Getting a handle on how many terns are nesting on Metinic (or any tern colony) is fairly simple: count up the nests and multiply by 2. However, Metinic has two species of tern, Arctic and Common, and figuring out which nests belong to which species can be tricky.
Like most terns, Common and Artic Terns both lay their eggs in simple nests called scrapes. The eggs of both species are essentially identical – oblong, with brown spots on a blue, brown, or olive background. Common Terns typically nest in grassier areas and have 3 eggs per nest.
Arctic Terns prefer more open rock or cobble and usually lay only 1 or 2 eggs.
However, it is possible to find an Arctic Tern with 3 eggs in its nest, or a Common Tern with eggs up on a rock, so these differences are not enough to determine whose eggs are whose. The best way to tell who laid a particular batch of eggs is to watch who does the incubating.
To do this, we begin by locating a group of nests visible from a blind window. We place a pink flag next to each nest whose species is unknown. This helps us find the nests from the blind, since the eggs are very well camouflaged and the blinds are usually some distance away.
Next, we hop into a blind, pull up a scope, and watch. Usually within a minute or two, adult terns will settle onto the nests and resume incubation. With a scope or binoculars, we can tell the adults’ species.
After that, we replace the pink flags with either a red flag (for Arctic Terns) or a blue flag (for Common Terns).
So why is all this important? Once we’re done flagging nests, we’ll gather all the flags back up and tally up the number of Common versus Arctic Tern, then use our census number to estimate how many of each species we have on the island this year. For example, last year we marked 64 Common Tern nests and 37 Arctic Tern nests, or 63% Common and 37% Arctic. Our mid-June census found 330 total nests on the island, so we estimate there were 209 Common and 121 Arctic Tern pairs nesting on Metinic last year.
We’re excited to see how our numbers compare to last year!