Friday, 21 June 2013

Breeding terns update

The main tern nest census was carried out on a fine day last week, and showed good increases in the number of nests and clutch sizes since our last visit.

As we rowed out to the platform, we could see that something was distressing the terns – they were repeatedly diving at an unseen intruder towards the eastern end of the platform – and we immediately suspected a pillaging corvid. We climbed the ladder to the platform and carefully picked our way (tern eggs are almost invisible on the shingle substrate) to the eastern end to find a Kittiwake spread-eagled across one of the nests. A quick examination revealed that the bird was very freshly dead, but showed no signs of external injury. Whatever the cause of death, this bird picked a less than peaceful refuge for its final moments... 

Common Tern nest in Dublin Port. Niall Tierney. 

Anyway, we promptly got back to the job in hand – the nest census. This is simply a count of the number of nests and the number of eggs per nest. This, along with the estimation of the number of adult birds present, allows us to see how the colony is faring from year to year. On the main platform, 418 Common Tern nests were recorded, with clutch sizes ranging between 1 and 4. There were 25 Arctic Tern nests, with clutch sizes of 1-2. There was an increase in the number of cached depredated egg shells, indicating that the (still unidentified) avian predator had been on the platform since the earlier visit (see previous post).

Two Arctic Tern nests in Dublin Port. Niall Tierney.

Once the census was complete, we quickly vacated the platform to allow the adults to resume incubation. Peak hatching is expected to be around the turn of the month. We’ll check back in on the colony again in the coming weeks to monitor breeding success and will post another update then. 

Tuesday, 11 June 2013

Breeding Terns in Dublin Port

Dublin Port has been home to a colony of breeding terns since at least 1949, and in recent years, has supported 550 and 55 pairs of Common and Arctic Terns, respectively. The monitoring programme that was started by the late Oscar Merne in 1995 will be continued by BirdWatch Ireland this year, and in subsequent years.

Tern platform in Dublin Port. Richard Nairn

Although the cold spring has resulted in delayed commencement of breeding for lots of birds, the Common and Arctic Terns in Dublin Port are now nesting. Last week’s colony census recorded 263 Common Tern and 8 Arctic Tern nests. Worryingly, a cache of at least 23 tern egg shells was found on one of the platforms, with the culprit most likely a corvid. 

Depredated tern eggs cached on the platform. Richard Nairn

As the colony reaches it full strength, it’s likely that the full complement of terns will be sufficient to drive off any intruders. As someone who has ringed in tern colonies, I can tell you that hundreds of angry terns have no trouble making their feelings known...

The joys of tern fieldwork!