From: Nick Rossiter <nrossiter@supanet.com>
To: Mike Crewe <Mikecrewe@runton.freeserve.co.uk>
Subject: Re: [UKBN] Honey Buzzard Movement Autumn 2000
Date: 07 November 2000 00:21
----- Original Message -----
From: Mike Crewe <Mikecrewe@runton.freeserve.co.uk>
To: UK Birdnet <ukbirdnet@dcs.bbk.ac.uk>
Sent: Monday, November 06, 2000 8:40 PM
Subject: Re: [UKBN] Fw: rubbish
> I have been desperately staying out of this debate, partly because it is
> becoming hopelessly entrenched, partly because I put forward some questions
to Nick right at the start which were never answered (so I lost interest)
Sorry Mike for this but I could not reply to every message. I could not actually
find yours in my mailbox. So you could send me a copy privately if you like.
> and partly because I wanted to sit back and see which way the discussion
> developed.
>
> I have to say I agree totally with Martin Kitching with regard to childish
> and defamatory comments made to and about Nick, although Nick has not
> exactly been conplimentary by failing to accept comments from people with an
> excellent local knowledge of their areas and their calculated comments with
> regard to the birds (e.g. Brian Unwin's comments on the state of the
> arriving birds and comments from the Gib Point warden - I would always take
> a local warden's word rather than anecdotal evidence phoned in by a visitor
> to a pager service).
Mike, this may be the case but discrepancies in accounts are always worrying. I should point out that pager records are not anecdotal (meaning narrative of amusing or interesting incident). They are original evidence (information given personally and tending to prove fact), far superior to recollections. They record what the observer thought at the time without subsequent interpretation (biased or otherwise). The use of pager records to challenge claims may not be entirely popular but it's a very valid technique.
> I admire Nick for his persistence and for the remarkable amount of time and effort he has put into his prepared argument. However, both Martin and Nick must be aware that the vast majority of comments opposing Nick's views have been sensible and
> reasonable, and not defamatory in anyway. Whilst I am sure there are
> supporters of Nick's theory who are staying 'off air' for fear of abuse,
> equally there are many (like me until now) who disagree totally with Nick
> and are staying off air because it has all been said by someone else - or
> perhaps some have just lost interest in what, for them, is a clear cut case.
>
> I think one issue that is getting bogged down is the argument over how big
> the UK population is. Yes, Honey Buzzards are incredibly hard to find and
> census (sorry James you're wrong!) and yes, I am sure that the UK population is larger than currently estimated - but not by the sort of figures that have been discussed. I think we should forgot the size of the UK population and concentrate on the point(s) at which the birds were first located, their onward direction and the overwhelming evidence from the weather patterns.
I'm not starting from this point. If you think of it, it's a biased starting point because it prejudices the whole of the subsequent discussion. It seems more likely from my field experience and the views of some other field workers that HBs have colonised the abundant forest areas in northern Britain than they have suddenly become drift migrants on a massive scale for the first time in around 200 years (see Unprecedented in almost 200 years ).
The secretive Spotted Crake is interesting here: 31 pairs in RBBP in 1998 rising to 72 in a targeted survey in 1999 and this for a bird with restricted habitat. I'm not criticising the RBBP -- secretive species require enormous inputs of human time and the targeted surveys necessary are not feasible every year.
> I am also interested in Martin Kitching's comment ... "the scale of the movement is unlikely to be repeated though" ... eh? Why??? If these are UK birds, surely this will be a regular feature! Perhaps the population has reached a critical mass, whereby it can no longer sneak out unseen? Surely if the arguments for a UK origin are to hold water, we can expect more of the same next year. Maybe not at the same sites or dates, but at least something resembling what we have seen.
I agree with Mike that there must be a long-term increase in sightings if my hypothesis is correct. The trend in the numbers of observed birds on migration though will be erratic (see below) so I (and Martin, quite rightly) are not going to make firm predictions for a single year such as 2001.
>
> So far, I feel that Nick has singularly failed to produce the following:
> 1. Convincing weather data to explain a UK origin.
