I wanted to add my two-cents worth on this interesting conversation.  There 
are a few points that I think previous respondents have not addressed.  One 
is that migration is an incredibly complex phenomenon, and though a great 
deal of research has gone into understanding it, no one can claim to know 
exactly what prompts individual birds to get up and go. Birding experience 
and historical records can help you guess, but individual birds show up in 
odd places all the time. This may occur more and more with changes to 
habitat and climate, which are happening blisteringly fast. What we know 
about migration patterns in the recent past might not tell us much about 
what's going to happen in the near future. Birds do not have time to evolve 
in response to global climate change, and they may respond in unpredictable 
ways. Lastly, a response to the comment that "neotropical migrants have no 
way of knowing that conditions may be suitable on their breeding grounds 
when they are thousands of miles away."  To this I say that we also have to 
consider the changes occurring on the wintering ground. I don't pretend to 
know how warming temperatures, shifting rain patterns, and a loss of 
habitat on the wintering ground will impact the migration habits of many of 
these birds, but we should not take a "temperate-centric" view on this. 
Birds don't just migrate to Colorado because it's a good place to breed; 
there are factors prompting them to leave their wintering ground as well.

Amber Carver
Littleton (Arapahoe Co.)

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