Yep - it's strictly biological, they will lay and if there is not a nest box available they will lay on the floor, in the run, under a building, behind a door - anywhere that is available and suits them. I've seen birds drop eggs standing at the waterer, etc - this year there was even a hen who...
It is entirely "normal" that none of your birds are laying just yet - at 14-16 weeks of age they are still young. In general, most will start laying somewhere between 18 and 28 weeks. As you have had the nest boxes blocked off, it is most likely you have instilled a pretty firm "roost to...