Passage migrant. Not recorded from December to February
It is not always possible to separate Common from Arctic Terns in the field unless they are close and seen well. However, most observers still wish to record what they see and log these birds as either “Common or Arctic”, usually expressed as “Commic Terns”. The following is a review of such records.
There has been a marked difference in the nature of spring and autumn passages. There are many breeding colonies of both species to the north and west of Pembrokeshire along the coastlines of both western Britain and eastern Ireland, which are probably the origin of most passing the county. Whereas there has usually been a strong passage of birds migrating southwards from these colonies in the post breeding season, comparatively few have been seen passing northwards on their way to the breeding grounds. Although there are juveniles as well as adults swelling the population in the autumn, this alone does not fully explain the discrepancy in the volume seen.
It seems more likely that having spent a pelagic winter season, most spring birds pass Pembrokeshire out of sight of land. This would explain why the majority that are detected are to seaward of the islands beyond the west coast, and why increasing numbers are seen from land further up the narrowing Irish Sea. It is interesting to note in this context that in the spring of 1984, when direct comparison was possible, twice as many were seen passing The Smalls compared to Skokholm. As with other species of migrant birds, spring passage is performed with a sense of urgency to reach the breeding grounds at an optimal time, so is a rapid event. By comparison autumn migrants do not need to travel to their wintering areas according to such a strict schedule, being able to pause and accumulate at rich feeding sources en route.
Many more Common than Arctic Terns are identified in the county, so probably make up the majority of reported “Commics”.
Many more pass through on autumn passage, July to October, than in spring, with occasional accumulations of 100 to 800 birds having been noted off St Ann’s Head, Skokholm, Broad Haven (north) and sea area between Point St John, St David’s Head and the Bishops and Clerks as far out as Bais Bank. However, the majority have been recorded passing along the north coast at Strumble Head. Normally up to 30 per day were seen but periodically larger passages occurred, the largest on record being 190 on the 11th September 1984, 458 on 2nd September 1988, 459 on 27th August 1990, 256 on 11th September 1992, 363 on 4th September 1997, 375 on 1st September 1998, 199 on 17th August 2002, 726 on 31st August 2005 (an additional 151 Common Terns identified as well) and 501 on 8th September 2009.
These large movements have occurred with moderate to strong south or south-east winds, usually accompanied by poor visibility due to rain or drizzle. The terns have arrived on a north-west to south-east track, suggesting they had come from the Wicklow coastal area. The exception was on the 27/8/90 when they arrived from a north-easterly direction, presumably caused by a previous accumulation in Cardigan Bay moving on en masse, there being a moderate south-west wind and good visibility at the time.
Spring passage has been recorded from the 29th March to mid June, with stragglers to the end of June, the majority of birds being seen in late April and the first three weeks of May. Most were seen to the west of Skokholm, Skomer and Ramsey, with very few along the north coast but small numbers off the south coast. Most sightings were of one to five birds but up to 18 together have been seen. Those seen moving along the south coast could conceivably have continued their migration up the Bristol Channel and through the Severn valley.
More about Commic Terns in Pembrokeshire