Are all hybrids sterile?
Contrary to popular belief, not all hybrids are sterile. Several species of frogs, canids, and snakes can interbreed to produce fertile hybrids.

In biology, reproductive barriers are mechanisms that prevent closely related species from interbreeding. There are two types of reproductive barriers: prezygotic barriers and postzygotic barriers. While prezygotic barriers prevent mating between two closely related species, postzygotic barriers reduce the chances of an offspring after mating occurs. Of the five prezygotic barriers, temporal barriers work in a unique way, as the prevention of interbreeding in this case is attributed to the fact that the two species in question reproduce at different times of the day, season, or year.

Temporal Isolation in Biology

So temporal isolation occurs when mating between two closely related species, with overlapping range, is prevented due to the difference in the time of sexual maturity―flowering in the case of plants. If behavioral isolation revolves around the difference in mating rituals of species and mechanical isolation around the difference in their genitalia, temporal isolation revolves around the difference in their time of sexual maturity.

Simply put, the two species cannot mate because their breeding season doesn't match. Maybe the two species breed in different seasons―one in winter and the other in fall, or maybe they mate in the same season, but at different times of the day―one during daytime and other at night.

Examples of Temporal Isolation

An oft-cited example of reproductive isolation resulting from differences in the mating season is that of the two toad species, the American toad (Anaxyrus americanus) and Fowler's toad (Bufo fowleri). Despite their overlapping geographic range―both are found in the eastern United States and Canada―and the fact that they have been successfully bred in laboratories to produce fertile offspring, they are not known to mate in the wild because the American toad mates in early summer, while the Fowler's toad mates in late summer.

Similarly, the western spotted skunks (Spilogale gracilis) and eastern spotted skunks (Spilogale putorius) cannot mate because the eastern spotted skunks are known to breed in late winter, while western spotted skunks breed in fall. Then there is the case two species of crickets, the spring field cricket (Gryllus veletis) and fall field cricket (Gryllus pennsylvanicus). While spring field crickets mature in spring, fall field species mature in fall.

As for species that are temporally isolated by the timing of the day when they are sexually active, the best example will be that of the two fruit fly species, Drosophila persimilis and Drosophila pseudoobscura. While the D. persimilis species is generally active in the early morning, D. pseuoobscura is active in the afternoon.

In plants, the timing, or rather the mistiming of flowering prevents interbreeding between two closely related species. Take the example of Canada lettuce (Lactuca canadensis) and grassleaf lettuce (Lactuca graminifolia). Both species are found in southeastern United States, but the fact that they have different blooming seasons―Canada lettuce flowers in summer and grassleaf lettuce in the early spring―means they rarely hybridize.

Besides the above examples, there are some unique cases wherein the two species in question cannot mate as a result of the difference in their breeding period, not in terms of seasons, but in terms of years.

It's the only one of its kind!
Of the seven recognized species in genus Magicicada, three follow a 17-year mating cycle, while four follow a 13-year cycle. While species with a 17-year cycle emerge every 17 years to breed, species with a 13-year cycle emerge every 13 years. In regions where their geographic range overlaps, their emergence coincides once in every 221 years.