A bit on citizen science and some other interesting tidbits of information can be found in this opinion piece from the New York Times:
Phenology data, from Pennsylvania and from other locations in more northern latitudes, tell us that many bird species are arriving earlier in the spring than they did 40 or 50 years ago. The results are the same regardless if trained scientists or citizens are reporting their observations over the long term. The difference is that scientists typically publish their data in peer-reviewed journals; citizen scientists publish (or crowd source) their data in public databases such as e-Bird maintained by the Cornell Lab of Ornithology (see http://ebird.org/content/ebird/about).
What we don't yet know is how this early arrival impacts the ability of birds to find food after a long migration. Are their spring food sources -- mainly insects -- hatching earlier? We know that the fuel reserves of birds can be largely depleted after their long flight; we don't know if they are able to sufficiently replenish their energy needs. If not, how is this impacting the health and survival of neotropical migrants -- our long-distance athletes?
And if spring arrival of birds and the emergence of insects are no longer in sync, what is happening to the population of insects? The trend has been for milder winters. Bats are in decline. And both of these two observations lead to more bugs.
Do the birds find shelter if the leaves on trees and shrubs aren't unfurling earlier? The earliest plants to leaf-out are typically woody invasive plants. Are the birds o.k. with these new types of temporary shelter? Birders create list of the birds they see on a given day at a given location. But they typically don't report the habitat conditions, so we don't really know.
Is there a competitive advantage for the "early bird"? Perhaps, but we don't know if the worms are available. These squiggly animals also migrate, but do so vertically through the soil. (This migration was described by Edwin Way Teale in his 1951 book North With the Spring.) Contributors to Journey North can track spring observations of earthworms (see http://www.learner.org/jnorth/worm/index.html), but I don't know of anyone linking these data with that of the timing of bird arrivals.
Do the birds build nests and lay eggs earlier? If so, is there a greater risk that a cold snap will diminish their reproductive success in a given season?
So many questions. So many birders and nature lovers/observers are needed to help us find out more information in our changing environment.