Our work changes a lot between various parts of the season - there's a lot of vegetation to be cut in May, a lot of eggs to be counted in June, chicks to be weighed and ringed in July and nestboxes to be scrubbed in August! One standard throughout the whole season however are our nest checks. Every morning and every evening we have specific study areas around the island that we visit and record any changes since our last visit - new nests, new eggs, chicks hatching, chicks dying etc etc. so that we have a few hundred 'nest histories' for each season.
I was somewhat surprised by one nest in my study area earlier this summer as it appeared to be a Roseate Tern egg in a site I knew had been used by Commons in previous years. The egg was a 'typical' Roseate egg however and the location did offer some degree of shelter and so could conceivably have been picked out by a Roseate. A second egg was laid, again a typical Roseate egg and a little reconnaissance from a distance confirmed it was a Roseate Tern sitting on the eggs.
Then, last week on one of my nest rounds I saw that the first of the two eggs had hatched - that is the first of those two typical-looking Roseate Tern eggs that had been incubated by Roseate Terns for all those weeks. The only problem was that it had a fluffy appearance and pink legs among other features, meaning it was a Common Tern chick!
A few days later and the second egg hatched - another Common Tern chick I hear you ask? Nope! A Roseate Tern chick! So this pair of Roseate Terns ended up with a brood of one Common and one Roseate Tern chick. There were no nearby nests that an egg could have been stolen or even accidentally rolled from, so the whole thing is a bit of a mystery! The adults are blissfully unaware however and are dutifully feeding their Common Tern chick as if it was their own - which I suppose it is now anyway! Given the similarities between the two species, particularly their diet, this chick stands as good a chance as any of fledging in the not too distant future!
Here are a couple of videos from this unusual nest - the larger and more active chick is the Common Tern and the smaller chick with a 'spiky' appearance is the Roseate Tern chick.