Arctic Terns have arguably the most impressive migration of any bird – they travel from the Arctic to the Antarctic and back again just about every year of their lives. While we’ve known for a while where the terns travel, how they get there has been something of a mystery. Do they travel along the coastline? Do they take a direct route from Maine to the Antarctic coast? Do they do a marathon flight from pole to pole, or make pit stops along the way? These questions were nearly impossible to answer until very recently, for the simple reason that following a single tern (or even a flock of them) is nearly impossible. Terns are small birds – a little over 100 grams, or 3.5 ounces – so they can’t be equipped with heavy satellite tags. They also do mostof their traveling over water, so the odds of spotting a tern on its migration are slim.
Enter the light-level geolocator, here to solve these problems and answer many of our migration questions
Four years ago, in the summer of 2010, the Refuge deployed 30 geolocator tags on 30 separate Arctic Terns. These tiny tags are lightweight enough that they don’t hinder the terns as they travel (in fact, they’ve been used on even smaller birds, like Purple Martins). They work by using detecting light levels and recording the time of sunrise and sunset every day. Since the length of day and the time of sunrise and sunset are slightly different at every point on the globe, this information can be converted into a rough map of everywhere the tag, and by extension the tern, has been.
A pretty miraculous little gadget, isn’t it? But there’s a catch: to reduce weight, the geolocators don’t transmit the data they gather, they simply record it. To get at all that information, the tag must be retrieved and physically attached to a computer. This means the tagged birds must be recaptured and the tags removed before they do us any good.
Recapturing a bird can be difficult. The best way to do so is to find the bird’s nest, and use a trap that springs while the bird is incubating. That, however, requires the bird to be able to find a mate. Last year on Metinic, Zak and I spotted a geolocator-equipped tern who had eluded capture for 3 years, but we couldn’t get our hands on him because he didn’t have a mate or a nest. He spent all summer trying to court various female terns with fish, but there were no takers.
This year, our lonely tern returned and found himself a mate. Syd and I set up a trap called a bow net trap, which is triggered by the tern attempting to incubate a set of fake wooden eggs (the real ones are safely stored in a blind so they aren’t crushed by a struggling tern). It took a couple of tries, but today we succeeded in capturing the handsome gentleman we have nicknamed Giovanni (Geo, for short).
Geo was released back to his mate and eggs, probably glad to be rid of his extra baggage. The geolocator will be returned to Refuge staff, who will hopefully be able to use it create a roadmap of Geo’s travels from the past four years. That could be up to eight traps between Maine and Antarctica – more than 70,000 miles!