If a person is prosecuted for destroying a condor egg, it is the reality of the egg being a living organism and member of the condor species that triggers the illegality of destroying it.
From my understanding of avian biology, by the time a fertilized avian egg hatches, the embryo inside has already (1) passed the point-of-no-return in its development, and incidently (2) been packaged with everything necessary to develop into a hatchling, needing nothing more than an atmosphere which is at the right temperature and, FWIU, occasional slight physical motion. As such, it is without a doubt a self-contained organism.
Different species of plants and animals have enough differences in their development cycles that analogies tend to be imprecise. Nonetheless, my impression is that nearly all sexually-reproducing species produce offspring by first building a "scaffolding" which will later be discarded, and then commencing work on the organism within. In some sexually-reproducing series, the scaffolding is produced before fertilization (e.g. in avians) while in others it is produced afterward, but in almost all species it is produced somehow (not sure about marsupials).
I dunno, I guess I'd like to think that personhood is something that can't really be produced in a test tube. And so maybe my other beliefs are conformed to that (rightly or wrongly). Besides, as I've noted elsewhere, if we both agree that a 40-week fetus is a baby, why not focus efforts on points of agreement rather than disagreement?