Date: 3/20/17 6:52 am From: Geo Kloppel <geokloppel...> Subject: Re: [cayugabirds-l] Migration Video and question
Colombia, Venezuela, the Guyanas, and northernmost parts of Brazil and Ecuador actually lie in the northern hemisphere, where days have been lengthening ever since our winter solstice. Right now (at equinox) the rate of photoperiod change has reached its maximum, noticeable even in equatorial regions. I presume that seasonal migrants are sensitive to that rate, which has been accelerating ever since December 21st, reaches its peak today and now begins decelerating toward the next (our summer) solstice. The amplitude of the cycling rate of change is subdued in the tropics, but it's the very same cycle that is so pronounced in the higher latitudes where these warblers breed each year, so I doubt that they lose track of it, even if they winter at or south of the equator, as some do.
> On Mar 20, 2017, at 8:22 AM, Peter <psaracin...> wrote:
> Folks.......I have a spring migration question and wonder if anyone out there can help. I understand that the lengthening days ignites hormonal responses in birds and, among other things, encourages "migratory restlessness" - an "itch" to begin their respective journeys north. But how does this mechanism work with respect to neo-tropical warblers? After all, for those spending their "winters" in northern So. America the days will be shortening!!! The "photoperiod" will be decreasing.
> What, then, is the trigger to get them on the move and heading northward?
> Thanks for the help.
> Pete Saracino