“EastEnders” – Common Tern style

March 27, 2023
Dublin Bay Birds ProjectDublin Port CompanySeabirds

April is just around the corner, which means Dublin Port will soon be heaving with nesting terns. As we eagerly await their arrival, we thought we’d share a short story we captured on a trail camera, of two Common Tern pairs nesting in Dublin Port last season…

When the camera starts rolling we can see three nests, two of which are being incubated. The third nest, in the foreground, has been abandoned.

The next day, a pair, one of which is colour ringed, show up. The colour ringed bird is ‘7P2’, and was ringed as a chick in Dublin Port back in 2019. In fact, ‘7P2’ hatched on the very platform on which our story takes place.

The next day ‘7P2’ and its mate get down to the business of egg making…

…and later claim a territory beside the abandoned eggs. At first, they don’t interact with the eggs…

…until the next day when ‘7P2’ pulls the abandoned eggs into the scrape and begins to incubate them.

However, ‘7P2’ and her partner are not the only pair which will end up showing an interest in these eggs.

On the 20th of June 2022, the new parents disappear for the night and aren’t seen back until the following morning.

In the meantime, attracted by the heat from the sun, the Common Tern chicks from the right-hand corner nest, move closer and closer to the previously abandoned nest.

That evening we see their parent incubating them – and the previously abandoned eggs…

The next morning the colour ringed pair returns. They briefly inspect the nest before leaving again.

The adult with chicks makes her move and reclaims the territory, leaving its own chicks behind and moving to the eggs.

The eggs and territory swap hands over the next few hours. And then something odd happens. The metal ringed adult from the right-hand corner is caught on camera carrying a punctured egg from the disputed nest. Had it broken in an earlier altercation and the bird was just removing it from the nest site so as not to attract would-be predators?

‘7P2’ returns to the nest, seemingly oblivious to what has just unfolded.

However, this fight is not over…

By the last day, the metal ringed Common Tern appears to have been victorious. ‘7P2’ will still be present over the next couple of days until the camera is eventually removed, but less frequently.

As the camera had to be removed, we don’t know if the metal-ringed Common Tern continued to incubate the eggs or if it lost interest.

When eggs are very close to another nest site, such as the abandoned eggs that ‘7P2’ adopted, it’s not uncommon for incubating Common Terns to adopt them. Several studies, including one on Rockabill Island, a tern colony managed by BirdWatch Ireland, have found that Common Terns do not appear to have the ability to recognise their own eggs, and so adopting eggs is preferable to potentially losing one of their own.

What seems more unusual is a Common Tern pair incubating eggs from another nest when they have chicks. Perhaps the chicks being in the abandoned nest with the eggs stimulated this behaviour? When food is plentiful, Common Terns have also been known to lay a second brood, so perhaps that fed into this scenario also? Common Terns are also incredibly territorial at the nest site, particularly when they have young and have been known to injure and even kill other chicks. Deterring a late arriving pair from breeding close by is pretty typical behaviour, and this, along with the chicks forays into ‘enemy territory’ may help explain some of what occurred.

However, the truth of the matter is we just don’t know why this played out the way it did. Which is fascinating in and of itself! Animal behaviour is far more complex than we sometimes give it credit for, and it is one of the reasons monitoring these colonies is not only vital from a conservation point of view, but endlessly enthralling!

Thanks to Dublin Port Company for their funding and support throughout this work!