Female birds lay their eggs one at a time – usually one a day or every other day. While some birds incubate their eggs throughout the laying process, ducks and geese do not begin to incubate until all the eggs in their clutch have been laid – a process which can take up to 2 weeks. Although this means that eggs are left exposed to very cold environments, Arctic waterfowl have not modified their laying strategy. Instead, their eggs are able to survive temperatures as low as 10ºC or less – the equivalent of being placed in a refrigerator – but only during the period before incubation starts. The eggs do not develop at all during this period. Because they are all incubated at the same time, the eggs in a clutch all hatch at the same time, even though one egg is 2 weeks older than another.