I've been mulling on this thought for a while, but as of last night, I have two three-week-old squabs to feed and I figure I probably need to find the answer. So far, I have been feeding them defrosted peas. Does this contain enough nutrients and minerals for squabs to live on, or do they really need chick feed or similar?
As a general rule, there are very few foods that are a source of complete nutrition. Always add some variety to diets, whether it's your own diet, or your Squab's.
As for the question itself, I'll refer you to a good nutritional site:
You can see by looking at the charts that although peas are a good source of a variety of nutrients, there are a few important ones that it's lacking (mainly Vitamins B, D, E, and a couple minerals). That would suggest that if this was all they had over the long term, they may become deficient in these nutrients, so I would definitely add more variety.
Note I'm completely ignorant of the nutritional requirements for Squabs. This is just to outline the weaknesses of feeding them only peas.