How many Common Terns

Pager watchers and viewers of the Spurn web site http://www.spurnbirdobservatory.co.uk/ will have noticed the huge numbers of Common Terns passing south past the peninsular each evening and sometimes north in the mornings; all of these birds come south into Lincolnshire but where they all go and just how many birds are involved through the autumn is still a bit of a mystery; along the Lincolnshire coast finding an observer in an evening in August is a rare event! in recent days I have been concentrating on a favoured roost site where there have been up to 5000 birds but getting accurate counts while feeing from spring tides and trying to cover a couple of miles of beach in fading light is never easy; observations show that large numbers of terns continue south offshore while the flocks are arriving at the roost site; on the evening of the 25th I counted birds passing at a rate of 500 per minute for 20 minutes before concentrating on perched birds; even observations of birds on the beach where birds such as adult Black Terns can be identified to individuals suggests that many rest in the roost and then move on before nightfall down the coast but how far do they go; birds are usually still moving an hour after sunset.

Checking in BWPi shows that the British population is only around 10,000 pairs but about 160,000 pairs nest in adjacent countries of Europe and Scandinavia with an additional 50,000 - 250,000 in Russia. On Migration BWPi update states

Autumn Migration

Johansson and Jakobsson (1997; based on counts) estimated 110 000 Common Terns passing south Sweden during autumn migration, plus presumed 20% more during night. Common Terns migrate over broad sector crossing south Sweden, attracted by large lakes along migration routes in south-west direction ( Johansson and Jakobsson 1997; see also Lemmetyinen 1968). At Hanstholm (north-east Jutland/Denmark), 54 000 terns (Common and Arctic) counted autumn 1984 (Brandt 1985). North and east European populations migrate westwards to Great Britain, route follows west European seaboard as that of other western populations. During autumn 250 000–400 000 Common Terns migrate over Netherlands; daily maximum (August–September) Wadden Sea >3000, along coast maximum nearly 5000 (Lensink 2002). Autumn migration at French North Sea coast peaks in late August/early September, up to 10 000 birds (e.g. 1994) per day, on average 2000–3000 birds per day (Dubois et al. 2000).

So clearly very large numbers of Common Terns may be passing down the North Sea each day and single day counts may well need to be summed to reflect the total number of birds passing through although a proportion of birds do obviously roost and return north each day presumably feeding up prior to migration.

In the bottom 2 shots I have spotted the tern in census fashion -- 1183 if you had a guess