Gannet – 2003-07 breeding

Morus bassana – HUGAN – Breeding resident.

Comparison with previous atlas:

Breeding confirmed12
Breeding probable
Breeding possible
No of tetrads occupied (of 478) (of 490)
Percentage of tetrads0.2%0.3%

Despite being relatively widespread around the coast, gannets are confined to breeding on just one small remote island, Grassholm – about 12 miles off the Pembrokeshire mainland. Here they nest at very high density, the only breeding colony for this species in Wales. Because of the importance of the Gannet population on Grassholm (in European and indeed World terms) the island has been designated a Special Protection Area for them (under the EC Directive on the conservation of wild birds (79/409/EEC).  

Counting the population is a special operation requiring skilled observers. The colony is now so dense that visitors are no longer able to land on the island as this causes too much disturbance. Because of this, assessments of the colony population size have to be made from aerial surveys usually done in late summer. This is done by counting the number of apparently occupied nest sites (AOS) observed in photographs. This process is now considerably aided by modern high quality digital photographic equipment and computer mapping software.

The most recent aerial survey during the 2003-07 atlas period was undertaken in July 2004 by ornithologists from the Centre for Ecology and Hydrology with a follow up ground survey (after the young had fledged) in September the same year to confirm the colony boundary areas. (Wanless et al 2005).

During the 1984-88 atlas period, the population estimate was 28,600 AOS in 1984–85, with an estimated 30,000 AOS in 1986 (Lloyd et al. (1991).

By 1999, the population was an estimated 30,688 AOS. In 2004 the population had risen to 32,094 AOS; an increase in colony size between 1999 and 2004 of 4.6% (a rate of approx 0.7% per annum). Comparison with other UK and Irish Gannet colonies in 2004 indicated that Grassholm is the third largest northern Gannet colony in Britain, supporting approximately 12% of the UK and Irish population, about 8% of the World population (Wanless, et al, 2005).

If the well documented population expansion and gradual spread of the colony across Grassholm continues, it is interesting to speculate where a new Gannet colony may form if they eventually run out of space! For a few years, (during the 2003-07 atlas recording period) one Gannet appeared to be resident on St Margaret’s Island each summer, making a nest and even laying an egg in one year. However, no mate was ever confirmed and successful breeding did not occur, so Grassholm currently retains its status as the only Gannet colony in Pembrokeshire and in Wales.  

Bob Haycock (BTO rep & Chairman of the Pembs Bird Group)

Records extracted from the Pembrokeshire Bird Reports, which may contain more detail than shown here.

LLOYD. C, TASKER. M. L and PARTRIDGE. K. 1991.The status of Seabirds in Britain and Ireland, T & A D Poyser, London

WANLESS. S, MURRAY. S, HARRIS. M.P. and EVANS. S, 2005. A count of the Grassholm Gannetry in 2004. CCW Contract Science Report No. 604.

Update: Since the atlas fieldwork period was completed, another aerial survey was undertaken in 2009. This survey produced a mean population estimate of 39,292 AOS. Between 2004 and 2009 numbers of Gannets on Grassholm had therefore increased by 22.4%, at a mean rate of 4.1% pa. Grassholm remains the third largest gannetry in the UK; by 2009 holding approximately 15% of the UK and Irish population and 9.5% of the world population. Murray (2009).

MURRAY. S. 2009. A count of the Grassholm Gannetry in 2009. Contract Report to RSPB/CCW

More about the Gannet in Pembrokeshire