Our towns have hundreds of thousands of more trees today than there were a century ago, and consequently, the kinds of birds we're apt to see today can differ from what was common years ago, and during the period of transition from fields to trees. Other factors are affecting bird populations. When I was a kid, seeing a cardinal was a big deal. Yet, cardinals are now almost as common as robins. It wasn't until the 1940s that they started really moving into the state.
Small things can cause big differences.
The once-rare Northern Mockingbird is now common. Of course, some species are declining because of destruction of habitat here or in their wintering grounds, parasitism, pollution, and climate changes. Skyscrapers and tall antennas kill many migrants each year. Still other species are increasing because man messed with Mother Nature. Witness the mobs of Canada Geese.