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Hi James and others,
There are a number of strategies that birds use for southern migration. The case mentioned of Ruby-throated Hummingbirds demonstrate the strategy of differences of fall migration timing of adult males from that of and adult females and young, perhaps to reduce competition for resources along the route. My observations of migrating adult females and young on Brier Island show that the available resources there may be vigorously defended by individual, non-adult males. So while it may lessen competition it certainly doesn't eliminate it.
For raptors, males and females migrate together in loose associations and their size differences (females are significantly larger than males) that allows each sex to exploit different sized prey. So rather than a timing difference it is a physiological difference that may enable the species to better exploit the available resources.
Other examples that I can think of are Empidonax flycatchers where most adults migrate before young birds and warblers where there doesn't appear to be much of a timing difference between males and females nor probably between adults and young. However, there may be an age specific difference in migration route selection or even in the type of migration that takes place. Minimal steps (or even a non-stop trip) for adults and multiple steps for young.
Lastly there are species such as Blue Jays which are known to employ several different strategies. Some birds will overwinter, some birds will be short distance migrants and some will be long distance migrants.
The risks/rewards you've listed in your email are varied and would certainly provide good explanations as to why these varied strategies evolved.
All the best,
Lance
From: naturens-owner@chebucto.ns.ca [mailto:naturens-owner@chebucto.ns.ca] On Behalf Of James Churchill
Sent: Monday, September 10, 2012 1:49 PM
To: naturens@chebucto.ns.ca
Subject: EXTERNAL: Re: [NatureNS] Intriguing question?
Hi folks,
Some additional thoughts, from another non-expert, on a neat topic...
I'd add that if zugunruhe (loosely, impulse to migrate) and migration behaviour are not entirely genetically controlled (which seems to be the case) males and females should be expected to experience different selective pressures in terms of timing of migration from breeding grounds to wintering grounds related to differences in costs and benefits to males and females of leaving vs. staying.
This might differ based on things such as the distance between breeding and wintering grounds, seasonality of risk (food availability, climate/weather patterns, predator abundance/distribution etc), mating system (as Chris and Rick allude to) physiological and/or genetic constraints and environmental/social conditions experienced on wintering grounds. I would guess, that these factors should also lead to differences in the relative timing between males and females among species but also among m/f of the same species in different geographic locations (i.e., no population-level selection; but perhaps depends on definition of 'population').
Since ruby-throats are polygamous, and males are reported as not providing parental care, there could be significant fitness benefit to males of sticking around so they could achieve additional fertilizations with late-breeding or re-nesting females (not sure if there is any evidence of this for hummingbirds?). But perhaps the potential benefit of this is outweighed by other factors such as resource availability during migration, securing a good wintering territory, or physiological constraints.
cheers,
James.
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#1F497D'>There are a number of strategies that birds use for southern=
migration. The case mentioned of Ruby-throated Hummingbirds demonstr=
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lt females and young on Brier Island show that the available resource=
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