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Do theories regarding the mechanisms for Vagrancy stand scrutiny, and what does vagrancy tell us about migration? (1 Viewer)

Jon.Bryant

Well-known member
I have been reading 'Vagrancy in Birds' and the sections on how birds navigate is very interesting and informative. I find however, the theories on compass errors, reverse migration, mirror-migration, and drift, as mechanisms for vagrancy not totally convincing.

Of course I am not discarding the theories completely, but wonder if they explain the majority of vagrants. The information in the book on Fork-tailed Flycatcher (suggesting that vagrants to the USA are reverse migrants) seems compelling, as is the reasoning why birds near the equator may confuse north and south. I find however, the book's reference to Cottridge & Vinnicombe's comparison of Red-breasted and Collared Flycatcher less compelling - the arguments is that Collared, having a more north-south migration route, would be unlikely to reach the UK in autumn, whereas Red-breasted which migrates northwest to southeast, would reach the UK by migrated in the reverse direction, explaining the higher numbers of that later that are recorded annually in the UK. I am not sure that these two species are the best species to compare. Collared Flycatcher for starters is very difficult to identify in autumn, so may well be under recorded. The main populations of Collared Flycatcher are also well to the southeast of the UK (Czechoslovakia, Romania, Slovenia), whereas Red-breasted Flycatcher has reasonable populations much closer in the Baltic states. There therefore seems to be an argument that the comparison is looking at a scarce bird that is hard to ID, and comparing this with a commoner bird that is easier to ID. In this case, these facts alone may explain the difference in autumn records between the two species in the UK. I also think that true reverse migration from the Baltic states, would probably take Red-breasted Flycatchers into Scandinavia, rather than to the UK. On similar grounds, I also cannot see how true reserve migration in Wood Warbler could take birds to the Bearing Straights (unless this was by going over the Arctic circle and then south to the Bearing Straights!). It is suggested in the book that Wood Warbler could reach this location by reverse migration.

Theories that birds may have compass errors due to difficulty in using magnetic fields in high latitudes, or because weather conditions block out sun or star bearings, would seem to me to be temporary problems and may explain some movements. Extreme vagrants appear however, to be hell bent in following their wayward trajectory, rather than following a path with occasional or even frequent errors in bearing. I would expect that if a birds was getting it sometimes right, it would end up south of the breeding range, rather than in northwest Europe, the Pribilofs, Nova Scotia, Japan etc. A prime example of birds being hell bent on a certain course is the 'invasion' of Siberian Accentors in Europe in 2016. In UK 16 birds were found in the northern half of England and Scotland, with no birds apparently relocating subsequently in a southerly direction. Although over 200 Siberian Accentors were logged in Europe in the autumn of 2016, with records spreading westwards as the autumn progressed, there was no evidence of birds returning east later in the autumn or spring - birds just seemed to disappear - it would seem plausible that birds that got as far as the UK continued west and reached their demise in the Atlantic. If so, this suggests an overwhelming imperative to travel west, rather than a navigation error.

Drift is also suggested in the book as a possible cause of vagrancy from the far east. Surely drift due to easterlies over central Asia, would result in birds heading south west, and could not explain vagrancy into north-west Europe or other high latitudes.

Drift is suggested in the book as the main cause of transatlantic vagrancy, but is this reasonable? Firstly, if we assume that some errant birds purposefully travel west across Eurasia, why shouldn't we also assume that a few birds also purposefully travel east (although such a journey from the eastern USA would include a large sea crossing). It could be that if the Atlantic crossing was less hazardous, some American passerines might be as common or in fact commoner in Britain as birds travelling to Britain from a similar distance to the east (e.g. Dusky Warbler) - the prevailing wind direction is westerly in the northern hemisphere for a start. Secondly, I wonder if migrants are that easily swept off course by strong winds - I have read that migrants in the USA have been recorded setting off at dusk and rapidly gaining height, but then dropping back out of the sky when encountering unfavorable winds at altitude. In adverse weather there would always seem the option to either not set off in the first place or to abort the migration. Vagrancy in Birds reports that birds have been recorded on radar drifting out over the Atlantic in numbers, but it is not explained whether this was in southerly winds with an easterly component, with birds making use of the southern vector, rather than setting off and being carried over the Atlantic in adverse conditions. Also some bird wintering the Caribbean may find that a longer sea crossing in south easterlies is beneficial to following the coast down through Florida and taking a longer route. It is interesting that an example in the book describes a Bar-tailed Godwit that gave up her intended heading in a headwind and turned to fly with the wind. But this could be argued to be slightly different to birds being drifted out to sea and then flying with the wind. The Bar-tailed Godwit was undertaking a planned and prolonged ocean crossing, during which weather conditions could change for the worse, with no option for the Godwit to stop short and land - the Godwit presumably would not have set off in adverse conditions, but was forced to change plan when conditions changed several days into the trip..

