Do any birds migrate along an arc? (1 Viewer)

[Jon.Bryant]

Fly from London to New York and the plane flys an arc - the shortest route loops north over the Atlantic, and is not a constant bearing.

Is anyone aware whether gps tracked birds have proven that they have the ability to fly an arc route during migration - the shortest route between breading and wintering grounds?

Just thinking about it, there are a few things that make me think it is unlikely.

1/ For birds migrating north-south there is no need - a constant bearing north-south is the shortest route. For slight variations from north south the benefit would be relatively small. For most birds I therefore think the forces of natural selection would not be strong enough to drive arc adoption.

2/ A precise arc may well cross inhospitable habitat (deserts, seas, frozen tundra, high mountains etc.). I imagine that there would probably be a benefit in migrating a more meandering route over less inhospitable habitat.

3/ The maths of flying an arc is quite complex as the bearing changes along the route, which is governed by the start and end point and has a flight length. It would seem unlikely that this calculation ability could be easily evolved in birds.

4/ Vagrancy seems at odds with the precision of arc navigation. What happens at the end of a planned arc (say started off on the wrong bearing)? - stop, continue straight, or try to extend the arc?

5/ There would seem some advantage in species that may be specialists in the breeding season, but more generalist in winter, to dispense during migration. The adoption of a precise arc may therefore be counter productive, leading to unnecessary competition on wintering grounds, when food resource may be more scarce.

I have seen some books which suggest arc routes, particularly to explain vagrancy patterns, but I think these are unconvincing arguments. It imagine that the odd apparent arc migration could in fact be a coincidental learned route. If anyone is aware of proof that birds can (and semi-regularly do) migrate along arcs, references would be appreciated.

Regards

Jon Bryant

 

[jurek]

Hi, there is quite a lot of publications online showing maps of radio-tracked birds, you can check yourself.

They seem to travel between stopovers, and be moved around by wind, plus many (not all!) landbirds avoid crossing long distance over oceans and seabirds avoid landmasses. Therefore not much traveling on arc route is visible.

Albatrosses crossing southern oceans were radio-tracked a lot and would be a good test, because they travel along the longitudes. However, they use winds and forage during their travels, so I don't recall any straight lines.

 

[MJB]

From viewing many mapped geo-tracked routes taken by migrants, particularly those that cross much of the eastern Palearctic, I get the overall impression** that Great Circle routes are generally followed (Just looking at a single track can be misleading due to positional uncertainties inherent in various geo-tracking methods), but there are a number of caveats, some already mentioned, particularly inhospitable and mountainous areas.

It is known that many landbird migrant species have endogenous 'clocks' that limit the time spent on, say, an east-west leg of outward migration, after which the bird makes an abrupt change of course (See various papers by such as Wiltschko & Wiltschko). Some migrant species follow the reverse path on return migration, which may enable birds to 'check off' prominent waypoints, but others return via loop migration, which in the example above, includes an early west-east leg, requiring two sets of waypoints to be 'stored' that can simplify migration success in subsequent years. Much depends on which aspect of a bird's navigation capability is prime at any period during the journey (sun-compass, star-compass, internal clock, internal magnetic compass) and how effectively the bird compensates for enforced deviations.

Not all species have the same suite of navigation systems and not all individuals in a species possess the inherent abilities to the same degree, which makes the interpretation of geo-tracked birds a bit of an art!
MJB
** It really needs geo-tracking experts to support or dismiss my impression!

 

[Jon.Bryant]

It is known that many landbird migrant species have endogenous 'clocks' that limit the time spent on, say, an east-west leg of outward migration, after which the bird makes an abrupt change of course (See various papers by such as Wiltschko & Wiltschko).

Interesting point about the endogenous clocks. I presume that this relates to species that need to avoid certain obstacles, such as the Greenland race of Wheatear, that crosses the Atlantic by the shortest sea crossing, prior to travelling south to Africa. Perhaps this also applies to Common Swifts (race pekinensis), Willow Warbler (race yakutensis), Amur Falcon etc, which breed in Asia but may need the dog leg migration to avoid a long Indian Ocean crossing. I have wondered whether a 'dog leg' route may explain a paucity or complete lack of proven vagrancy of these birds to UK/Western Europe.

Hi, there is quite a lot of publications online showing maps of radio-tracked birds, you can check yourself.

Thanks, but I have tried this without too much success. The plots either do not have much data - basically straight lines between widely spaced points, or are presented in such a way as to make it unclear - you end up having to draw a straight line between two points in Google Earth, then orientating the Google Earth view to see if it generally matches the migration track plot. What would be really good would be the raw data used to generate the track plot, but as MJB states how is this recorded? Given battery saving requirements, it is unlikely to be hourly data on speed and orientation. If location only is recorded at say daily intervals, birds can migrate quite a long way in 24 hours.

I take the point about landbirds avoid sea crossings, but It also dawned on me that shorebirds may have specific routes to incorporate stopover sites - i.e Australian and New Zealand waders making for the yellow sea, prior to heading up to Siberian breeding grounds. Seabirds often follow ocean currents - the ocean currents may meander or veer due to the spin of the earth, but this is a little different. What ideally you would need would be a landbird, with a long migration route over hospitable terrain, which migrates south west or south east (but not due south), and is large enough to carry a data logger than can capture info on a regular basis - probably a tall order.

