1. Size proportional to time. Generally larger birds incubate longest.
2. State of development at hatching. Even in altricial birds, limited resources produce smaller eggs that hatch quicker.
3. Predation rates. With high predation rates, there is selection pressure to get out of the nest as soon as possible. Hole nesters incubate for shorter times than do open cup nesters. In Costa Rica, Skutch found that birds that nested low to the ground incubated for 11-13 days, open cup nesters in trees incubated 14-20 days, while hole nesters took 17-24 days. Each type of nest on this less has a correspondingly lower vulnerability to predation.
4. Availability of food--most birds just can't afford to spend all their time on the nest.
5. Environmental temperature--the colder it is, the slower they embryo growth rate. Early spring clutches take longer to hatch.