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.