If you want a perfect hatch, you need to start with perfect eggs.  Factors that influence an egg's hatchability include whether the egg was fertilized, whether the shell is in good shape, and how the egg was stored.  Below, I've included the section on seasons and parentage from Permaculture Chicken: Incubation Handbook --- be sure to check out the book to learn more about selecting eggs with a good shape and shell quality, how to store eggs before they go in the incubator, and what to expect with mail order eggs.

Time of year deserves consideration as you plan your hatch.  I prefer to incubate chicks in February and March so they're out on pasture during the peak grass-growing season of March through June, then I start another batch in late summer to take advantage of the secondary pasture peak in September through November.  If you're revitalizing your laying flock, it's important to have chicks out of the shell by early to mid April to ensure they are old enough to lay before days shorten in the fall.  Otherwise, you'll be feeding non-laying pullets all winter.

However, chicken biology dictates a slightly different hatch pattern.  As you can see in this chart, eggs tend to be most fertile in March through June, with a secondary peak in fertility in September and October.  Don't feel obliged to follow the chart's lead, but do be aware that your results won't be quite as good if you're trying to raise chicks in the dead of winter.

A related issue is the health of the eggs' parents.  Older poultry manuals admonish you to keep your breeding stock out on lush pasture, since the high nutrition forage results in eggs that are more likely to hatch.  In my own experience, I've found that breeds of chickens that produce brighter yolked eggs (meaning that the hens ate more bugs and greenery) have higher hatch rates.

While we're on the topic of parents, feed isn't the only important factor.  Hens and roosters who have just become sexually active or who are more than two years old will produce fewer fertile eggs.  I once tried to hatch eggs from four year old hens and had abysmal results, convincing me that even if these hens are still laying, most of the embryos aren't viable enough to mature.

Of course, you must have a rooster if you want chicks (even though a hen will lay unfertilized eggs when no males are around).  Some chicken keepers report issues with their roosters not mating frequently enough to ensure that all eggs are fertilized, but I've never had that problem.  On the other hand, I keep fewer hens than the recommended dozen per rooster, so my flock's patriarch doesn't have to work too hard to do his job.

Inbreeding is a more tricky issue for the backyard chicken keeper.  You may need to trade roosters with like-minded friends every year, or buy a round of straight run chicks to raise a new rooster annually, ensuring that your hens aren't mating with their brothers or fathers.  Inbreeding tends to lead to lower hatch rates and to a higher proportion of sick chicks out of the eggs that do hatch.

This post is part of our Chicken Incubation lunchtime series.  Read all of the entries: Permaculture Chicken: Incubation Handbook Why incubate? Choosing hatching eggs Filling in the incubator Chicks hatching