I’ve recently published a new paper with colleagues from the National Audubon Society in Maine (US). You can read the full paper here, or read below for the story about how the study came about and a summary of what we found.
During a workshop in Iceland I attended in 2017 with other puffin researchers, I discussed with colleagues Steve Kress and Paula Shannon, who have been monitoring the small seabird islands off the coast of Maine (US) for decades. On one of these, Matinicus Rock, which hosts scores of auks and terns during breeding, they explained that Manx shearwaters have recently started to establish a very small colony, of about 5 breeding pairs. This picked my curiosity because the vast majority of Manx shearwaters, which are medium-sized seabirds from the Procellariiform order (i.e. cousins of petrels and albatrosses), breed on offshore islands along the eastern shores of the North Atlantic, mainly in the UK. Very few birds are known to breed in the western Atlantic, and very little is known about those birds. That they had started to breed on a well-monitored seabird island seemed like a perfect opportunity to learn more about these birds.
I was particularly intrigued by the migration movements they birds may take. The clockwise migration patterns of British Manx shearwaters are well known, discovered by colleagues from the Oxford Navigation Group about 10 years ago, when migration tracking technology emerged. After breeding from April to August, the birds head out south until they reach western Africa, they then cross the Atlantic to reach the coast of Brazil, and follow it south until their wintering grounds near the Patagonian shelf. In February or March, the birds head back North, towards northern Brazil and then continuing northwards with a detour towards North America, crossing the ocean further north to return to the UK. So what route would US-breeding birds take? If they are British birds deciding to settle in the US, would they follow British birds’ migration route (which would include a very long detour from the US to the eastern Atlantic before flying south), or did they develop a new route? Or perhaps they wintered in a different place?
Steve and I decided to track a few of the individuals breeding on Matinicus Rock to find out. After obtaining permission to track 20% of the island’s breeding population (i.e. two birds), in summer 2018 Paula fitted a miniature geolocator (weighing only 1g) on two birds (from two nests). The loggers recorded the approximate position of the birds twice daily, and whether the logger was wet or dry (indicating sitting on the sea or being in flight) every 10 minutes. In June 2019 the birds were captured again in their nests, the loggers retrieved and downloaded. This new paper describes what we found.
The key finding was that both birds wintered in the same area as British birds but used a totally different route to get there (see map below). Instead of following a clockwise pattern like the British birds, these birds followed the British birds’ northwards migration route in reverse. After breeding, the headed south from Maine until they reached south America, then followed the coast to the Patagonian Shelf. In spring they followed this in reverse, similar to the British birds’ returning to the UK. The southwards migration took both birds about 40 days, including some stopovers, whereas on the way north they were much faster (~17 days). This may be because the wind patterns were less favourable during the southwards migration (the birds likely flew into headwinds). In terms of timings, the southwards migration was similar to that of British birds, with birds leaving the colony soon after breeding around mid-September and arriving in the wintering grounds in late October. Conversely, Maine birds left the wintering grounds from mid-March to early-April, whereas British birds leave several weeks earlier, likely because they have to go further.
Although we only tracked two birds, so we may have missed the full picture, the results raise interesting questions. First, where do these birds come from? One hypothesis is that they are visiting immatures from British colony which then decide to settle down to breed on the island, rather than to return to Britain to breed. The second question relates to the mechanisms by which migration routes develop. Evidence from tracking British birds suggest their migratory direction is inherited (all birds fly to the same place, even newly-fledged birds) but our findings suggest that perhaps it is more flexible than we thought, if those birds come from British colonies. We’re planning to continue studying these birds in the future to find out more.