On our last blog earlier in the month we were discussing colour-ringed wader re-sightings, but now the tern season is upon us, and its time to update you on the breeding terns in Dublin Port.
The annual breeding season monitoring started earlier this month, and the nest census revealed healthy numbers at the two subcolonies on the mooring dolphins in the port. We've made a few visits so far in order to count the nests and work out clutch sizes.
Common Tern colony on the ESB "Dolphin" in Dublin Port Helen Boland
Our first visit was all about nests and clutch sizes, and we recorded a total of 417 Common Tern and 66 Arctic Tern nests. Censusing the platforms is quite straightforward pre-hatching, as you're not trying to count mobile chicks, scurrying into the tiniest of gaps! During the visit we encountered a number of predated eggs, which, like last year, look like corvid depredation. The tern eggs had been snatched, brought to a safe (from mobbing terns) spot, and eaten by the culprit. It's a fairly natural occurrence in the early part of the season, and as tern numbers build, so does their capacity to deal with intruders.
|Corvid predated Common Tern egg Richard Nairn|
When rowing out to the colony, we saw a small number of adult Common Terns "belly dipping" in the water in an effort to dowse the eggs to keep them cool in the strong June sunshine. This is a behavior often exhibited by tropical tern species, such as White-cheeked and Whiskered Terns in warmer climes, as the eggs can over-heat and the chicks more or less cook inside the shell! People often think that when a bird is sitting on the nest that the sole purpose is to keep them warm, but in fact it can often be done to shelter the eggs or nestlings from overheating.
|Arctic Terns will nest anywhere. Richard Nairn|
Our second visit revealed an increase in nest numbers: 487 and 91, Common and Arctic Terns, respectively. A total of 315 chicks, 42 of which were Arctic Terns, were ringed. We counted 555 Common and 58 Arctic Tern chicks, but there were still some eggs, which will hatch over the next few days.
|Common Tern chicks, ready to be ringed. Helen Boland|
All in all, its turning out to be a reasonable year for the port terns. Mean clutch size is on par with their Rockabill peers.
We will continue to monitor the fortunes of the colony and report back soon on progress.