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Red-throated Diver - SmålomGavia stellata
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Red-throated Diver
Photo: Alf Tore Mjøs, Norway May 2006

Red-throated Diver is a common and numerous spring migrant at Skogsøy with a peak sometime in early to mid-May. Counts of over 600 would appear to be reasonably regular - possibly an annual event, although this is hard to define due to lack of consistent coverage. Small numbers are observed from the end of March, gradually increasing through April before the peak in early to mid-May after which numbers rapidly drop off. A rough estimate of the total number of Red-throated Diver passing Skogsøy, based soley on my own data and assuming a 10-hour migration day for this species, is around 7000 individuals.

With this kind of passage it seems that a significant proportion of the Norwegian population (estimated at 2000-5000 pairs) passes each spring. Of course, a proportion of these birds may belong to other populations. Red-throated Diver winter around the North Sea and as far south as the Bay of Biscay.

Skogsøy thus "outperforms" other bird observatories such as Utsira and Fair Isle. However, significant numbers are recorded earlier in the year further south at localities such as Cap Gris-nez in France.

The species is very regular in the autumn, although the timing of the passage is more drawn out and maximum daily counts are somewhat lower than in the spring. Small numbers are noted during the summer months but larger numbers do not occur before early September, passage then continues through towards the end of November after which I have not recorded any.

Numbers generally peak late morning / early afternoon and by then many flocks are passing high up so it can be challenging to spot everything - looking low over the sea and high up in the sky is best suited for more than one observer.

The following diagram shows the average recorded in a given three day period (green line, scale on right hand axis). In order to give an idea of the frequency of observation observer effort is presented as the blue line (number of visits) and the orange lines (number of days the species was recorded). More comprehensive data is available on the Skogsøy downloads page.


Maximum counts of Red-throated Diver at Skogsøy

Red-throated Diver occurrence at Skogsøy, SW Norway
Number of Red-throated Divers by week number at Skogsøy (artsobs)

Red-throated Diver populations are poorly monitored due to their dispersed populations in both the breeding and non-breeding seasons
. Regular counts at suitable points during the migration seasons would seem to be the most effective way of finding out about population trends.

Migrating Red-throated Diver, Skogsøy, Norway, May 2020
Migrating Red-throated Diver, Skogsøy, Norway, May 2020


Red-throated Diver, May 2019

Migrating Red-throated Divers, Skogsøy, May 2019

Red-throated Diver migrating in strong headwinds, October 2018
Red-throated Diver migrating in strong headwinds, October 2018

Migrating Red-throated Diver at Skogsøy, April 2018
Above and below: Migrating Red-throated Diver at Skogsøy, April 2018
Migrating Red-throated Diver at Skogsøy, April 2018



Record:
The record day count is 1332 individuals (01 May 2020).

Weather:
In common with the other diver species the best numbers during both spring and autumn are almost invariably seen during headwinds. This may be due to birds being forced lower over the sea and further inshore; alternatively it could be that the divers actually prefer headwinds as these give them more lift. During tailwinds there is often a tendency for flocks to fly higher and further out - making them harder to observe.

References:
Tveit, B.O., Mobakken, G. og Bryne, O. 2004 Fugler og fuglafolk på Utsira. Utsira Fuglestasjon
Ellis,P., Harvey, P.,Heubeck,M.,Okill, D., Osborn, K.,Pennington,M.,Riddington, R. Birds of Shetland. 2004.
Bakken, V., Runde, O. & Tjørve, E. 2003 Norsk Ringmerkingsatlas Vol. 1, Stavanger Museum, Stavanger



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