Honey Buzzard as a 'Scarce Migrant'
in Britain in 2000
References:
Fraser, P A, & Rogers, M J, (2002), Report on
Scarce Migrant Birds in Britain in 2000, British Birds 95(12)
p.610-612.
Gantlett, S, & Millington, R, (2000), Honey
Buzzards in September 2000, Birding World 13: 363-364.
Mitchell, D, (2000), The Honey Buzzard Invasion: a
Once in a Lifetime Event, Birdwatch 101 p.60-61, 2000.
Roberts, S J, & Lewis, J M S, (2003), Observation
of Honey-buzzard Breeding Density in Britain, British Birds 96(1)
p.37-39.
Rossiter,
Nick, The Honey Buzzard Movement in Britain in Autumn 2000,
http://nickrossiter.org.uk/hbweb/hbsept2000.htm (2000-2003).
Wernham, C, Toms, M, Marchant, J, Clark, J,
Siriwardena, G, & Baillie, S, (2002), The Migration Atlas,
Movements of the Birds of Britain and Ireland, BTO, T & AD
Poyser.
Introduction:
The report of Fraser & Rogers (2002) covers the
largest movement of Honey Buzzards through Britain for at least 200
years. Some obvious queries arise:
We had been promised
in 2000 an authoritative account of this movement including its
origins. An eminent 'birds and weather' man had been asked to be a
co-author with Barry Nightingale. The request for information
(British Birds 93(10) p. 507) started:
"The unprecedented incursion
of European Honey-buzzards Pernis apivorus during
mid-September to early October 2000, concentrated on the English
east coast but also extending well inland, will be documented,
analysed and discussed in a forthcoming paper .... "
The obvious
questions:
Is this it?
Or has it proved impossible to write, given the
difficulty of establishing an origin other than Britain?
Notes:
The total reported is
1,975 compared to the previous highest maxima of a revised 200 for
1999 and 167 for 1993.
The total is
calculated as a grand total of the totals for each county. No
attempt has been made at removing duplication of birds as they pass
across different counties "as this would be an impossible
task".
From exact dates for
1,611 individuals, 1,277 (79.3%) were recorded in the last ten days
of September and 240 (14.9%) in the first week of October.
For the year as a
whole, Sussex was attributed the highest total (680) followed by
Essex (221), Kent (186), Yorkshire (130), Hampshire (113) and Dorset
(101).
A number of pulses
were detected in late September:
20th September --
east coast of England -- Cleveland-Thames
22nd-23rd September
-- Essex, NE England, some NW England
25th September -- NW
England, Midlands, SE England
27th September -- E
England N of the Humber
29th-30th September
-- the major arrival, mostly between Sussex and Dorset. Numbers
seen on these two days were two or three times greater than the
numbers seen on 20th, 22nd-23rd and 25th.
Many birds were seen
at roost sites (arriving or leaving).
An estimate was made
of the time spent in Britain:
Assuming:
a 'migratory day'
from 10:00-17:00
flying speed of
50km/hour
Then 350km would be
covered each day.
Consequently a Honey
Buzzard could pass through Britain in three days:
day 1 -- arrive and
roost
day 2 -- flying
throughout
day 3 -- depart
Therefore it is
speculated that the departure from the south coast on 29th-30th was
not of birds that had filtered slowly through the country.
The counts were complicated by the presence of
substantial numbers of Common Buzzard Buteo buteo.
Comments:
The model produced of
a three-day passage through Britain in 2000 is very much open to
question. In particular:
Spaaret al did find a ground
speed of 50km/hour (flap-glide).
However, this figure
of 50km/hour needs to be adjusted for the weather which is ignored
in the British Birds report.
Virtually throughout
the period from 19th September-28th September the winds were adverse
ranging up to 25-40km/hour (weather).
The birds were in effect 'blocked' until 29th September
by a persistent low-pressure system just to the west of Britain
generating vigorous southerly/ south-easterly winds and much
precipitation.
Even if the birds
flapped strenuously into a 30km/hour wind, they would only make
20km/hour over the ground (140km/day) and would become very tired
even at this rate of progress.
The birds are
therefore likely to make slow progress with frequent stops and would
try to delay a sea crossing.
Five to ten days to
travel 300-400km is a more likely rate of progress.
The grand total
produced of 1,975 assumes a rapid passage through Britain: 'the
three-day model'. Since this model is untenable in view of the
prevailing weather pattern, the calculated total needs to be
reviewed.
The obvious direction
is downwards.
