Another, more frustrating conflict will come from homeowners who own bird feeder. As you no doubt have already experienced, birds are not the only ones who enjoy the mix of seeds and corn used in bird feeders. They also attract squirrels like mad, and the little guys are quite inventive when it comes to getting their hands on some of that feed. They can climb up just about anything and hang from the most precarious positions for as long as they feel they need to. Squirrels tend to be a bit greedy about unprotected food sources and, if you have set up your feeder for the pleasure of some early bird watching you may be disappointed to find all your feed gone but no birds in sight.
A good idea is to attach a baffle to your bird feeder. This is a plastic dome with a rotating system. It blocks access to the seeds inside but uses a mechanism to dump small amount of seed onto the ground below. It seems like a concession, but getting squirrels to give up on a free source of nuts and seed is an uphill battle few homeowners are going to win. Giving a small amount of feed while leaving enough for the birds is a healthy compromise. After all, there are few repellents that will drive away the squirrels without also driving away, or harming, the birds you are intending to attract.
Alternatively you can mix in foods that the birds will like but which squirrels will find uninteresting. These include safflower seed, nyjer thistle, and white proso millet seed.
Some also find that the best solution is also the simplest. Just create a distraction by leaving out a feeder specifically for the squirrels and keep it full so that they are not tempted to pilfer from the bird feeder. Most animals are lazy, after all, and will go for free food every time. Note that this can be as simple as nailing up a board with a stalk of corn affixed to it. As long as a single squirrel cannot carry the whole thing away, they will be content to share and leave the birdseed to the birds.