Recent years have also seen growing evidence of American passerines reaching deep into Europe - e.g. Red-eyed Vireo in Italy, Yellow Warbler in Denmark and Red-breasted Nuthatch in Germany. This seems to me to indicate that some birds are compelled to travel east. If birds had been displaced by weather, surely they would instead land as soon as they had completed the sea crossing, and then either try and relocate southwest or continue a southerly route on the wrong side of the Atlantic.

The fundamental problem with putting theories to causes of vagrancy would seem to me various. For starters vagrants can only ever be recorded where birdwatchers (with the requisite skills to ID the bird) are looking. All those far eastern vagrants that reach Britain, have not reached Britain in a single flight, but in a series of leaps and bounds, probably including stays in northern Europe. Looking for vagrants in the forests of northern Europe must be a lot harder than looking for vagrants at coastal locations (and particularly in places with limited cover). This presumably explains why records of some eastern vagrants are frequent in Shetland, but less so across their probably migration routes through Europe.

Vagrancy is also presumably impacted by topography and environment - birds may skirt high mountains, deserts or large bodies of water. It would seem reasonable to assume that topography and habitat could easily funnel birds, meaning that some places are hot spots for vagrants, whereas other places in the 'shadow' of mountains, lakes and deserts may receive few birds. I wonder how the Baltic may funnel birds migrating east-west, to the north and south? Perhaps the Baltic explains why certain eastern vagrants are commonest in Shetland, having followed a track along the north shores into western Norway, and from there across the sea to Shetland. There may even be an issue that east-west movement is less energy intensive - I can see potentially that a taiga species could struggle to migrant for a prolonged period through sub-optimal feeding habitat such as tundra or steppe grassland. but may be able to make a more prolonged errant migration through the boreal forests of northern Eurasia.

With all the unexpected occurrences of vagrants now being recorded across the globe, I am starting to think that as birds inherit a migration direction from the parents, perhaps vagrancy is simple caused by a small proportion of birds inheriting the wrong bearing. The pattern of vagrancy may then be to do with where people look and filtering effects from terrain. This would mean that virtually everything is possible, but certain movements become a game of chance depending on obstacles to be surmounted and prevailing weather conditions. On this basis perhaps a Paddyfield Pipit, can indeed reach Cornwall, but with the likelihood of the same luck and repetition slim.

Some level of poor copy could even be an evolutionary mechanism, particularly for birds that specialise in the breeding season, but are more generalists in the non-breeding season - a degree of poor copy would potentially ensure dispersion and reduced direct competition in winter. Bad, rather than poor copy of inherited direction, could possible lead to discovery of new non-breeding habitat, as appears to have happened with some species - German Blackcaps that now winter in the UK, perhaps Richards Pipits wintering in Spain/Southern France, maybe Yellow-browed Warblers that possible now winter in unknown areas of southern Europe or Africa (as suggested may be the case in the book), or perhaps the records of Pine Bunting wintering in Italy.

Of course if 'bad copy' does occur and is a reason for vagrancy, then in many occasions the outcome will be tragic, rather than discovery of new wintering grounds. This could explain why vagrancy is more common in first-years than in adults. If the direction of migration is hard-wired for life, then for an adult to turn up as a vagrant, it would have had to travel an errant and potentially arduous route at least twice (assuming it returned to a hard wired place of birth in the breeding season). Perhaps inexperience of first-years is not the reason why the majority of vagrants are first-years - for the majority it may unfortunately prove to be a one way ticket, which they do not survive and repeat as adults.

If anyone knows or any research or work on disproving or proving poorly inherited map bearings, or any articles on whether inaccurate migration could be a mechanism to avoid competition on wintering grounds, I would be very interested to hear about it.

Any comments on whether the above musing are dribble or plausible, are also welcome.

