From: Crooks, Peter <p.crooks@ucsm.ac.uk>

To: Mike Crewe <Mikecrewe@runton.freeserve.co.uk>; UK Birdnet <ukbirdnet@dcs.bbk.ac.uk>

Subject: RE: [UKBN] Fw: rubbish

Date: 07 November 2000 13:07

Mike Crewe wrote...

> So far, I feel that Nick has singularly failed to produce the following:

> 1. Convincing weather data to explain a UK origin.

> 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.

> 3. Convincing evidence that the 'unknown' UK breeding population (which I

> accept exists) is anywhere near big enough to produce the numbers seen.

> 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).

> 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.

> 6. Any convincing evidence at all that these were NOT Continental birds!!

>

Replies to these questions are dealt with separately. > > I was going post something along similiar lines in response to previous comments in the on-going discussion, but Mike seems have covered all the main aspects of Nick Rossiter's discussion paper that I continue to have concerns about.

>Just as an aside though - a few more assumptions, 'back of an envelope calculations' and 'ballpark numbers'.

>1. The Swedish HB population is 5,000-10,000 pairs - NR quoting the European Breeding Atlas. (I'll make no comment on what this says about the relevance of counts at Falsterbo as Stuart Reeves has already covered that).

Peter, the European Breeding Atlas: p.xxi says: "The Atlas project has produced a high-quality dataset. Inevitably there are limitations to the data. Those wishing to use the data for further research must take into account these limitations …:1) there was no prescribed set of fieldwork methodologies; ..4) some data are partly extrapolated data".

I am not sure that anybody knows the size of the Swedish population. It would be very unfair to call the figure of 5,000-10,000 pairs a guess but there is certainly a high degree of uncertainty over the population size. My purpose in quoting the only figure available was partly to show this uncertainty.

>2. NR argues the recent British movement involved 400-600 birds.

Agreed. Birding World came up with a similar total of 500.

>3. NR has variously quoted HB breeding success rates of 1.4, 1.56 and 2.0 young reared per pair. If these breeding success rates are assumed to apply to the Sweden breeding population that would conceivably result in anywhere between 7,000 and 20,000 juveniles fledged per year in Sweden.

If you look at the literature in BWP and BWP Update (see British Population is Still Relatively small ), you will see that the Swedish productivity is very low at around 0.34 juveniles per pair. Indeed they are very worried about their HB population, particularly with the shooting of migrants in Malta and other countries on the Sicily run. The high productivities found for instance in Britain at the moment will fall as the birds are forced to select suboptimal habitat as the population grows.

> In this context the recent British movement (using NR's figures) therefore only represents between 2% and 8.6% of the total number of>juveniles that could conceivably be departing from southern Sweden.

Or more likely, on previous Swedish productivity figures, 1,700-3,400 juveniles were fledged so the movement in Britain represented 12-35% of the Swedish juvenile numbers. If you allow for the inevitable losses on any mooted North Sea crossing, then of course the percentage will be higher. There are complications. The Norwegian population (see British Population is Still Relatively Small ) needs to be added to the Swedish totals, particularly when movements through Denmark are considered (this will boost the totals). The uncertainty in all Scandinavian populations also makes all calculations liable to considerable error. Counts over Falsterbo in recent years seem to suggest a decline has taken place.

Does anybody have figures for sample actual productivity in Sweden or Norway in 2000?

Thanks very much, Peter, for your comments. Regards ... Nick

> Pete Crooks