I'm so very sorry for your loss.
Yes, birds are a common way of communicating that our loved ones use. Sometimes folks will be sitting outside, and birds will bravely come right up to them, and even land on them. Others have had deer come very close and stay still, etc.
Ben, I know everyone has differing opinions, but I think our bodies are just an envelope or shell we use while on this earth. Our eternal spirits are also housed in these bodies. When we die, our spirits continue to live in a spirit body. They've just left the physical body that housed them for awhile. Whether that physical shell is cremated or buried whole doesn't really matter to that living spirit who has moved out of that temporary house.
Many people who die during surgery or an accident, then come back into their bodies, report how when outside their bodies they often don't even recognize who that "poor person" is that just died. They feel so light outside their body.
These folks who have passed are still alive and well; they've just moved into "another house," so to speak. Our loved ones can communicate with us, but we have to learn to recognize "how" they communicate with us.
Please read the book "Hello from Heaven" by Bill Guggenheim, and look over this website. Click on the button on the left of this page -- "About This Site."
Then click on "What is an ADC?" and "Sample ADC's." This may whet your appetite to learn more about how your mom can communicate with you. She wants you to know she is alive and well on the Other Side.
Blessings, and may you find the answers to your questions.