Thank You, Christian, for the well-reasoned (and tactful) post. I agree with you completely. I thought it was worth pointing out that long-distance migrants are generally not able to simply respond to favorable weather conditions and migrate early. Long-distance migration is an incredible feat requiring an enormous suite of complex physiological adaptations (especially for passerines). Long-distance migrants rely on photperiodic (day length) cues, not only to indicate when they should leave their wintering grounds, but also to initiate the physiological/hormonal cascades required to achieve the body conditions required of migration. Furthermore, neotropical migrants have no way of knowing that conditions may be suitable on their breeding grounds when they are thousands of miles away. For these reasons, it is more likely that short-distance migrants are able to respond plastically to conditions on or near their breeding grounds, both because they have more information and because short-distance migration doesn't require the complex "machinery" that long-distance migration does. Adaptation by long-distance migrants to earlier springs will require many generations of positive selection on those earliest-arriving individuals in order to slowly shift their entire migratory program and their responses to photoperiod cues.
I guess the main points are, 1) As Christian pointed out, understanding distribution and status (of which, arrival time is part) is a key to good birding and recognizing what is unexpected and what is not, 2) Most long-distance migrants are not capable of simply getting up and leaving their wintering grounds in response to conditions on the breeding grounds, and 3) Some species are more capable of early arrivals than others.
Certainly, the status of migratory birds is always in flux, and we will continue to see changes in the status and distribution of our familiar species. That is part of what makes birding fun and part of what makes it scary these days. As Christian said, extraordinary arrival dates warrant skepticism, but carefully documenting these events will allow us greater insight into how and to what extent species are capable of adapting to accelerating climate change.
Final note: Southern Nevada, where I review eBird records, has had a number of record or near-record early migrants this spring including Hammond's and Ash-throated Flycatchers and Wilson's Warblers. Note that these are all short-distance migrants.
Enjoy your Spring!
Respectfully
Carl Lundblad
Moscow, ID (often in CO!)