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
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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