MODERN BIRDS AND MIGRATION

["Matthew Troutman" <m_troutman@hotmail.com>]

<<First, I am trying as a layman to establish how old bird migratory 
behaviour is. Is it as old as the modern bird species? Or might it be 
inherited behaviour from the predecessors of the modern species?>>

Migration has been speculated to be present in non-avian dinosaurs, 
which is of course not the much more complex aerial migration seen in 
some temperate (Northern and Southern Hemisphere) neornithine, 
neognathus birds (that is, most living birds except for ratites and 
tinamous).  Aerial migration is a very physiologically costly exercise 
(see George and Berger 1966; _Avian Myology_ for a discussion on the fat 
reserves in avian pectoral muscles) and, indeed, some physiologists have 
linked the evolution of modern bird endothermy with aerial migration 
(this is based on the presumption that _Ichythyornis_, a very close 
relative to neognathus birds, was a tern-like migrator and migration of 
some sort is primitive for ornithurine, that is all modern birds and 
their nearest relatives, birds).  Until future interpretation suggests 
otherwise, ornithurine birds date at least 120 million years, possibly 
more.  I would suspect that aerial migration itself probably evolved at 
that point, or a few million years later. 

<<Secondly, how old are the modern species, on average? Are strongly 
migratory species, such as the Bar Tailed Godwit, importantly younger or 
older as a species than other, less strongly migratory species?>>

The fossil records of all Recent bird genera may go back only a few 
thousand years or up to 35 million years in the case of some buteos.  
Sometimes the more recent (and more derived) genus has less migratory 
tendencies than its ancestors and vice-versa.  There is no clear trend.  

<<Thirdly, is it known or is there a good theory as to why birds migrate 
in the way they do? The Godwits migrate vast, distances, for example,
from Northern Europe to New Zealand. Why so far? If it's just food or
water there's plenty of that a lot nearer than New Zealand. Migratory 
behaviour is predictable, regular, systematic, in fact, highlyevolved. 
Are there many species that travel similar distances to the Godwits, or 
are they unique?>>
 
Long migration distances in birds is certainly not unique amoug 
vertebrates (or even animals).  So far, the best theory (in my opinion) 
is that long migrations are charged by the changing seasons in temperate 
zones and at the poles.  Moving away from birds for a second, look at 
the variation in spring and summer habitats for North American/South 
American vespertillonid bats.  Some individuals stay near Panama, 
whereas others summer as far north as near the Artic Circle!  It all has 
to do, probably with how their ancestors migrated, because a bird or bat 
seems to not be able to migrate without doing so with its parents and/or 
"flock".  As for long distances travelled by terns and godwits, it was 
probably a gradual trend for some individuals to travel far North and 
South.  Another factor might be certain individuals or flocks watching 
other individuals for flocks.  

Migration in birds, at least, is also aided by a type of "navigation" 
that has to do with the brain and ear morphology with other things.  

<<I have an idea that, when this type of systematic migration started, 
the distances between the points of departure and destination were much 
closer than they are today: I am thinking of a geographical model of 
such as that of Pangea, shown on the Website "The Dynamic Earth" of 
which the address is:
http://pubs.usgs.gov/publications/text/dynamic.html#anchor19309449
 
 But the dates given in this model are 225 million years ago for the 
start of the breakup of Pangea, the Geography of which might offer a 
feasible geographical scenario for the start of migratory behaviour.  By 
the Triassic the distances between the continents might already be  too 
far for such as the Godwits to discover such places as New Zealand by 
accident: it seems to me that they must have established their migratory 
behaviour before such distances were a fact.>>

This would only be true if birds (specifically modern birds) are 225 
million years old.  Few, even detractors from the dinosaur/bird (OK, 
maniraptoran/bird) link, view modern birds, or EVEN birds to be that 
ancient.  Larry Martin and Samuel Tarsitano believe that birds diverged 
from other archosaurs at an early date like this (and there is some 
possible fossil evidence to back this up, assuming that _Protoavis_ is 
an ancestral bird or a proavis and Martin and Whetstone's interpretation 
of crocodylomorph braincases holds up as well as Tarsitano's "primitive" 
braincase features in birds).  However, the best evidence for the first 
birds lies 150 million years ago and these species (_Archaeopteryx_, 
Korean bird) are not far removed from non-avian dinosaurs or even 
maniraptorans.  The best date, assuming a maniraptoran ancestry of 
birds, for the origin of birds is probably 160 million years ago.  
Assuming a crocodylomorph proavis, probably 225 million years ago.  The 
maniraptoran ancestry of birds is more likely however, since all fossil 
evidence supports it and anatomical evidence.  

Another point that should be made is that migration in some modern bird 
species may not be linked to common ancestry, some birds do not migrate, 
but rather to the climate in which a bird lives in.  Aerial migration 
probably evolved several times with Aves.  

<<But surely: such modern bird species do not date from the Permian, or 
even the Triassic? Surely they date from much later?>>

See above.  The first neornithine birds (the ancestors of ratites, 
according to Peter Houde, were fliers and were also migratory) date from 
the Late Cretaceous, around 100 million years ago (Alan Feduccia thinks 
otherwise, but all fossil evidence suggests not). 

<<So: why do birds migrate in the manner in which they in fact do? And 
when did they start to display this particular type of behaviour? And 
how do we account for the sometimes enormous distances involved?>>
 
See above.  Probably greater variation in regional distances and gradual 
(perhaps punctuated) invasion of farther northward and southward (or 
eastward to westward in the cattle egret and many, many other birds and 
bats).  

<<Thankyou to anyone having the patience and taking the time to respond 
to this probably quite complex query. Feel free to respond to me 
personally if I am so off-topic that I should not be on this NG, but as 
I stated at the top, I cain't find nowhere else to go...>>

No problem.

>> David.

Matt Troutman


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