If you cannot reach a wildlife rehabilitator right away
Keep trying. Wildlife rehabilitators are very busy, especially during the spring and summer. If you get an answering machine, leave a message and make it easy for them to reach you. Call around to others in the area. Be willing to arrange transportation for the animal once a rehabilitator is found. Remember that there is no government funding for wildlife rehabilitation in Ontario, and all rehabilitators are funded almost entirely by donations.
While you may be tempted, you cannot keep the baby bird or care for it yourself. The vast majority of information on raising wildlife found on the internet is incorrect and, even worse, harmful to baby wild animals. Learn more here.
If you cannot find a wildlife rehabilitator able to admit the baby bird you have found, and you have had the baby for 24 hours or more, and reuniting the baby with their mother has failed, unfortunately options are limited. In most cases, you will be referred to contact your municipal animal services agency. Learn more here.