I think the sudden bad weather from 18th-20th September was a very good trigger. Temperatures dropped and the first great rains of the autumn occurred. The weather in Northumberland was incredibly stormy in this spell with amazing 'cloudbursts'. Ian Newton (Population Ecology of Raptors, Poyser 1979) says at p.198: "The passage of a cold front to the north is often followed by a strong movement in autumn". On 20th September this was exactly the weather pattern over northern Britain.
See also Accompanying Species for an account of other species involved and a pointer to evidence from ringing.
> 2. Convincing weather data to discount a Continental origin - given that we
> don't know the origin of the birds (and it doesn't have to involve Falsterbo
> Nick, they could have come from much further east via a more southerly
> route, for example) it is difficult to point to weather trends in one part
* of Europe and make a comment.
The SE wind could have brought birds over from the continent. It was not that strong on the continental side -- our Dutch correspondent (Norman) said force 6 (see Eurobirdnet comments ) and in Denmark it was no more than 29-43km per hour (force 4-6) around 20th at an exposed west coast locality (see Movements in Denmark ). The wind did not bring over any unusual volume of other migrants.
I would have thought that HBs, being strong fliers, would not be helpless in a force 6. Indeed if you look at the Danish evidence, you will see small parties of HBs recorded moving SE into a SE wind of up to 40km/hour.
I would also ask why in 200 years of ornithological observations, no significant drift movement of Honey Buzzards has occurred into Britain before (see Unprecedented influx in almost 200 years ).
The weather conditions were not exceptional. I think the ball is really in the court here of those who support the continental origin. Where did the birds come from? Why did we get a drift movement for the first time? Why was it unobserved on the continent? Why was the movement so late, occurring after the end of migration at normal observation points? There are also only two recoveries of foreign-ringed HB in the UK -- from Germany in 1973 and Sweden in 1976. One of these was 4 years old (see Ringing Data for Raptors ). Further details would be welcome. An event happening for the first time in around 200 years needs a convincing explanation, migration-wise, and this has not been forthcoming.
> 3. Convincing evidence that the 'unknown' UK breeding population (which I accept exists) is anywhere near big enough to produce the numbers seen.
Mike, we should wait for the HB census results and interpretations.
> 4. Any evidence at all that a small (on a World scale, Nick) population such
> as the UK's could produce the very wide plumage variation noted in the birds
> involved (this point not addressed at all yet). (Ref. early discussion on
> variation in Common Buzzard plumage which is much greater on the Continent
> than in the UK (in areas of comparable size) and this is already known to be the case with Honey Buzzard).
This is an interesting observation. The juveniles raised in my study area have always been pretty mixed (since 1993 when I first noticed them). I suspect that we may be seeing, as stated in my article, some colonisation from the continent from the large populations in Holland, Belgium and above all from north-west France (an easy route into Britain). There are a number of ringing recoveries where raptors bred in one part of western Europe turn up in another part in a subsequent breeding season (Marsh Harrier, Osprey for instance).
At least some birds may be arriving from these sources, most passing quickly into the lush northern and western forests. Such stray birds would have been shot in many parts of eastern Britain before 1990.
As an aside, I like your comment on the smallness of the British population. At 250 pairs (for argument's sake), only Ireland (0 pairs, apparently), Portugal on the southern limit (10-100 pairs) and Luxembourg (100-150 pairs) would have smaller populations in the more maritime parts of western Europe. Note the big ranges everywhere with this species but it's one of the commonest large raptors in Europe. See British Population is Still Relatively Small for further details.
> 5. Convincing evidence that the UK population could normally sneak out the
> back door unseen (whilst presumably increasing almost exponentially each
> year) then suddenly flaunt itself en masse to all and sundry in a single year.
It's not actually that unusual for a secretive species to suddenly appear to be much commoner -- look at the Hobby which I mention in my article. At low density, a species can be very inconspicuous particularly if people are inexperienced in locating them.