 

[pianoman]

Regarding the arc being the shortest distance between two points: That's really only when drawn on a flat map, which has to show a round globe on a flat surface, thus distorting the continents. On a Mercator projection map for instance, the shortest route looks like an arc, but that is because of this distortion. On other projections, the arc can be less pronounced.

On an actual globe, the shortest route would be if you stretched a thread from one point to another. It is a straight line in one sense, though of course it follows the curvature of the surface. That's the way birds fly in the real world!

 

[KC Foggin]

Interesting thread and learned something I never knew about. Thanks one and all

 

[MJB]

Interesting point about the endogenous clocks. I presume that this relates to species that need to avoid certain obstacles, such as the Greenland race of Wheatear, that crosses the Atlantic by the shortest sea crossing, prior to travelling south to Africa. Perhaps this also applies to Common Swifts (race pekinensis), Willow Warbler (race yakutensis), Amur Falcon etc, which breed in Asia but may need the dog leg migration to avoid a long Indian Ocean crossing. I have wondered whether a 'dog leg' route may explain a paucity or complete lack of proven vagrancy of these birds to UK/Western Europe.

No, the endogenous clock doesn't usually relate to avoidance of obstacles. One example is the migration of Central European Starlings that initially fly west, usually in flocks, then over central France, make a sharp left dogleg that carries them SSW into Iberia. Another is the migratory divide of White Storks in Central Europe, where birds west of the divide fly arcing west into south to Spain and then over to Africa via the Straits of Gibraltar. Birds east of the divide fly east, often following the Danube as far as Hungary, then arc east into south through SE Europe, crossing the Bosphorus into Turkey, then south across Syria Lebanon and Israel before doglegging left into Egypt and then on to south of the Sahara. Mind you, White Storks, being long-lived, probably rely increasingly on identifying previous waypoints, for they mostly migrate during the day.

Re the Indian Ocean crossing, Amur falcons fly directly across from India to East Africa, feeding on dragonfly swarms en route, these insects taking advantage of the Inter-Tropical Convergence Zone that heads WSW across the Indian Ocean each year during the boreal autumn (Moreau 1938, Anderson 2009, & Dixon et al 2011: the last-named used satellite tracking). The Amur Falcon's return migration, on the other hand is mostly overland, spring records in Arabia being numerous and almost absent in autumn. Some cuckoo species and rollers also cross the Indian Ocean this way.
MJB
Anderson, RC. 2009. Do dragonflies migrate across the western Indian Ocean? J. Trop. Ecol. 25: 347-358
Dixon, A, B Nyambayar and P-O Gankhuyag. 2011. Autumn migration of an Amur Falcon Falco amurensis from Mongolia to the Indian Ocean tracked by satellite. Forktail. 27: 86-89
Moreau, RE. 1938. Bird migration over the Indian Ocean, Red Sea and Mediterranean. Proc. Zool. Soc. Lond. Ser A. 1938: 1-26 (?)

 

[DMW]

One species with an interesting migration is Aquatic Warbler, which migrates wsw in Autumn to the French Atlantic coast before turning south.

 

[occasional]

What ideally you would need would be a landbird, with a long migration route over hospitable terrain, which migrates south west or south east (but not due south), and is large enough to carry a data logger than can capture info on a regular basis - probably a tall order.

Does Eleanora"s falcon fit the bill ?

 

[MJB]

Does Eleanora"s falcon fit the bill ?

They have been geotracked crossing directly from NW Africa to Mozambique & Madagascar in SE Africa. Juveniles undertake this migration seprately from the adults two weeks after fledging...
MJB

 

[Muppit17]

They have been geotracked crossing directly from NW Africa to Mozambique & Madagascar in SE Africa. Juveniles undertake this migration seprately from the adults two weeks after fledging...
MJB

Looking at the research paper, I don't think this species fits the bill based upon the published tracks. It is certainly not a grand circle and the initial phases appear to be aggregating in the Mediterranean and then a dog leg across the Sahara.

It looks as if it is a route based on food availability not minimising distance travelled.

 

[Jon.Bryant]

Just reading Vagrancy in Birds, which I think answers my question. In the book it suggests that there is no real evidence that birds migrant along an arc as this requires 'map sense', i.e a knowledge of where you are and where you want to go (and I suppose an understanding that the world is sphere), to be able to calculate a varying bearing. The book suggests that migrants have an element of map sense, but this is acquired knowledge - after completing a migration birds can memorize a set of waypoints to assist in future migration. The book explains that certain species such as Marsh Warbler have quite complex migrations and staging posts, which even first-winter birds adopt. This suggests that birds can also inherit way points, which may explain why some species can perform dog-legs along a migration route. I suspect however that the route between waypoints is likely to be 'straight' (ignoring avoidance of adverse habitat [deserts, high mountains, long over-water crossings] and effects of drift, plus correction).