Honey-buzzard Migrants are British Born and Bred
The imminent delisting of Honey-buzzard as a scarce migrant in the annual British Birds reports has great significance for the accepted status of the Honey-buzzard in Britain. The delisting was reported in Scarce Migrants in Report on Scarce Migrant Birds in Britain in 2014: Part I: Non-passerines, British Birds 109(12) 724-748 by Steve White & Chris Kehoe, including Honey-buzzard account pp.735-736 p.735 p.736 and in Abstract p.724.
Relevant extracts from the report are:
[Main account: pp.735-736] As the British breeding population slowly increases – estimated at 25-39 pairs, from which at least 27 young fledged in 2014 (Holling et al 2016) – it becomes increasingly difficult to maintain any meaningful distinction between passage migrants, wandering non-breeders and breeding birds. In 2014, the first occurred in Argyll on 3rd May and in Greater London on 5th, with spring movements continuing until early June. Autumn passage was somewhat larger, extending into early October, the last in Sussex on 11th October.
An unknown number of Scandinavian migrants do presumably occur annually but a large proportion of the 130-150 birds that have appeared in recent years (around 60% of them in autumn outside of influx years) may simply be part of the breeding population, although it is perhaps just a coincidence that the numbers in autumn each year broadly reflect, or at least do not exceed, the size of the British post-breeding population. For this reason we no longer intend to report on this species in future Scarce Migrants reports, although we will attempt to collate and publish records for major influxes such as that in autumn 2000.
[Abstract p.724] Records of migrant and breeding Honey-buzzards Pernis apivorus have become increasingly blurred and the species appears in this report for the last time.
Comments by NR:
There are many questions arising from the delisting. So far the credo has been that all Honey-buzzard flying around in the UK are Scandinavian migrants. That did give some consistency even if it was plain daft in the light of the physics of broad-winged raptor migration. Now a brick has been removed from the wall and we seek a new set of consistent principles, which are likely to disappoint many people who would rather the Honey-buzzard was forgotten about altogether!
I'm not sure the authors have thought through their comments with any thoroughness. The migrants are reported on a broad front, often to the north of the official Honey-buzzard population area in southern England and south Wales. So if they are equating passage numbers with breeding numbers, it would appear that many migrants are over-shooting their breeding areas, which is surely unlikely for experienced adults. There is also the ridiculous assumption that all passage birds are spotted and identified correctly by birdwatchers and that all breeding pairs are discovered and known for what they are. For a bird that migrates on a broad front at considerable altitude, most Honey-buzzard on passage will be missed by the recording system. For a secretive breeder, difficult to identify and nesting in remote areas where few birders go, it is probable that only a fraction of the nest sites are found. So if the count of passage birds roughly equals that expected from the breeding population, it is a coincidence.
It cannot be presumed that any Scandinavian migrants occur in Britain annually. Ringing returns, observers on oil rigs, detailed analysis (by NR) of the 2008 and 2008 movements, the physics of broad-winged raptor migration, observations of birds resisting drift to W on the continent, and lack of correlation between continental populations and movement sizes, all provide no support for this speculation.
What is very welcome though is the shift in thinking away from the notion that every Honey-buzzard migrant in Britain is a continental bird. Seeking a new set of consistent principles is going to be fascinating, without admitting that the UK breeding population of Honey-buzzard far exceeds that reported by the RBBP and that the 2000 and 2008 movements are not influxes but just exceptionally visible passage of British-bred birds, blocked by adverse weather. The recent Scottish Birds paper discussed elsewhere provides welcome support for the existence of a much higher population of Honey-buzzard in northern Britain than previously suspected by some 'authorities'.
Of course a vital question is: how many migrants are there across Britain. The official figures in the Scarce Migrants report show a mean of 136 from 2010-2014. However, many more are reported on BirdGuides, where final totals for Honey-buzzard in UK for 2016 are: April 1, May 45, June 15, July 39, August 79, September 65, October 14, November 0; total 258. These are counts of records not of birds. Some reports refer to the same locality and others, on the other hand, involve multiple birds. It's an impressive total, particularly for a species which is 'so rare' in the UK. All the sightings are opportunistic by BirdGuides subscribers so IMHO represent a small proportion of those actually on the move. Nearly all the birds seen are either UK-breeding or UK-bred: there is no evidence for a significant fly-over of the UK by continental birds as confirmed in the Scarce Migrants article. If you assume that only 10% of birds actually moving are recorded on BirdGuides and make some deduction for juveniles in September, a UK population is indicated of around 1,000 pairs!
It is of course also worth questioning the accuracy of the official totals, which are far below the gross reported numbers. From a scientific point of view it may well be a triumph of precision over accuracy: we have a few very nice (precise) descriptions but the official totals are highly inaccurate when compared to the real number of birds frequenting the kingdom. In information retrieval terms, such as a search on Google, the request is so precisely (tightly) specified that the items retrieved are indeed relevant but many items are missed, giving low recall: an inaccurate reflection of the real world. The information on p.302 Separating Common Buzzard and European Honey-buzzard, by Dick Forsman in Flight Identification of Raptors of Europe, North Africa and the Middle East, Christopher Helm (2016), could almost have been written for bird record committees in Britain who seem to have a thoroughly out of date approach to identifying Honey-buzzard, particularly the juveniles, resulting in a high-precision approach that does not even address the critical feature necessary to identify the species. High precision inevitably leads to low recall through scientific trade-offs. This material by Forsman will be reviewed shortly by NR.
(c) Copyright Nick Rossiter 2017