There are only a few instances where you can enter Canada with nothing but your birth certificate, and it is still recommended that you have additional documentation when you travel abroad from Canada.
As a citizen, you can potentially re-enter Canada with nothing more than a birth certificate. This is especially true if you are a child, because you have not yet gotten a passport.
Older Canadians, regardless of how they got their citizenship, should carry their Canadian passports with them when they travel abroad to facilitate re-entry and to ensure identification at customs.
Permanent Residents will need a few documents to re-enter Canada. The first and probably most important is the Permanent Resident Card. The PR Card is about the size of a credit card and can be used for quick interactions at customs to prove your Permanent Residency.
Generally, when you travel abroad you should carry your passport. As a Permanent Resident of Canada, you will still retain the citizenship of your home country and will carry one of their passports, this will be insufficient evidence for Canadian border control.
If you will be outside of Canada for an extended period of time, you may want to consider contacting Citizenship and Immigration Canada and securing a Travel Document to make it easier to re-enter the country. As a Permanent Resident, spending too much time outside of Canada could make the CIC think that you had abandoned your claim to live in Canada.
Foreign residents who wish to visit Canada will most likely have to obtain a visa and a passport. There are some instances where you don’t necessarily need a visa, but you certainly cannot enter Canada with a birth certificate alone.