There is so much evidence for global warming coming from so many sources that it can't be denied. One more of those uses data that has been crowd-sourced by a citizen science organization called eBird. It's a database maintained by amateur birdwatchers who can report sightings of migratory birds via the organization's website. "Since 2002, eBird has collected more than 48 million bird observations from roughly 35,000 contributors," which makes it a great source of raw data for scientists trying to find unusual patterns.
Assistant professor of biology Allen Hurlbert, Ph.D., explains: “Timing of bird migration is something critical for the overall health of bird species. They have to time it right so they can balance arriving on breeding grounds after there’s no longer a risk of severe winter conditions. If they get it wrong, they may die or may not produce as many young. A change in migration could begin to contribute to population decline, putting many species at risk for extinction.”
The study concludes:
Across all species and geographic locations, species shifted arrival dates 0.8 days earlier for every °C of warming of spring temperature, but it was common for some species in some locations to shift as much as 3–6 days earlier per °C. Species that advanced arrival dates the earliest in response to warming were those that migrate more slowly, short distance migrants, and species with broader climatic niches.
Since the variation is per degree, this seems to say that rising temperatures will progressively have more and more impact on bird migration and further destabilize the adaptations that took a long time to evolve. Some species might not survive those pressure, especially when combined with other threats like habitat destruction and toxins in the food chain.
If you want all the details, the whole study is available on PLoS: Spatiotemporal Variation in Avian Migration Phenology: Citizen Science Reveals Effects of Climate Change
If you are interested in migratory birds, check out eBird.org.