Honey Buzzard as a 'Scarce Migrant' in Britain in 2000
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.
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?
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:
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.
The model produced of a three-day passage through Britain in 2000 is very much open to question. In particular:
Spaar et 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.
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!
© Copyright Nick Rossiter 2003