Summer holidays in Dublin Bay

We know quite a lot about the waterbirds that spend the winter in Dublin Bay, with decades’ worth of monitoring through I-WeBS (the Irish Wetland Bird Survey) and its predecessors having shaped our knowledge and formed a large database of information that can be drawn on. But year-round counts by the Dublin Bay Birds Project since 2013 have revealed a few surprises for summertime in the bay, showing that significant numbers of waterbirds are present outside of the core monitoring period for I-WeBS.

Black-tailed Godwits staging in Dublin Bay Liam Kane

I-WeBS counts have shown that Dublin Bay regularly hosts 33 waterbird species, and wintering numbers regularly exceed 30,000 waterbirds. Among those wintering are large numbers of ducks, geese, waders and gulls, with Light-bellied Brent Goose, Knot, Black-tailed Godwit and Bar-tailed Godwit occurring at internationally important levels. 

The general pattern is that bird numbers across the bay fall sharply after March, and are at their lowest levels in May and June. And then, from late June onwards, we start to see the numbers rising again, and several thousand birds are added each month, until the annual peak in January. 

Average number of waterbirds and seabirds recorded during monthly 
low tide counts between July 2013 and June 2015. Bars refer to the 
total number of birds recorded. Line refers to the average number 
of species recorded. I-WeBS core season = grey, I-WeBS off-season = green. 

In spring, the majority of our waterbirds head for their breeding grounds, and by the end of March many species have disappeared almost entirely. As the days draw out and the temperatures rise, we see the waterbirds in the bay becoming scarcer by the day. Spring soon gives way to summer: the lifeguards are posted on the beaches; the Little Egrets display their wavy plumes, and the skies are filled with melodious Meadow Pipits and Skylarks, raucous Common and Arctic Terns, and the buzzing chitchat of Swallows and Martins on their high-speed aerial chases. But our year-round surveys have shown that it’s not just terns and songbirds that provide the soundtrack for Dublin Bay’s summer. The mudflats are far from bare and silent - Bull Island, Sandymount Strand and the Tolka Estuary provide a rich species list for any waterbird lover.

Colour-ringed Bar-tailed Godwit "DH" in summer plumage Kim Fischer 

In summer, Brent Geese, Wigeon, Teal, Shoveler and Pintail are absent, as are Red-throated Divers, Red-breasted Mergansers and Great Crested Grebes. And like the waterfowl, Knot and Grey Plover are winter-only inhabitants of Dublin Bay, with the vast majority of them recorded between September and March. 

But this isn’t the case for all waders and the bay is particularly important for some species during the summer months. Whimbrel, a passage migrant in Ireland, is practically absent from the Dublin Bay I-WeBS dataset, as it only appears in the I-WeBS off-season. Highest numbers occur in May, when birds are moving northwards to breed, and in July, when they are en route to their wintering grounds. 

Black-tailed Godwits occur in the bay in internationally important numbers through the I-WeBS season, but April and August see significant numbers too. The spring influx starts in March and the birds are still present in April, before disappearing off to Iceland to breed. By July there are birds back in Dublin, and by August their numbers have risen to exceed the international threshold once again.  

Oystercatchers, too, are a Dublin Bay staple, being present in each month of the year. Numbers during the winter are much higher, but the threshold for national importance is exceeded in four out of the five I-WeBS off-season months, showing how important the site is for them during the summer.   

Average number of birds counted per month 
on monthly low tide counts between 
July 2013 and June 2015. The orange lines refer to 
the 1% threshold for national importance, and the grey 
line refers to the 1% threshold for international 
importance. Note that Whimbrel numbers in Ireland
 do not reach the international threshold and no 
thresholds of national importance are available. 

So, through this project, we have been able to appreciate the year-round importance of Dublin Bay for some waterbirds, and have been able to see just how important it is as a staging site in spring and autumn. Although the official I-WeBS period is September to March, the I-WeBS Office will very happily accept any counts taken in the off-season, as they may shed new light on the importance of other sites during this period.