The effect of ambient temperature before and during laying on egg volume in birds has been studied by many authors. The objective of this paper is to show that trends in daily temperatures changes can also influence egg volume. The study was carried out near Warsaw, Poland in 1994 and 1995. We ascertained the laying sequence, size and volume of 1070 eggs in 211 clutches of the Tree Sparrow. The effect of the trend in temperature on the mean egg volume in a clutch explained 0.4% of its variation (0.03% to 4.2%, depending on the brood-period), and the effect of actual temperature explained 0.9% (0.5% to 1.6% depending on the brood-period). The joint effect of temperature and its trend explained 3.1% (1.7% to 8.9% depending on the brood-period) of variation in the mean egg volume in a clutch. The authors discuss possible mechanisms of the effect of temperature and temperature trend on egg volume.