You certainly don’t see every single bird that visits the area of your yard, so how do you get a rough headcount? Experts disagree on details, but they’re in the same ballpark. Count the birds you see in your yard in a typical ten second span (count those at your feeders, flying nearby and those just watching). Do it several times and take the average. Multiply that average by 5 or 6 (that’s where they disagree) for a very approximate number of birds who look to your yard for support. his method figures in the birds you don't see - those that are deep in the vegetation, in a nest or high in a tree.
Be as ignorant of property lines as the birds themselves. They could care less about who owns a certain tree or hedge. “If you can see it, you can count it” should be your guideline.
Birds
like plenty of space for feeding, and it prevents the spread of disease. Jockeying for
space probably increases a bird's stress level, making it more susceptible to
disease. While images of dozens of birds crowded onto a single feeder are
attractive, the likelihood of contact between sick and healthy birds grows. To
prevent overcrowding, use feeders that minimize physical contact between birds, and
provide additional places to feed. Increase the number of feeders
too. By far your most important task is to keep both the feeders and the
feeding site clean.