If you can't always observe for the same time each day, you can still compare results by standardizing your data - calculate the number of visits per minute or per hour. If the feeder is very busy, you might want to have several students observing and recording, and average the results between the teams. Students could investigate how much food is actually consumed by measuring or weighing the amount that is put out and comparing to how much is left after a day or week. Look at seasonal trends by comparing data from fall, winter and spring. You don't have to collect data every day - one day a week is enough.
Bird feeders can attract other animals, including bears.
This experiment is best done between December and March when bears are usually hibernating. This is also the best time to see birds. Bring feeders inside during other times of the year, especially at night.