Hard to tell from that photo, but at least if it is a roo it won't be messing with your supply of unfertilized eggs anytime soon. There is a particular structure to rooster feathers on the neck that hens don't get (unless they're so imbalanced hormonally that they are completely rooster looking); if you compare this bird to your others and notice any different structure it's a good sign it's a roo. While females can also get those fancy neck feathers that have fringing to them with a narrow centre of normal feather type, roosters get very long fringes in comparison. I hope that makes sense, lol. If a boy that age is 'getting his roo feathers on' you will likely also see the shoulder/wing ones and rump hackling starting to emerge. They're a lot different from normal feathers. Anyway, best wishes.
These are chicks; they won't be developing hackle feathers for some months. You usually see saddle feathers before hackle feathers, but I wouldn't look for either before about 10-12 weeks. Do not try to sex chicks by using neck feather development.
I would guess your mystery bird is a female Black Sex Link. Here's an image search so you can see what they look like: http://tinyurl.com/lpe2zo4
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