From Redfern, C.P.F. & Clark, J.A. (2001) Ringers, Manual. BTO, Thetford, p.205 :
"Ringers should master the various techniques and the terminology used in Svensson’s Identification Guide to European Passerines, Baker’s Identification Guide to European Non-passerines (BTO Guide 24), Prater et al’s Guide to the Identification and Ageing of Holarctic Waders (BTO Guide 17) Jenni & Winkler’s Moult and Ageing of European Passerines and Ginn & Melville’s Moult in Birds (BTO Guide 19) as a part of their training (see Chapter 4). Other useful works are Busse’s Key to sexing and ageing of European passerines, and Pyle et al’s Identification Guide to North American Passerines. Ringers should always use the latest editions of recommended guides because details change as knowledge increases. In addition, new techniques for ageing and sexing are regularly published in Ringers’ Bulletin and Ringing & Migration."
I have only been referring to Svenson as that was what the OP originally discussed. For all these recommended sources, the latest editions are old, but the instructions are clear that we must use them (latest editions), so for updates we must rely on what is published in Ringers Bulletin.
This is a scientific study run by the BTO. The BTO defines the protocols for that study regardless of what else is published. The idea of scientific observation (data collection) is that it should be consistent, reliable and repeatable, so having a defined set of protocols is essential and it is essential that all observers do their best to adhere to those protocols.
Working together or independent is totally irrelevant. Any scientific protocol will draw on independent sources, not be entirely based on the workers own efforts. Why should any set of workers choose to ignore all the other good science in the literature that could underpin their own work ?
As it happens, the BTO have been requesting help from ringers (through their own Ringers' forum) in a collaboration with Svenson to develop and publish a new edition of his guide.
See above.
"Take account of that" ? Yes - by widening the confidence limits around any result from any analysis, i.e. reducing the reliability of the results. That is precisely what I am saying about methodology, saying that we should be striving to standardize methodology so that analysts generate more reliable results. "Take account of that" can only mean accept that the data are poorer than they might have been, there is no way they can fudge the results to "what they would have been if the data were much better".
Until they are accepted as part of the protocol of the scheme, yes, so that the data collected are consistent. Also, some of these newly published studies are later contradicted, found not to be reliable, or only to work in certain populations. It pays the BTO to wait until the methods have been validated before they accept them into the Ringing scheme protocol. Many of us have come across little things in Svenson we do not believe lead to correct ageing (but they are the correct way of coding age according to the protocol), such variations are common.
Read the manual
Mike.