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.
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 Divers, Skogsøy, May 2019
Red-throated Diver migrating in strong headwinds, October 2018
Above and below: Migrating Red-throated Diver at Skogsøy,
April 2018
Adult Red-throated Diver, Þingvellir,
Iceland, May 2022
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
