The Common and Arctic Terns – hybridisation and migration
Terns in the UK
Terns …these beautiful, gregarious, raucous birds are with us throughout the summer. There are five species that nest in the UK: Arctic, Sandwich, Common, Little, and Roseate.
The Arctic is the most numerous. Their nesting colonies are confined to north east England and Scotland, with something in excess of 50,000 breeding pairs.
The Sandwich and Common Tern populations are around 10,000 breeding pairs. The Little Tern has around 2,000 breeding pairs, and the Roseate a mere 200 to 300, which are confined to RSPB reserves.
Globally, the populations of these species, and other Terns, are widespread, which makes accurate estimation of global populations difficult. Although the populations of some species are threatened and Red Listed e.g. the Roseate Tern, others remain on the Global Green List i.e. birds of least conservation concern.
In the UK, the British Trust for Ornithology has recently reported that the Arctic Tern has moved from the UK’s Amber to Red List, due partly to the devastating impact of Avian Flue on their colonies, which are thankfully now showing signs of recovery. While the Common Tern has been moved from the Amber to Green List1.
Habitat
Terns tend to nest on gravel banks and small islands and are not particularly site faithful, and so counting and comparing the populations of colonies year on year is difficult.
Locally, Dungeness and Rye Harbour Reserves are good places to see them between April and September, although the nesting populations fluctuate year on year. All the photos in this blog were taken of Common Terns at Dungeness.
A pair of Common Terns mobbing a Herring and Lesser Blackbacked Gull that had chosen to scrap near the Tern’s nest site.
Terns are seabirds, although Common Terns also colonise inland lakes, and hence are probably the most easily seen Tern. They nest on gravel banks and small islands, and have benefited from managed areas where artificial islands and rafts have been created to attract them. Nesting also benefits from being predator proofed against foxes and mustelids (weasels, feral mink etc).
Description
Dagger like bills, long forked tails, slender wings, silver grey plumage with black cap, often seen hovering in groups before plunge diving for small fish, noisy and persistent mobbing of intruders that dare to near their nest sites, people included … these are the UK’s Terns.
The appropriately named Little Tern is small, has a yellow bill with black tip, and a white forehead. The Sandwich Tern has a black cap, and black bill. The Common and Arctic Terns are very similar in appearance, with red bills and black caps. There are differences in structure and plumage, but they are quite subtle.
Common and Arctic Terns
These birds are similar in appearance, the main distinction being one of range and behaviour.
The Common Tern nests in the south of the UK, while the Arctic Tern is a bird of northern latitudes.
The Arctic Tern is remarkable for its exceptional migration from the Arctic to the Antarctic and return, every year. For some individuals, this can amount to as as much as 70,000 kms / 44,000 miles round trip1. Enjoying the long days of the northern summer, and long days of the Antarctic summer all in one year means they experience more daylight than any other species.
The Common Tern is more modest in its endeavours, migrating only as far as Spain and western coasts of North Africa.
Structurally of course, there is no reason why the Common Tern could not undertake a far greater migration distance. As with the Arctic Tern, its long narrow wings and streamlined body are designed for ease of flight.
Range Overlap and Hybridisation
Although there is a clear distinction in their breeding range in Europe and in North America, there is sometimes a small overlap, and there are records of hybridisation where this overlap occurs.
Detailed research on this has been undertaken in North America by Mostello et al 2. The team observed a particular male Arctic Tern that paired with the same female Common Tern over a period of 8 years. The pair produced at least 9 young during this period. These young were ringed, and genetically tested to confirm their parentage. At least one of the young returned to the same colony, paired with a Common Tern, and produced young.
This long-term study produced some fascinating and detailed results, but the team did not broach the subject of how the young of two species that migrate in such a vastly discrepant way behave when they themselves migrate. This left me wondering, and led me to try to unpick the complexity of migratory behaviour, and how it relates to this particular question – and no better place to start than Ian Newton’s work4. The following is based on this:
The Genetic Basis of Bird Migration
How birds prepare for migration, time their departures, respond to environmental and social cues, and how they navigate to and from their nesting and wintering ranges, is complex and not fully understood. But one clear point is that the genetic makeup of the bird determines whether the bird actually migrates or stays, the specific direction it takes if it migrates, and the end point of its journey. This has been clearly demonstrated experimentally in different ways in many studies.
Some of this experimentation utilised birds of the same species taken from different populations, each population of which showed different migratory behaviour e.g. Blackcaps from Finland migrated long distances, Blackcaps from Germany migrate short distances, and Blackcaps from the Canary Islands little or no distance. These behavours, as measured by levels of activity, remain fixed even when the birds are kept in captivity, and can be clearly demonstrated experimentally.
When birds from two different populations are crossed, the young do not become either short migrators or long migrators, but consistently demonstrate behaviour somewhere in between that of their parents. Direction of migration can also be measured experimentally, and the same intermediate response occurs.
Along with evidence from other types of experiment, and from observation the behaviour of birds in the wild, Newton concludes that there is:
…. a continuum of variation, presumably controlled by a single attribute of the same genes …. The frequency of birds with high migratory activity may be very low in populations classed as residents, while the frequency of birds with low activity may be very low in populations classed as migrants.” pp 359.
Therefore, some Common Terns will migrate further than others, and some Arctic Terns will migrate less than others, but all Arctic Terns migrate much further than Common Terns. This is determined by the genetic makeup of the individual birds.
Cross an Arctic Tern with a Common Tern and a compromise of direction and distance will apply to the offspring. If the result of this compromise works well, and their winter location is good, the birds will return to their place of birth, nest, and pass on their genes to their offspring.
But what happened to the offspring of the Arctic and Common Tern chicks in the study?
Well, no one knows, but one thing is clear which is that at least one of the offspring succeeded in navigating to somewhere, overwintered successfully, and returned to its original colony to breed and pass on this new migratory behaviour to its own offspring.
Whether a new migration route will be established for a small group of birds, and in time this becomes a core route for one of the two species is unlikely. Rather, if one or both of the Tern species is to slowly change its behavour in response to our changing environment, it is likely to occur as a result of natural selection and the existing variations in migratory behaviour within each species.
Terns aside, this actually has profound implications for the adaptability of birds, and the ability of species to change their migration behaviour – they can and do adapt and change, sometimes over very few generations – a small sign of hope perhaps for some of our bird species in the face of climate change.
Footnotes:
- BTO News, Issue 352, Autumn 2024
- Tracking of Arctic Terns Sterna Paradisaea Reveals Longest Animal Migration. C. Egevang, I. Stenhouse, R. Phillips, A. Petersen, J. Fox, J. Silk Published in Proceedings of the National Acadamy of Sciences of the Unity States of America, 11 January 2010 Environmental Science, Biology.
- “Common Tern Sterna hirundo and Arctic Tern S. paradisaea hybridization producesfertile offspring”, 2016, Carolyn S. Mostello, Derek LaFlamme, and Patricia Szczys, SEABIRD 29
- “Bird Migration” (2010) Ian Newton, Willian Collins Books.
Tracking Arctic Terns: https://pubmed.ncbi.nlm.nih.gov/20080662
Common Tern and Arctic Tern hybridization: https://seabirdgroup.org.uk/journals/seabird-29/seabird-29-39.pdf