I've never had a problem with Mareks in any chickens, but sebrights are a bantam and as a rule for all bantams a little more delicate as chicks. As adults they have been robust birds for me, rarely sick if at all. As with most chickens, how friendly yours will be is based upon how it is raised...
When I hatch eggs, I just call it an embryology experiment, and when you look at it that way you are more open to the thought of defects in each embryo's (chick's) genetic defects and why they didn't hatch.
No they wouldn't have hatched, any eggs that don't hatch after the 24 hr mark or so really aren't meant to make it (without assistance and there's ALWAYS a reason why the chick didn't hatch) because they probably had some kind defect like a twisted internal organ that didn't allow them to peck...
Well I know a little about pre-vet stuff but I only took one class on it so far, the advanced one isn't until the next semester. But I believe you're supposed to check for any sharp points in the swollen area, use two finger to do so and don't apply pressure if there is immediate danger to the...
Oh man, I can't wait for my sebright to molt and get his new feathers, he's just amazing but his comb is more henny that rooster-ery. Here's him a while ago <3