Migration of broad-winged raptors is also a complex business to study. Ian Newton (same book, p.199) says that "Visual counts cannot therefore be expected to give anything but a minimum (and possibly misleading) estimate of the numbers of birds migrating. They do however give an idea of relative numbers and species composition at different places, of the migration seasons of particular species, and of sex or age differences. Repeated over enough years, such counts also help to reveal long-term population trends though year-to-year fluctuations are usually great".
Ian Newton also says: "Winds are crucial to the volume of migration that can be seen by an observer on the ground. They influence not only the numbers of birds migrating, but also the proportion of migrants which pass over the observation point (as opposed to passing elsewhere or on a broader front), and the proportion of those over the observation point that pass within visual range". It appears that tail winds enable raptors to pass at high altitude on a broad front while adverse or cross winds encourage a concentration at a lower altitude. Ian Newton also says that the proportion of juveniles can be exaggerated by observers because they fly at lower altitude than adults.
If you look at the Falsterbo pages (Fågelobsar i Skåne, http://www.skof.se/obs/index.html , click on Rovfågelsträcket, then Se Diagram in turn for Pernis apivorus and Buteo buteo), you will see incredibly bunched movements which could be easily missed. If these bunched movements occurred with a tail wind, an observer might see very little. Only in the last two to three years has perhaps the HB population in Britain been large enough to give much of a show but perhaps the exit conditions have been favourable. Also there is not the experience in Britain yet with raptor movements although rapidly rising populations of our breeding species deserve much better study. Also note that the Common Buzzards move mainly one month later than the juvenile HBs.
* 6. Any convincing evidence at all that these were NOT Continental birds!!
I would counter: any convincing evidence at all that these WERE Continental birds!!
Following Stuart Reeves comments on other routes near Falsterbo, I've checked data from a number of Danish observatories (see Movements in Denmark ). The conclusion is that a Danish origin is also most unlikely. Just where is the source?
Reverting to your question, I lay out a number of reasons in my article why the movement through the UK seemed strange for an eastern origin. I pose some questions but I do not think they have been answered to my satisfaction (others may differ). You can also look at some of the remarks on Eurobirdnet ( Eurobirdnet comments ) when the influx started from Svensson and others. I regard the idea, that the birds must have come from the continent because of the numbers involved, as weak. The larger the movement into the UK, the less reasonable it seems that no source has been located on the continent. It's a massive number of birds to have moved from abroad with nobody noticing them moving or leaving. The large numbers are a problem for all the hypotheses -- the improbable versus the unlikely, some might say.
In some respects, it is the inability of people to prove a continental origin that has caused the heat in the debate. Frustration has been all too evident.
I'm actually in no hurry to resolve the issue and am quite happy to rest my case. We should all be continually on the look out for further information and I'm sure a conclusion satisfactory to most people will be arrived at eventually, even if it's after another few HB breeding seasons! What is clear in my mind is that we have four hypotheses: 1) direct origin from Scandinavia, 2) origin from Scandinavia via Germany, 3) origin from eastern Baltic (east of Falsterbo, anyway), 4) origin from Britain.
As pointed out by others, the variable in evaluating these hypotheses is the background of the writer. If you're used to raptor populations 'exploding' on the scale seen in parts of eastern and southern Scotland and Northumberland over the past 10 years, seeing near exponential growth in a population is not unreasonable and particularly where the habitat trends are also in the bird's favour (as they clearly are with the HB). In intensive agricultural areas, it may be difficult to imagine what is happening further north.
I also take a long historical perspective and on this basis, it appears more likely that the HB, like other former breeding raptors, has simply restored itself, being highly mobile, back towards (or in its case, even beyond) its former status. On the same basis the lack of any precedent for significant drift migration to the UK over almost 200 years means the source cannot be regarded simply as the continent without real evidence. The continental origin is a hypothesis. Hypotheses are not proofs. Re-stating a hypothesis with no further evidence, as has been done, does not make it any better founded.
Mike, thanks very much for your comments .. cheers .. Nick
>
> Well, that will do for starters, look forward to some answers, and hoping
> the arguments stay out of the gutter (which Nick has to be congratulated
> for, against some very heavy opposition! Sorry if this bounces about a bit,
> I am writing as I think this through and that's the weird way my mind
> works!!
