Sunday, 26 August 2012

East Coast Terns have a tough time of it in 2012

The summer of 2012 has broken meteorological records, most notably in the amount of rainfall dumped on us and the lack of sunshine.  Amongst the seabirds, east coast tern colonies are amongst the largest and usually the most productive in the country, and although the 2012 breeding season broke records, a lot of them were not good news stories.

Terns are small graceful, swallow-like seabirds which perform spectacular plunge-dives to catch the fish that make up their diet. Most are greyish or white in colour with black caps and all five Irish species can be told apart on the basis of size and bill colour.  Little Terns are the smallest, with yellow bills tipped with black; Sandwich Terns are the largest with black bills tipped with yellow.  The three middle-sized terns, namely Roseate, Common and Arctic, have black or red bills.  All species winter along the west and southern African coast and migrate north to breed in Irish waters were they are reliant on a good supply of small fish, notably Sprats, sandeels and whitefish (Pollock and Saithe).

Ireland's five species of breeding tern: all have uniquely coloured bills that help distinguish them © Michael O'Clery 
On the east coast, we have seven principal tern colonies, working north to south:
  • Baltray, Louth – Little Terns
  • Rockabill, Dublin – Roseate, Common & Arctic Terns
  • Dublin Port – Common & Arctic Terns
  • Dalkey Islands, Dublin - Roseate, Common & Arctic Terns
  • Kilcoole, Wicklow – Little Terns
  • Wexford Harbour – Little Terns
  • Our Lady’s Island Lake, Wexford – Sandwich, Roseate, Common & Arctic Terns

All tern colonies are protected and most are managed or wardened to some extent by staff and volunteers from BirdWatch Ireland, National Parks & Wildlife Service and Louth Nature Trust.

Of all the species, Little Tern colonies fared poorest; two colonies are on ‘mainland’ beaches whilst the Wexford harbour birds nest on a sandbank.  All colonies, totalling 250-300 pairs, were sitting on eggs (2 or 3) when they were washed away in early June by high tides backed by easterly winds.  Some pairs re-laid eggs a week or two later but again lost these to more wind-backed waves, and those at our flagship Kilcoole colony were not able to lay a third clutch and gave up.  A small number of pairs at Baltray survived the later storms and 24 young fledged.

Little Tern  © John Fox
Record numbers of Roseate Terns nested on Rockabill, over 1,200 pairs; most of these reared a single chick, which was not bad considering the rain . . . though they do have cosy nest boxes to shelter in thanks to the woodwork students at Balbriggan Community College.  The Common Terns and Arctic Terns did not fare so well and fledged well under a single young per pair.

Roseate Terns on custom-made nestboxes on Rockabill island, off Skerries, Co. Dublin © Maeve Maher-McWilliams  
Dublin Port held record numbers of Common Tern nests (over 500) but once chicks started hatching, a mystery predator(s) started killing them.  Although not caught in the act, we believe rats had managed to swim out to the colony, which is located in the River Liffey, and climb the vertical wooden pilings and concrete walls that support the platforms.  Virtually all chicks were lost in this killing spree.  We now have the winter to work out how to prevent rats from accessing the colony next summer.

Dalkey Island supports a small mixed colony of terns and is overseen by members of BirdWatch Ireland’s South Dublin Branch.  Access to the colony was very limited due to the stormy weather and most early nesting attempts were lost.  However, some Arctic Terns may have fledged young on another islet.  One pair of Common Terns, from either Dublin Port or Dalkey, laid a clutch of eggs in late July on a yacht in Dún Laoghaire Harbour belonging to a member of the Royal Irish Yacht Club.  These hatched successfully and were moved to a specially adapted pontoon moored beside the yacht.  At the time of writing, they are thriving and we have discovered one of the parents was ringed as a chick at the Dublin Port colony in 2002: ten years old and still going strong!

A storm-wave washes completely over the main tern colony on Maiden's Rock, Dalkey, Co. Dublin © Michael Ryan
Our Lady’s Island Lake in Co. Wexford is home to our biggest Sandwich Tern colony and occasionally this has held 2,000 pairs.  This summer nesting numbers were down, at just under 1,700 pairs.  However, as an early nesting species they seemed to produce a decent number of fledged young.  Arctic Terns fared poorly; many nest close to the lake edge and were flooded or washed away with rising water levels due to all the rain.  Another mystery predator, this time probably a Stoat, killed a significant number of Roseate Tern chicks fairly late in the season and overall a maximum of 91 chicks probably fledged from the 126 pairs that laid eggs.  This ‘productivity’ was similar to that at Rockabill.

Let’s hope for a better summer in 2013?

Dr. Stephen Newton
Senior Conservation Officer - Seabirds
BirdWatch Ireland

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