Cheers

Jon
 
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I am not a big fan of the theory of reverse migration either. It seems to work in some cases, but even the classic flycatcher example has its shortcomings. In addition to what you mentioned - it seems to predict that Collared Flycatchers are vagrants to the north of their breeding areas in autumn, but in reality they are almost completely absent in the eastern Baltic area in that time, much more scarce than in spring. Somebody suggested that perhaps north-south migrants are less prone to reversed migration than NW-SE migrants, but not much of the flycatcher example is left after this patching. In fact, other southern species too are typically more spring than autumn vagrants in northern Europe.

The Siberian Accentor case is a very good example of the mysteries of bird vagrancy. It is documented in Stoddart 2018, British Birds 111 (2) 69-83, and another good analysis is in Finnish in Tringa 44 (3) 80-91. One important thing is that the occurrence of accentors differed clearly from the occurrence of, for example, Yellow-browed Warblers. British Isles get their good share of the latter species, as of most eastern vagrants, but much the biggest numbers of accentors were seen in Finland and Sweden - even Norway got less. And this is not peculiar to the year 2016 - this same pattern is visible in the earlier records, although the species was much more rare before that year, and has been since. Anyway, I agree that the accentors somewhat disappeared in early winter, but they did not seem to continue to over Atlantic Ocean.

Vagrancy in Birds is so new book that its literature section should still list most of relevant references. I have the book and I have read it but for some reason can't find it now for checking.
 
From very little I know about genetics, it would be very difficult to precisely 'code' bird migration orientation in genes. So I think vagrancy is unavoidable, and as the common belief says, it is also a lifesaver for the population in case the environmental conditions suddenly change.

Another thought is that recent radiotracking and geotracking of birds showed astonishing vagrancy which nobody expected and was completely overlooked. Examples are a Richard's Pipit which migrated to Central Asia from Europe, a Bonelli's Eagle, who travelled north across many countries of Northern Europe without being seen once, and a Ring-billed Gull which similarly moved all over Europe and then bred in Belarus. So I suspect that bird vagrancy will prove to be much more diverse and common once we get the real-world data, not just suspicions.
 
From very little I know about genetics, it would be very difficult to precisely 'code' bird migration orientation in genes
The Vagrancy in Birds book gives some examples of where migration points are presumed to be hard coded into a birds genetics. One of these examples is Marsh Warbler, where first-winter birds will use a set of staging posts in Africa during the autumn migration (they apparently do not just simply follow a north-south route, but stage in certain areas of the continent).

In fact, not all birds migrate directly north-south, with many taking a specific bearing. If first-winter birds are not assisted by parents, it is hard to see how they could determine the direction without the instinct being being hard coded. Some species such as Northern Wheater even do 'dog-legs' during their first migration. Quite amazing to think that the first-winter Northern Wheatears I saw north of Beijing will winter in Africa, but must fly a dog-leg route to avoid a long Indian Ocean crossing.

The book also raises the question of how do birds no when to stop, unless they are hard coded with some sense of 'finally I've made it'. I recollect that teh book suggests this could even be knowledge of the magnetic field at the destination point.
 
Anyway, I agree that the accentors somewhat disappeared in early winter, but they did not seem to continue to over Atlantic Ocean.
Yes you are right. I was really meaning the few birds that reached western Europe did not seem to return east, so perhaps were driven to continue west. The northern birds are quite long distant migrants, so it seem odd to think that the rest ran out of steam and perished soon after being discovered. Perhaps they did return East, but migration spots were less heavily watched when they made the return - it still seems a bit odd, as I cant recall any sightings in late winter of spring.

Having seen Siberian Accentor in the frozen parks around Beijing, I think they are tough enough to survive a winter in western Europe, so quite fascinating why where they disapeared to in later autumn/early winter.

I suppose Yellow-browed Warbler could be taken as a better and more typical example. Lots arrive every autumn, but very few winter in Western Europe and spring records are scarce. The Vagrancy in Birds suggests that there now may be a population wintering somewhere in Southern Europe/Africa, but then why so few spring records? Perhaps the birds we encounter are simply compelled to travel west at all costs, and never to make the return journey?
 
I have carried on reading the book on Vagrancy in Birds and there is a section on natural dispersal, but this seems to be considered a secondary driver compared to navigational error or weather induced vagrancy.

Interestingly, there is a section on colonizing new wintering sites, which references work on Eurasian Blackcap, which indicated that offspring of parents with different wintering sites (and hence migrating on different bearings), inherit an intermediate bearing.

