Chick can spawn in random locations sometimes. I often have them walking around outside after spawning through a block wall. I also pick up eggs outside the wall.

One tile seems extremely short distance. I only use the hearths, but I haven't found the maximum distance from that yet. I have two hearths on my main floor, and the chicken coop in the "basement" under the hearths. The total space under there is probably 12x12, and there is nowhere that they are not warm. The bottom of the hearths are 3 blocks high, so they're definitely overhead.