Hi Sharon,
First of all it's a Rook not a Crow. The bulge you are seeing is known as gular skin or gular pouch. It's an expandable piece of skin attached to the lower mandible which allows temporary storage of food. Not to be confused with the crop which is part of the digestive system. So nothing to worry about.
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Tony
My Flickr Photostream
Ah thank you so much for the info. I did question whether it was a crow due to its size. First time I've had one in my garden and have never witnessed this on them before. Glad he/she is okay.