Thinking of things from a mathematical point of view and not as a biologists, I am not sure how this works. For simplicity if I assume
  • we have a static population, with each generation replacing the last.
  • we start with a first generation of birds migrating on bearings between say 85.5 and 94.5 degrees equally distributed
    • 10% migrate at 86, 87, 88, 89, 90, 91, 92, 93 and 94 degrees (90% or the population)
    • 5% migrate 85.5 and 94.5 degrees (the remaining 10% of the population).
  • birds mix randomly on the breeding grounds (i.e birds do not form bond on the wintering grounds, or breed in parts of the range where all birds migrate on a specific course). If this was not the case the development of isolated subspecies would seem a likely outcome.
The chance that a second generation bird has parents that both migrate at 85.5 (or 94.5) degrees is now 1 in 20 x 1 in 20, so 1 in 400. The chance that the first generation migrated between 89 and 91 degrees was 30%, whereas in the second generation, there are numerous combinations that create intermediate bearings within this range - the theoretical percentage of the population migrating between these bearings in the second generation is increased to 43.5%. In essence, with each generation the bearing of the offspring would converge on the average value (due south). Rather than maintaining a uniform distribution and dispersal across the wintering grounds, if offspring migrate in a direction intermediate to their parents, then over generations this averaging would lead to a convergence on more restricted wintering area.

In this case it would seem that a level or poor copy would be required to "re-seed" dispersal across a wider wintering area - assuming that the birds with poor copy and bearings outside the normal return to breed.

Although vagrant adult passerines are considered exceptionally rare, it would be interesting to determine if birds such as the adult Siberian Blue Robin found on North Ronaldsay in 2017 are hard wired, and have hence migrated backwards and forwards along an erroneous migration route at least 1.5 times, perhaps passing on at least part of their erroneous course to any offspring.
 
There are at least 160 accepted records of Red Eyed Vireos in Britain and must be at least another 100 more from Ireland and other European destinations and another 150 from the Azores all in autumn and winter but never a spring or summer record of this species. Not one trapped at a ringing station. Where Do they all go?
 
There are at least 160 accepted records of Red Eyed Vireos in Britain and must be at least another 100 more from Ireland and other European destinations and another 150 from the Azores all in autumn and winter but never a spring or summer record of this species. Not one trapped at a ringing station. Where Do they all go?
Although 160 sounds a lot, in the UK 5 or so birds is a good autumn. When you think about trying to relocate 5 birds, I think the odds are quite low of rediscovery. We then need to think about longevity and mortality rates when thinking about the potential for spring birds. You will probably find that less than 1/2 of first-winter Red-eyed Vireos survive the migration and winter (even when wintering in their intended prime location).

I think there are only four options for Red-eyed Vireos -
  1. The transatlantic crossing is too onerous and the birds have insufficient energy reserves to complete any urge to migrate. The birds therefore perish nearby to the point of discovery.
  2. The birds try to re orientate back across the Atlantic, but as this would be against adverse winds the odds of success would be probably zero.
  3. The birds are hell bent on continuing an easterly migration route and end up in mainland Europe, where they would be harder to relocate (but not impossible - and there are now two Red-eyed Vireo records from Italy which may indicate that this pattern occurs). With this option the question is whether the birds ever feel 'I've reached my wintering grounds', or just soldier on, driven by an urge to complete their migration. If the later then the question is whether they eventually run out of energy reserves and succumb to the perils of colder continental weather.
  4. They continue their migration south on the wrong side of the Atlantic, but if they then winter at the same latitude as in South America, they would presumably want to winter in sub-Saharan Africa. The odds of a transatlantic crossing followed by a trans-Saharan crossing would seem low.
Sadly, I think that most vagrants probably perish, but there is hope that the Dusky Warblers, Yellow-browed Warblers and even Siberian Rubythroats that have wintered in the UK and Europe do not. Do these survivors then return to their place of birth, and then do they try and do the same migration again the next autumn?
 
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Although 160 sounds a lot, in the UK 5 or so birds is a good autumn. When you think about trying to relocate 5 birds, I think the odds are quite low of rediscovery. We then need to think about longevity and mortality rates when thinking about the potential for spring birds. You will probably find that less than 1/2 of first-winter Red-eyed Vireos survive the migration and winter (even when wintering in their intended prime location).

