Global warming or climate change is known to have many effects on plants, but there has been relatively little research on global warming and animals, partly because too few long-term studies have been carried out. In northern areas, mean temperature is believed to be rising, and indeed during the last decades mean spring temperatures in Finnish Lapland have increased. In 1975-1993, mean air temperature during the main egg-laying period of a pied flycatcher Ficedula hypoleuca population correlated positively and significantly with mean egg volume of that population. Since larger eggs enjoy improved hatching success, global warming may alter birds' reproductive strategies, because warmer weather may allow females to invest more resources in reproduction. This in turn may help birds rapidly conquer new areas when they become available and compensate for rising mortality rates to be expected elsewhere where warming means desiccation.