Bats use three separate tricks to decouple autumn mating from spring birth. Many vespertilionids store sperm in the female reproductive tract over winter and only ovulate in spring. Some species delay implantation, holding a blastocyst unattached in the uterus. Miniopterus uses a third route: fertilisation and implantation both occur in autumn, and it is the growth of the embryo that is slowed, a state called delayed development or embryonic diapause.
Which mechanism a species uses is tied to how it hibernates. The retarded embryo resumes normal growth as the female warms in spring, and birth is timed to the emergence of abundant insects. Bent-wing bats then gather at a small number of large maternity caves, where thousands of females give birth within a narrow window.
That concentration is the species' vulnerability. Because reproduction depends on a handful of specific caves, disturbance, quarrying or tourism at a single site can affect a disproportionate share of a regional population. The species is assessed as Vulnerable.