I think there are only four options for Red-eyed Vireos -
  1. The transatlantic crossing is too onerous and the birds have insufficient energy reserves to complete any urge to migrate. The birds therefore perish nearby to the point of discovery.
  2. The birds try to re orientate back across the Atlantic, but as this would be against adverse winds the odds of success would be probably zero.
  3. The birds are hell bent on continuing an easterly migration route and end up in mainland Europe, where they would be harder to relocate (but not impossible - and there are now two Red-eyed Vireo records from Italy which may indicate that this pattern occurs). With this option the question is whether the birds ever feel 'I've reached my wintering grounds', or just soldier on, driven by an urge to complete their migration. If the later then the question is whether they eventually run out of energy reserves and succumb to the perils of colder continental weather.
  4. They continue their migration south on the wrong side of the Atlantic, but if they then winter at the same latitude as in South America, they would presumably want to winter in sub-Saharan Africa. The odds of a transatlantic crossing followed by a trans-Saharan crossing would seem low.
Sadly, I think that most vagrants probably perish, but there is hope that the Dusky Warblers, Yellow-browed Warblers and even Siberian Rubythroats that have wintered in the UK and Europe do not. Do these survivors then return to their place of birth, and then do they try and do the same migration again the next autumn?
There's a basic problem with (3) in that there is no evidence that Autumn American passerine vagrants are anything other than storm-driven/ship-assisted (and a good deal of evidence to support that they are, up to and including multi-species arrivals in a single afternoon during the passage of a front (two Swainson's Thrushes, Black-billed Cuckoo and Upland Sandpiper on Scilly in 1990), and multi-individual single species arrivals within hours on the track of a storm (two Yellow Warblers landing respectively in Southern Ireland and on the Isle of Portland). In either case they have no drive to migrate East: their migrations were Southerly orientated and were interrupted by a mighty blast or convenient mid-ocean perch. There is no reason to assume their subsequent movements would continue Eastwards: a fan of mainly Southerly movements would be far more likely. Red-eyed Vireos in Italy may have made their initial European landfall in Norway.

Incidentally, although I have no American passerine experience in Southern Africa I have seen both White-rumped Sandpiper and Franklin's Gull in Namibia, so hard evidence of continued Southward migration in transatlantic vagrants exists even for an occasional traveller like me.

John
 
There's a basic problem with (3) in that there is no evidence that Autumn American passerine vagrants are anything other than storm-driven/ship-assisted (and a good deal of evidence to support that they are, up to and including multi-species arrivals in a single afternoon during the passage of a front (two Swainson's Thrushes, Black-billed Cuckoo and Upland Sandpiper on Scilly in 1990), and multi-individual single species arrivals within hours on the track of a storm (two Yellow Warblers landing respectively in Southern Ireland and on the Isle of Portland). In either case they have no drive to migrate East: their migrations were Southerly orientated and were interrupted by a mighty blast or convenient mid-ocean perch. There is no reason to assume their subsequent movements would continue Eastwards: a fan of mainly Southerly movements would be far more likely. Red-eyed Vireos in Italy may have made their initial European landfall in Norway.

Incidentally, although I have no American passerine experience in Southern Africa I have seen both White-rumped Sandpiper and Franklin's Gull in Namibia, so hard evidence of continued Southward migration in transatlantic vagrants exists even for an occasional traveller like me.
I suppose that this is the point of my thread in the first place. A Siberian Thrush arrives in Shetland and is obviously not purely a weather driven arrival - there would be plenty of options for it to put down and ride out adverse conditions - there is something else going on. A bird arrives from America in Europe and it is must be (solely) weather induced - why the difference? Why do a few eastern birds apparently want to migrate west, but a few western birds not apparently want to migrate east? The book on Vagrancy in Birds explains that it is in fact hard to truly correlate weather with arrivals of birds from America - it is thought that conditions sparking migration need to be matched with weather pushing birds east across the Atlantic. There are plenty of storms that do not bring birds to the UK and it is argued that this could be because conditions were unsuitable for birds to initiate migration in the first place. I think the point to think about, is just because birds arrive in suitable weather, does not necessarily mean they are unwitting travelers - they could equally be birds with a urge to travel east, making the most of weather conditions providing suitable tail winds. This could explain why birds are found in Europe (Red-breasted Nuthatch, Yellow Warbler etc last autumn), when they could have easily made landfall earlier (and possibly/probably did).

