Among the factors that can change what shows up in your yard are:
- The neighborhood food supply. Feeders are only a fraction of the food source for birds. A change in the natural food supply can make a lot of difference in their local bird populations. For instance, last season’s acorn crop was apparently not a large one hereabouts. The mast, the annual production of tree nuts in a region, can vary from year to year, and a low mast year can drastically affect populations of Blue Jays as well as mammals, such as squirrels and chipmunks. Blue Jays are major consumers of acorns (and may plant more oak trees than do squirrels, which get all the credit). Wild Turkeys are also big acorn-eaters; wonder how they are doing.
- Neighborhood habitat. Changes in the places where birds nest, roost and feed can vary the population. For example, some birds like evergreens for roosting and nesting. If someone cut down, or disease killed, a noticeable number of evergreens in the area, certain species might become less common.
- Predator changes. A hawk or perhaps even an owl deciding to nest in the middle of an area popular with song birds would discourage the songbirds from hanging around. Even something as seemingly innocuous as a family cat – and worse, feral cats – can affect the bird population in a small area. (The number of birds killed by pet cats each year would astound most people.)
- Man-made environmental changes. Obviously, major sources of noise, light, or air pollution can affect local bird populations. Set up a rock crusher next to a stand of trees that have sheltered birds for years, and you will undoubtedly chase them all away. But less obvious changes can occur. For example, spraying trees for caterpillars may kill many of the insects the birds feed on, forcing them to look elsewhere.
- Disease. Of course, disease can affect local and regional populations of birds. We saw just recently many reports of Pine Siskins dying, probably from Salmonellosis. West Nile virus, to which corvids seem more susceptible to than many other species, may be lowering the jay population.