One of the key pieces of information required to accurately asses population trends within a species is the average survival rate - i.e., how likely any individual is to survive any given year. Gathering the data required to determine survival rates is fairly straightforward, but it requires time. The easiest way to go about it involves marking a certain number of individuals (in this case with small metal bands bearing a unique identifying number) and then tracking them over time noting how long each bird is present within that population. There is actually quite a bit of math involved (handled by authors Greg Robertson and Danielle Fife), and catching and re-catching birds every year isn’t necessarily easy, but at least it’s lots of fun! Our efforts over several years at Nasaruvaalik Island have finally paid off, and you can read all about it here in Polar Research!
While Arctic Terns appear to have relatively high survival rates, our analysis suggests that recently noted population declines at some colonies are likely caused by either low juvenile survival or emigration to other colonies. More work is needed to clarify these issues, but this latest paper provides a very useful baseline with which to assess fluctuations and trends within the species.