There is another reason that makes me think that birds reaching our shores are in fact migrating with the weather. Purely weather induced events normally result in large arrivals - Vagrancy in Birds references a fall on the Atu in the Aleutians including 180 Eye-browed Thrushes, 225 Olive-backed Pipits, 193 Rustic Buntings, 366 Brambling and 700 Wood Sandpipers. It would be impossible to credit such a fall to compass errors or random dispersal, and the weather conditions fitted weather induced displacement. If birds from the USA are unwitting visitors to the UK, driven off course by the weather, then are they the only birds somehow plucked out of the throng of migrants travelling down the east coast of the USA, or are they the extremely lucky ones with a great horde of birds driven out into the Atlantic and a watery grave? In the latter case, what is special about the few birds that reach us - only the fittest 1%, the 1% with the highest fat reserves or the 1% that manage to find rest-bite on a vessel? Having spent a bit of time a Cape May in autumn, the shear amount of birds migrating can be immense. If birds in one of these events where fooled by the weather into a watery grave (with just an odd bird making it to Europe, and then presumably never returning to the breeding grounds), it would seem to me that the impact on the population could be quite substantial. I prefer to think that birds are not that easily fooled by truly adverse conditions, and that weather is part, but not all of the reason that a few birds from North America reach Europe each year.

On this issue of South Africa, I recently pleased to find an Elegant Tern in Namibia! I think terns, waders and gulls are quite different to passerines, as many of them migrate in flocks. My Elegant Tern was with Sandwich Terns and presumably all the European vagrant Elegant Terns (which seem to now regularly return each summer) are now in fact migrating back and forth to Southern Africa with Sandwich Terns each year. There are other vagrants to Southern Africa, which I presume are direct vagrants (even if ship assisted). I cannot imagine that Snowy Egret, Little Blue Heron, American Purple Gallinule etc. reached western Europe and then migrated down to Southern Africa - particularly the gallinule as the Atlantic crossing normally is enough to finish them off.

I have been thinking about group migration in relation to Kumilien's Gulls. If Kumilien's Gull is a hybrid between two races of Iceland Gull, and hybrids migrate with their parents, why are Kumilien's Gulls apparently rare on the west coast of america and Thayer's rare on the east coast? Does this suggest a stable hybrid swarm, with few new hybridization events (and hence newly created Kumilien's following their Thayer's parents to the west coast)?
 
Forgot to say that according to my guide to Southern Africa (which seems to include vagrants) there are no North American passerines mentioned. So you are not the only one with no American passerine experience from Southern Africa. I think that most american passerines reaching the UK, do not migrate to such southern latitudes as the equivalent of Namibia and South Africa. The absence of birds in Southern African does not therefore necessarily mean that option 4 is not occurring. As I say, I think most birds would find the Sahara a big hurdle after an Atlantic crossing. However, It would be hard to determine if birds north of the Sahara (in say Morocco) had reached there directly on indirectly.
 
There's a basic problem with (3) in that there is no evidence that Autumn American passerine vagrants are anything other than storm-driven/ship-assisted (and a good deal of evidence to support that they are, up to and including multi-species arrivals in a single afternoon during the passage of a front (two Swainson's Thrushes, Black-billed Cuckoo and Upland Sandpiper on Scilly in 1990), and multi-individual single species arrivals within hours on the track of a storm (two Yellow Warblers landing respectively in Southern Ireland and on the Isle of Portland). In either case they have no drive to migrate East: their migrations were Southerly orientated and were interrupted by a mighty blast or convenient mid-ocean perch. There is no reason to assume their subsequent movements would continue Eastwards: a fan of mainly Southerly movements would be far more likely. Red-eyed Vireos in Italy may have made their initial European landfall in Norway.

Incidentally, although I have no American passerine experience in Southern Africa I have seen both White-rumped Sandpiper and Franklin's Gull in Namibia, so hard evidence of continued Southward migration in transatlantic vagrants exists even for an occasional traveller like me.

John


Both these species have very different patterns as vagrants. I know White Rumped Sandpipers are more common but they seem to have a very strong coastal bias.
In the European context White Rumped Sandpipers very rarely turn up at inland sites except 2 last "autumn" in the UK and maybe more - Rutland Water and Buckinghamshire? I think the first was in August-ish so more likely an adult which may have summered in North Europe. Franklin's Gulls just seem to turn up anywhere and have been found in Australia, Central Asia, Egypt and maybe India too? Franklin's Gull is maybe at least 3 times as rare as Bonaparte's Gull in Britain yet in the Netherlands there are twice as many records of Franklins in the Netherlands.
 
