HUGE plaudits to frontline ornithologists Alexander Lees and James Gilroy for their absorbing and authoritative new book, Vagrancy in Birds.
From Victorian collectors to today's high-octane bird-chasing twitchers, the enigma of vagrancy has long been a source of obsession for birders worldwide, but this study really drills deep into the phenomenon.
It's complicated. As the duo admit, defining the term is itself "a challenge".
After reflection, they conclude that "the geographic range of a species should encompass something like 99.9 per cent of individuals - anything outside this range might be defined as a vagrant."
When the sightings of a vagrant bird is reported in the popular Press, the general line is that it has been "blown off course", but there are some sedentary species - the short-toed treecreeper is cited - that do not migrate.
As such, they cannot accurately be said to have been blown off course since they do not have a 'course' to take.
Most vagrancy occurs in species that are, by nature, long-distance migrants, and weather can, indeed, be a factor (as can drought or food shortage) in how they come to end up in the 'wrong' place.
But the authors emphasise that some form of disruption or distortion in their navigational compasses - whether using sun, stars, patterns of light or the Earth's magnetic field - is often likely to be just as influential, if not more so.
The text occasionally slips into somewhat pompous and ugly scientific jargon - for example, using the terms, 'exogeneous' and 'endogeneous', to distinguish between external and internal causes of vagrancy. But mostly it is accessible, and the narrative clips on at a lively rate.
There is one catchily-titled section - "the sky as a complex habitat" - which gives the lie to our generally one-dimensional view of wind.
The authors note that "we, as humans, have very little intuitive grasp of the windscape that birds experience in the airspace above us - a complex three-dimensional habitat comprising an ever-changing maze of jets, boundaries and eddies, invisible to the eye, but keenly felt by any bird ascending through the atmosphere.
They continue: "Pockets of turbulent air that buffet aircraft sometimes cause vertical lurches that can leave your stomach in your mouth.
"Given that these forces are strong enough to shake the metal hull of an aeroplane, it is easy to imagine just how disruptive the winds could be to an airborne bird."
There is a word of caution about reaching mistaken conclusions on vagrancy.
Time was when an Iberian chiffchaff seen in the UK might have been regarded as a vagrant, but it may simply have been that, in the past, many were overlooked.
Nowadays, records of this species are fairly commonplace, probably because of "enhanced observer awareness".
Similarly, more regular appearances of black-winged stilts within our shores may be driven by habitat creation.
By contrast, we are unlikely to see increases in little bustard, tawny pipit and ortolan bunting because they have contracted both in range and population size towards regions where agricultural modernisation has been less aggressive.
As well as exploring the biology of vagrancy, Lees and Gilroy explore the whole spectrum of the world's bird groups to pinpoint instances of individual species within each that have gone awry in their travels. This is all fascinating - the sort of content that you return to again and again.
A word, too for the selection of photographs. Where possible, the authors have, to their credit, plumped for the quirky rather than the conventional which adds significantly to the overall vibrancy of their study.
What are the routes by which a vagrant might reach that out-of-context destination where it reaches the eye of some doubtless delighted twitcher? The authors are a bit vague on this.
They note: "Human agency plays an increasingly major role in influencing patterns of avian vagrancy worldwide.
"Vagrants that cross barriers by piggybacking on human transport networks may have an important role to play in allowing species to respond to climate change and should perhaps therefore be viewed with a more open mind than many currently afford them."
But what does this mean? It needs to be clarified in a future edition.
What happens to vagrants after they have left the scene where they were twitched? Very few are seen again, and the authors are probably as disappointed that they remain in the dark just as much as the rest of us.
They continue: "It is a common assumption that most vagrant birds are ultimately doomed aside from the rare cases where individual birds are able to repatriate and return to their normal ranges.
"While most long-distance vagrants undoubtedly meet an untimely end, some individuals can survive for protracted periods in their new ranges - even decades in some case - provided they are able to meet the basic requirements for survival.
"By virtue of never breeding and consequently escaping the physiological stresses of reproduction, it is possible that these lost individuals can survive even longer than their life expectancy in their native range."
The subject of vagrant-hunting - or twitching - has been covered in other books, so Lees and Gilroy do not dwell on the motivators for what many people (non 'bird literate'!) see as somewhat eccentric behaviour.
However, they acknowledge that, in an era of escalating climate change, there is a moral issue to be addressed.
"Vagrant hunting is often a very high carbon hobby, especially when it involves long-haul flights, short-haul flights aboard small aircraft or epic long-distance car journeys.
"There is thus a pressing need to reduce the carbon footprint of both amateur and professional ornithology.
"Local patch birding has always attracted stalwarts, but, in an age when emissions need to be reduced, his represents an opportunity to reduce the activity's carbon footprint.
They add: "Species can be 'vagrants' at local as well as national and continental scales."
Vagrancy in Birds is published next Thursday (December 9) by Bloomsbury Helm. Price is £40 or £36 Ebook.
Jim Wright
