter is spent off the coasts of Africa, South America or Australia, with some birds not moving far from their breeding grounds at all.
Although they are also predominantly ground-nesting birds, they are less exclusively coastal and often nest on or near inland water bodies. They can also nest in places with longer vegetation than Arctics as they have longer legs. Common terns are happy to use manmade structures to nest on as well, whether purpose-built rafts or warehouse roofs.
Like Arctic terns, they are vulnerable to the effects of climate change. Because they usually nest on the ground, an increase in extreme weather events is likely to put their colonies more at risk of flooding and failure.
Arctic vs Common
So, how to tell the difference between them? If you are lucky enough to get good views of a perched bird, it is understandably easier to tell them apart. But there are some clues if watching birds in flight, too.
Arctic terns have a blood red beak with little or no black at the tip. Common terns’ beaks are a slightly more orangey-red colour (although this isn’t always obvious). Crucially, they have a black tip to the bill that is missing on the Arctic terns.
Common terns have longer legs, and, like the beak, they are more orangey-red than the blood red legs of Arctics. Conversely, Arctic terns have a longer set of tail streamers than those of common terns. When standing, the tail protrudes beyond the folded wingtips, whilst it is goes no further than the wings in commons.
Arctics are slimmer and daintier than their cousins. When you look at a perched common tern, it will look noticeably stockier and less streamline. It will also have a slightly flatter head compared to an Arctic tern’s more rounded looking crown.
Arctic terns have a much bouncier flight style than common terns and seem daintier in the air. When actively fishing, they also tend to hover, then drop a bit, then hover, then skim down to the surface of the sea. Common terns are more likely to circle or hover then dive directly down to the water without the stepped approach.
Whether or not you get good enough views to tell the difference between these two species, they are always a joy to watch and listen to. At their breeding colonies or fishing they might look like graceful swallows of the sea, but their constant chattering and chittering means they have bags of character. We only get them for a few months of the year and now is the perfect time to get out and make the most of them.