Although 160 sounds a lot, in the UK 5 or so birds is a good autumn. When you think about trying to relocate 5 birds, I think the odds are quite low of rediscovery. We then need to think about longevity and mortality rates when thinking about the potential for spring birds. You will probably find that less than 1/2 of first-winter Red-eyed Vireos survive the migration and winter (even when wintering in their intended prime location).

I think there are only four options for Red-eyed Vireos -
  1. The transatlantic crossing is too onerous and the birds have insufficient energy reserves to complete any urge to migrate. The birds therefore perish nearby to the point of discovery.
  2. The birds try to re orientate back across the Atlantic, but as this would be against adverse winds the odds of success would be probably zero.
  3. The birds are hell bent on continuing an easterly migration route and end up in mainland Europe, where they would be harder to relocate (but not impossible - and there are now two Red-eyed Vireo records from Italy which may indicate that this pattern occurs). With this option the question is whether the birds ever feel 'I've reached my wintering grounds', or just soldier on, driven by an urge to complete their migration. If the later then the question is whether they eventually run out of energy reserves and succumb to the perils of colder continental weather.
  4. They continue their migration south on the wrong side of the Atlantic, but if they then winter at the same latitude as in South America, they would presumably want to winter in sub-Saharan Africa. The odds of a transatlantic crossing followed by a trans-Saharan crossing would seem low.
Sadly, I think that most vagrants probably perish, but there is hope that the Dusky Warblers, Yellow-browed Warblers and even Siberian Rubythroats that have wintered in the UK and Europe do not. Do these survivors then return to their place of birth, and then do they try and do the same migration again the next autumn?


Agreed 160 overall is not a lot. However, there are spring and other late winter records of other American passerines in the UK, Iceland and mainland Europe and most of these species are 10X as rarer than Red Eyed Vireos. I am not sure if there are any post December records of Red Eyed Vireos anywhere. The ones in the UK seem to be healthy generally and some have stayed for days. Most of those found in mainland Europe seem to be healthy and active too. I started a thread about spring records of American passerines at this side of the pond and there were some interesting records in that.

A late December record of Yellow Billed Cuckoo in Morocco from 2021 and a May 13 record from the Azores of this species in 1966. It wintered there or was trying to get back or accidentally flew north east from South America?

A Pallas Warbler ringed at Spurn in October? 2016 was later re-trapped at one of the Welsh Islands in May 2017 - where was it for the winter?

A couple of years ago a Dusky Warbler ringed in Denmark was re-trapped in Belgium some days later.
 
Also forgot to mention that Vagrancy in Birds states for the Azores 'While tropical storm systems can bring large influxes of vagrants to the Azores, arrivals there show far less clear association with these fast-moving systems. Some of the largest 'falls' of American vagrants have occurred independently of storms, during periods of relatively light westerly winds, particularly when they form a continuous vector across the eastern Atlantic. They surmise that birds may be reaching the Azores due to drift, but is that convincing? Perhaps we are back on a small percentage of birds inheriting the wrong orientation for migration, and making use of favorable winds to achieve their erroneous objective.
 
Both these species have very different patterns as vagrants. I know White Rumped Sandpipers are more common but they seem to have a very strong coastal bias.
In the European context White Rumped Sandpipers very rarely turn up at inland sites except 2 last "autumn" in the UK and maybe more - Rutland Water and Buckinghamshire? I think the first was in August-ish so more likely an adult which may have summered in North Europe. Franklin's Gulls just seem to turn up anywhere and have been found in Australia, Central Asia, Egypt and maybe India too? Franklin's Gull is maybe at least 3 times as rare as Bonaparte's Gull in Britain yet in the Netherlands there are twice as many records of Franklins in the Netherlands.
The White-rumped was at a waterhole in Etosha.

Cheers

John
 
The UK Northern Mockingbird a couple of years ago was interesting...I remember someone on BF (sorry, can't remember who) correctly predicting it might follow the coast northwards, thinking it was on the east coast of America...which it duly did, turning up eventually in Northumberland.
 
I've photographed plenty of wintering Eastern Phylloscs in recent years - in fact year-listers seem to reckon on nailing Yellow-browed, Hume's and Pallas's early in the year, if not Dusky! So there's little mystery about what some of them do. Rarer birds also attempt to winter viz Dusky Thrush in Derbyshire a few years ago. Who knows when that actually reached Western Europe....

