Heidi Faulkner, Project Assistant says:

Hi Murray, Although it’s impossible for us to know the cause of each specific increase and decline of birds, there are several common causes for bird population fluctuations:
* Habitat changes frequently affect bird numbers. If there has been any change in your neighborhood–such as trees being cut down, new houses being built, or different crops being planted on nearby fields–that could be the reason you are seeing more or less birds.
* Natural food supplies–such as pine cones, berries, seeds, and insects–fluctuate from year to year, causing birds to shift ranges to take advantage of food surpluses or to compensate for food shortages.
* Weather fluctuations often cause birds to shift ranges, especially in winter.
*Predators such as hawks or domestic cats sometimes move into an area and cause birds to move elsewhere.
You can see what others are reporting in your area on our Bird Summaries by State or Province page here.