I am skeptical of bird feeders for a long list of reasons. People have mentioned vulnerability to cats. I also have seen Cooper’s hawks at feeders, but this is more part of nature so it bothers me less. People have also mentioned the spread of disease.
Another problem I’ve noticed is that feeders often keep alive species that might die or be driven farther south in cold winters, and these species can have detrimental effects on local bird populations. The big two I notice are brown-headed cowbirds, which are nest parasites and harm warbler populations, and house sparrows, which compete with cavity-nesting birds for nesting sites. Both of these can be supported by feeders in much larger populations than they could exist in the wild. You can reduce this effect by buying different seed, but probably not eliminate it. I’ve noticed that cowbirds in particular but most birds will turn to less-than-preferred food sources in the coldest, snowiest weather, and that’s when this stuff is critical.
I also am worried about changes to behavior and ecological effects on other species. I’ve read studies establishing that feeders do significantly change bird behaviors. Birds need to learn skills to survive in the world, and they have culture, i.e. passing on skills to the next generation. Time spent at the feeder is time not spent foraging in the wild, and the birds are learning a completely different set of skills to thrive at the feeder (often involving unnatural types of competition with other species they would not compete with as much in the wild.)
There can also be cascading effects on plant species. Year-round resident birds like blue jays, chickadees, nuthatches, and the like often cache seeds, and this act of caching is an important seed distribution mechanism for native plants. I have found a number of native plants with heavy seeds (i.e. not able to travel far in the wind) that came up too far from a source population to have been carried by wind alone, that I strongly suspect originated from such caching. When we feed birds at feeders, we take them away from paying attention to native plant seeds. Also, in many cases we end up having them cache and then spread into the wild various non-native seeds, since much of the seed used at feeders, even “high-quality” seed, is not native. I have seen a number of grasses coming up in my surroundings that I strongly suspect originated from bird feeders, such as Sorghum bicolor, which can be invasive. And even when the seed is native, purchased seed lacks the true diversity and adaptability to the local habitat of the seed that would naturally be in the area.
If people want to feed birds in winter, the best thing to do is to grow locally-native plants. I have done this with great results. For example, I’ve seen the mourning dove population in the surroundings of my home soar by a factor of 5-6 as I’ve planted and favored native plants with large seeds, that they prefer. I’ve seen dark-eyed juncos and native sparrows like white-throated sparrows and song sparrows increase too, and go after slightly smaller seeds. The native plants also support birds during the nesting season, through increased insect food, something feeders generally don’t do at all.
The native plants then also spread out into the surroundings. So for example, I live near railroad tracks. I planted some native wild lettuces, Lactuca biennis and some Lactuca canadensis, on my side of the tracks, and the wind blew the seeds onto the other side of the track where they are now spreading up and down the tracks. These plants have nice, big seeds that birds like, and during the growing season they support aphids, which warblers love in migration.
So it’s not just that there are problems with feeders, it is that there are other things we could be doing that address the problems facing birds in a much deeper and more powerful and more sustainable way, like gardening with native plants and removing problematic invasive plants from our property and surrounding wild areas.
If people put the amount of money and energy they put into bird feeders into more natural solutions, imagine the good we could achieve!