As for American passerines migrating on: They don't have to go over the Sahara, they can go round the coastal strip and then vanish into the huge forested areas of West Africa. It isn't at all surprising that they don't get found there, finding some of the residents can be hard enough.

Individuals arriving from the East may be explained by something I once read (don't ask me where) that quoted Russian ringers as noting that recoveries of migratory species showed not vagrancy vectors but a 360 degree radiation of birds failing their first migration test, with reducing numbers based on distance (due both to the widening area involved and losses en route) and, of course, the likelihood of eventual utter disaster for individuals found heading to the Polar Arctic or out over seas. This of course suggests that those Eastern birds that continue on their way after being found in Western Europe probably do fall in the ocean. That's life.

The really odd events are things like the multiple arrival of Siberian Accentors on a really narrow vector - in Britain nothing as far South as Norfolk, first impact Shetland but the majority of records from the Tweed to the Humber. People quoted a very long and unusual weather blast from their home area but why was it a single species that was transported (the number of Sibe Accs found indicates a lot of birds were caught up)? I'm glad I'm not waiting on the next such event, anyway!

John
 
there are spring and other late winter records of other American passerines in the UK, Iceland and Mainland Europe
This is quite complex to untangle. For a starters why are some American Sparrows much more frequent in the UK in spring rather than autumn. Presumably these are mainly spring migrants moving up to Newfoundland, then overshooting and travelling through Greenland and Iceland to reach the northern isles where records seem to predominate. So we need to think about direct arrivals verses indirect arrival by birds moving up from wintering grounds on the east side of the Atlantic.

Then we need to think about where species normally winter. Yellow-rumped Warblers for instance is in fact quite hardy with some birds wintering right up the eastern coast of the USA. In fact the furthest it winters south is Central America - so even the most extreme migrants only reach as far south as say the Gambia. Given where YRW winter in the USA, it would seem reasonable that a few birds have managed to winter in the UK. Some Ovenbird and Black-and-White Warblers winter in Florida, so perhaps the winter records in the UK are not that surprising. Perhaps the Scillies Northern Waterthrush is a bit more odd, as the bird normally winters no further north than the Caribbean and Mexico. The Scillies is however, its own little micro climate surrounded by the relatively warm waters of the Atlantic, subject to the gulf stream influence. Perhaps Scillies on a good day is not too dissimilar to the Caribbean in winter? Then we have the truly odd like the Golden-winged Warbler found in Kent in February, when it should be wintering no further north than extreme southern Mexico.

I think however, that Red-eyed Vireo is a whole different kettle of fish, with the complete population apparently wintering in the Amazon. As I say, the whole population winters at a latitude, which on this side of the Atlantic would be south of the Sahara. This perhaps explains the lack of winter and spring records in Europe.

I also think that it is easy to underestimate how hard it is to relocate an individual bird. It can be hard enough to find a bird again when it moves a few hundred yards, let alone a few miles or more. I was surprised when birding at Beachy Head to see Firecrests working along hedgerows circa 1km inland - are these birds dispersing inland and are the birds seen at Belle Tout Wood just a small fraction of the birds that move through? Where for instance did the Hertfordshire Eastern Crowned Warbler come from or go to? Presumably after it was ringed at 8:30, it was still somewhere in the area, perhaps working its ways through the hedgerows and woods along its intended migration route never to be seen again.
 
Russian ringers as noting that recoveries of migratory species showed not vagrancy vectors but a 360 degree radiation of birds failing their first migration test
I think this is what I am talking about. Do a small percentage of birds migrate the wrong way in any case and at a completely random bearing, and then topography, climate etc shape our understanding of the pattern. As you say not many birds orientating north will survive in the arctic and there are probably not many folks there to chart the birds demise. The Arctic would have a big filtering effect, but so too would the Atlantic, high mountain ranges, deserts of even atypical habitat for the vagrant where they would find it harder to feed up and survive.

Regarding topography, it is interesting that some eastern species favor the Northern Isles and some don't. It would be interesting to determine whether this is due to birds skirting around the Baltic and the entrance to the North Sea. See figure below
Map.JPG

Perhaps this is why Pechora Pipits and Lanceolated Warblers favour the Northern Isles and why species with more southerly ranges such as Eastern Crowned Warbler and Pale-Legged Leaf Warbler have yet to make it there.
 
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