The final large exit
as the winds became lighter on 29th-30th September and 1st October
consisted of birds blocked earlier by the adverse weather and winds.
Released, the birds then decisively moved S over the Channel.
Taking the numbers
exiting the south coast as the total moving through Britain is the
best way forward.
However, simply
aggregating the county totals along the south coast may not be
reliable as birds may coast before crossing.
In addition it looks
as if the county totals presented by Fraser & Rogers (2002) are
not the same as presented at the county level. For instance Fraser
and Rogers present a total of 680 for Sussex whereas the official
report
for Sussex indicates a total of 488 after eliminating
obvious duplicates but still possibly with some degree of
duplication. The Sussex report should be the more reliable estimate.
Other reports on the
south coast have not been further examined here. Adding together
Sussex as revised (488), Kent (186), Hampshire (113) and Dorset
(101) gives 888. Small numbers were also seen in Devon and Cornwall
so the total number exiting the south coast may be as high as 900.
However, the possibility of duplication needs to be considered and
the movement may have been smaller if the numbers in the other
counties are also inflated in the same way as those at Sussex.
An accurate total
cannot be derived from such an uncontrolled migration watch. Placing
it in the range 600-900 seems a reasonable outcome.
This new estimate of
600-900 is significantly higher than the first estimate of 400-600
in the present study Rossiter (2000-2003) and not out of line with
earlier estimates of 750-1,000 by Mitchell in Birdwatch (2000) and
500 by Gantlett & Millington in Birding World (2000).
How does this relate
to the British population of Honey Buzzards? Roberts & Lewis
(2003) estimate a population in the low hundreds. Rossiter
estimates 350 pairs. If we take the latter and assume a
productivity of 1.33 juveniles per pair ( 2000
Results), we get 470 juveniles bred this year. The post-breeding
population will be 1,170 birds but most adults will have gone by
late September. Assuming 25% of females and 0% of males remain, then
the movement should have involved about 560 birds which is somewhat
below but not inconsistent with the actual number noted in the
movement.
Wernham et al
(2002, p.692) emphasise that we are still building up a picture of
the Honey Buzzard in Britain: "Neither the breeding status nor
the movements of this species within Britain are fully understood,
largely because breeding pairs are unobtrusive ... some observers
withhold information ...".
It must be assumed
that no reports of significant continental movements or
concentrations have been found by the authors of the British Birds
report.
If indeed 1,975
birds had arrived from the continent it appears amazing that no
corresponding movements have been seen in Scandinavia, Germany or
Holland (leaving) or France (arriving).
For instance the
occasional large movements of Common Cranes Grus grus through
Britain have been accompanied by large concentrations on the other
side of the Channel and the times of movement have been consistent
with the timing of normal movement through Germany (Wernham et
al, 2002, p.699-700).
No controls were made
in Britain in 2000 of foreign-ringed Honey Buzzards ( Ringing
in Britain; Annual Report of Bird Ringing
in Britain and Ireland published in the
BTO's journal Ringing and Migration).
It appears that the
vast majority of the birds originated in Britain.
The distribution of
Honey Buzzards in 2000 is similar to that in earlier years with most
along the south coast and in eastern England.
The presence of
Common Buzzards is surprising. In Britain they are sedentary and
foreign-ringed birds are very scarce (Wernham et
al, 2002, p.238-240). Scandinavian Common
Buzzards also move about one month later than Honey Buzzards
(Falsterbo
2002). The difficulty of separating Common Buzzards and Honey
Buzzard juveniles needs to be considered.
The map in the British Birds report omits
Lincolnshire although a report was written by the county bird club
on the movement (see report
). If we add the 97 birds recorded in Lincolnshire we
then get a new grand total of 2,072. Do we believe this is now the
new total? Is this way of assessing the movement credible? No
indication is given of the origin of the birds.
Conclusions:
An
approximate figure for the total movement is 600-900 birds.
The
British breeding population of Honey Buzzards was estimated as 350
pairs in 2000. With productivity at 1.33 juveniles/pair and assuming
the movement comprised all the juveniles of the year, 25% of the
adult females and 0% of the adult males, then about 560 birds should
have been observed. This figure is slightly below but not
inconsistent with the size of the observed movement.
Two
years ago many people doubted that Britain supported enough Honey
Buzzards to produce the movement observed in autumn 2000. They were
wrong!
An
approximate figure for the total movement is 600-900 birds.
Two
years ago many people doubted that Britain supported enough Honey
Buzzards to produce the movement observed in autumn 2000. They were
wrong!