Tubenoses have smaller egg clutches and much longer and more variable incubation and fledging times than passerines with similarly sized eggs, resembling swifts in these developmental factors. Tubenoses and swifts have generally secure nest sites, but their food sources are unreliable, whereas passerines are vulnerable in the nest but food is usually plentiful. In the particular case of the storm petrel, it has a body temperature perhaps 3°C lower than other small birds, and this may also contribute to the lengthy incubation.