We have a hummingbird feeder and a tube-type bird feeder. We put finch food in the tube feeder, and as a result, we get a lot of finches (duh!). Chickadees also visit, and towhees come for the stuff that falls onto the ground. Every now and then a jay stops by, but the feeder is surrounded by a sort of large-mesh cage so big birds can’t use it very well, nor can squirrels.
My posting about the Lawrence Berkeley Laboratory turkeys a few days ago got me thinking about the idea of displacement: when 60 turkeys move into an area, the food that they eat is subtracted from the food that other animals used to eat. More turkeys means less of something else.
In the case of the finch food, we’re providing extra food: more finches doesn’t necessarily mean less of something else. But it might. For one thing, the finches don’t eat just from our feeder, they also forage for other food (or at least, I assume they do; certainly they have to on the very rare occasions that the feeder is empty for a day or two). Also, they need nesting sites, and the sites that the finches choose aren’t available for other birds. In our case, I’m pretty sure the displacement effects are small: the main effect of our feeder is that, in addition to what birds we would have otherwise, we also have finches. But I’m not positive. And I know that the general assertion that providing food for birds helps the birds that eat it but doesn’t hurt anything else would be false: for example, crows attracted to campgrounds by the food that people leave also eat the eggs of other birds, so feeding the crows (intentionally or not) decreases the populations of other birds in the area.