The importance of weather in relation to food resources, in determining reproduction, remains poorly understood for mammals, particularly for species that do not depend on food resources limited by spring weather conditions. We studied the effects that weather and food supply had on timing of spring reproduction and observed litter size in 2 squirrel species, the Siberian flying squirrel and the European red squirrel, using long-term data sets spanning 20–30 years. Both species subsist on foods from tree masting, and these are available for squirrels from autumn until early spring. Good food conditions in winter and spring before reproduction had positive effects on spring reproduction in both species by advancing the onset of reproduction, and in flying squirrels, slightly increasing litter size. Higher temperature in late winter and, surprisingly, increased precipitation in late winter resulted in early reproduction in flying squirrels and red squirrels, respectively. In addition, higher early spring temperature was positively related to litter size in red squirrels, likely reflecting low survival of small juveniles in cold weather. Our study supports the view that spring reproduction in these species is determined by food supply before breeding. Our results also highlight the fact that reproduction is also dependent on weather.

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