Wednesday, 22 July 2015

A Fly on the Wall Look at the Dublin Port Tern Colony

Due to tern chick predation by a mystery predator, we deployed a motion-triggered “trail camera” in an effort to identify the visiting culprit. 

When we retrieved the memory card some days later, we were disappointed not to have identified the predator, but we did capture an intriguing insight into the activities and routines of the terns when completely undisturbed on the colony. 

In the three days, the camera was triggered 2,088 times, and after meticulously scrutinising each photo, we discovered that the platform is not just home to terns at this time of the year. 

Nocturnal squatters started to appear around 8 pm each evening and stayed overnight, roosting on the boundary planks and within the nesting compartments until shortly after 5 am each morning. Although the camera was directed into just one nesting compartment, it was clear from the sequence of photos that Starlings use the structure as a night time communal roost with very little, if any, reaction from the nesting terns.

Adult and juvenile Starlings get some sleep among the nesting terns
Ricky Whelan
The early bird gets the....sand eel (bird centre foreground) Ricky Whelan

We also managed to make some observations on the hours that the terns keep. They became active between 4 and 4:30 am each day, and the earliest food delivery occurred at 5:19 am. As expected, the adults spent their day diligently coming and going with food, resting and occasionally brooding young chicks and eggs. Sporadically, the terns do what’s known as a “dread.” Dreading is when the entire tern colony suddenly and silently (unusually for terns!) takes to the air and vacates the nesting area and performs a few looping laps of the nesting site, before returning to business as usual. Dreads are pretty much over as soon as they start, and in general, last less than about 10 seconds. This behaviour is associated with the terns being startled by an avian predator, but often occurs with no apparent stimulus.

Dreading adult terns leave the nesting colony briefly Ricky Whelan

Peace at last. Common Tern chicks and adults roosting for the night
 with Starlings tucked in along the left hand edge Ricky Whelan

As the light fades, the frequency of prey deliveries declines and everyone is settled down for the night by about 10 pm. 

So, while we didn't manage to get confirmation on the mystery predator, (which we suspect is Mink or Rat), we did get some interesting insights into the tern activity on the platforms when they are left to their own devices. The effect that this predation has had on the overall colony success this season has yet to be measured, but will be quantified by the end of the season.

Tuesday, 7 July 2015

Tern Talk

School’s out, but we’ve still got plenty of work to do. As the seasons pass, we see huge differences in the composition of the birdlife in Dublin Bay. And new arrivals mean different survey types to adequately determine how they are all doing. Tern time is always a very busy time for us… which may explain the lack of blog posts recently! 

Many of the waders and wildfowl are now long gone, with only a fraction of their winter numbers remaining. Last February, we had a total of 38,854 waterbirds in Dublin Bay during a low tide survey. The corresponding number in June was just 3,566. But that doesn’t mean that Dublin Bay is not important for waterbirds in the summer -far from it, in fact, but that’s for another post!

But it’s tern time for us now, and it will be until around about the time that the kids go back to school and the terns go back to Africa (and beyond). So, right now, we’re flat out doing nest censuses, ringing and colour-ringing chicks and assessing foraging locations within Dublin Bay. And before too long, we’ll be assessing the numbers of post-breeding terns roosting on Sandymount Stand in late August and early September. At which point, the waders will be flying in and the wildfowl won’t be too far behind them. So, it’s always worth taking the time to enjoy the breeding season before it passes...

Common Terns  Niall Tierney
Arctic Tern nest at the foot of a mooring bollard Niall Tierney
Arctic Tern nest Niall Tierney
Arctic Tern nest Niall Tierney
Arctic Tern nest Niall Tierney
Erythristic Arctic Tern eggs Niall Tierney
Common Tern nest Niall Tierney
Common Tern nest Niall Tierney 
Newly hatched Common Tern Niall Tierney

Common Tern chick Niall Tierney
Common Tern chicks Niall Tierney

All photos taken under NPWS licence.