Monday, 7 September 2015

Arctic Terns and Oystercatchers on Rockabill, 2015

So we've had two end-of-season species reports so far - the Black Guillemot population on Rockabill is bouncing back after a rough winter in 2013/14, and our Kittiwake population hit a record high, though productivity was a bit below average. Still, all things considered there has been much to be positive about in our previous two blogs!

With that in mind, we've paired up Arctic Terns and that one pair of Oystercatchers in this blog. . . . . just so we can keep the negativity to a minimum. . . . . . . . . . 

Arctic Terns

Arctic Terns are one of the longest migrants in the world - thought to see more daylight in a year than any other species as they migrate from one pole to the other - breeding in the northern part of the northern hemisphere and returning to the South Pole for the winter. This feat is ridiculously impressive when you consider that this little bird weighs less than 100g! I personally weigh much more than 100g, and would struggle to get to the south pole despite all of the modern conveniences and technology us humans enjoy. . . . . . . .

Arctic Tern on Rockabill, 2015. (picture taken under NPWS license)

We have three species of Tern on Rockabill, and Arctic Terns have always been the least numerous. They peaked at around 230 nests five years ago, but generally have ranged between 100 and 200 pairs. In recent years that number has been closer to 100 pairs, with only around 50 nests recorded at any one time. Our Arctics prefer the rockier, outer parts of the island away from the chaos of the middle of the colony, with their main stronghold being the Bill and a few other nests around the helipad. Unfortunately, being on the Bill means they are only a hop, skip and a jump from the Gulls that hang around on the other end of the Bill waiting for fishing boats to go by. You can see where this is going. . . . . . . . 

Nesting Arctic Tern (picture taken under NPWS license)
Arctic Tern egg - more round, blotchy and blue/green that Common Tern eggs. (picture taken under NPWS license)
A predated Arctic Tern egg.
Unfortunately a small number of those Gulls have perfected the art of taking Arctic Tern eggs within hours of them being laid. The most we found on the Bill this year was two nests of one egg each, and they didn't last long! Having carried out counts of adults and pairs on the Bill around the time egg laying began we know there were more pairs trying to breed but the Gulls had other ideas. We had something in the region of 50 Arctic Tern nests at any one time on the Rock - the usual ones around the Helipad and a few more that may have originally tried laying on the Bill. The fact that they laid on the rockier parts of the island meant that occasionally heavy rain flooded many nests, and being on the outer edges of the colony meant they were easier for the Gulls to get as there were few neighbouring terns to help them defend their nests. We managed to ring a small number of Arctic Tern chicks (they didn't even get that far last year!), but again rain and predation hit them hard and only a small number are likely to have fledged in the end. No Arctic chicks fledged last year, only a handful fledged this year, but we've learned from this and we'll be putting specific recommendations in our report to help ensure things improve for them from next year onwards.

Arctic Tern chick (picture taken under NPWS license)
Arctic Tern adult and chick (picture taken under NPWS license)

Brian and Andrew with two Arctic Tern chick siblings. (picture taken under NPWS license)


Some of you will remember the story of our Oystercatcher pair, but for the sake of completeness here's a quick recap: A pair of Oystercatchers were the first species to lay eggs on Rockabill this season - only the day after we arrived! Unfortunately, and I'm sure much to their surprise, around three weeks later they were completely surrounded by Common Tern nests. That wasn't a problem for the adults - Oystercatchers are much bigger than Common Terns, with much bigger bills, and in general there seems to be a certain amount of 'nesting bird ettiquette' at large breeding colonies -  "You stay out of my nest, I'll stay out of yours, and if you see a predator flying over let us know!!". Everything was going well and both eggs hatched in early June. Unfortunately Oystercatcher chicks tend to run from their nest pretty much once they're able to run and our two Oystercatcher chicks ran into the path of an aggressive nesting Common Tern, and both died from the injuries. This was the first time we've had Oystercatchers nesting on the Rock, and as with the Arctic Terns we'll be making specific recommendations in our report to make sure they don't meet the same fate if they come back next year.
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So thats the really bad news out of the way! It really shows how complicated things are at a colony like this and so many factors come into play over the course of a season that determine whether things end on a good or a bad note. You'd think the success of our breeding Terns would be tied together, given how similar they all are, but the location of nests combined with weather, has led to huge problems for our Arctic Terns. You'd also think Oystercatchers would be able to hold their own in a colony of much smaller birds, but again nest location and the vulnerability of their chicks meant they were unlucky this season.

Stay tuned this week to hear about how our Roseate and Common Terns did in 2015. . . . . . . . .

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