The analysis covered 1332 Tree Sparrow eggs, 1053 of them hatched, in 257 clutches of which 114 were fully successful, 119 partially so, and 24 were deserted. The mean egg mass in deserted clutches was lower than in clutches where hatching success was total or partial but not significantly so. The mean hatching success per clutch increased with the mean egg mass. The individual eggs in fully hatched clutches weighed significantly more (2.21 g) than those in partially hatched clutches (2.17 g), and were also heavier than “infertile” eggs (2.14 g). During nest inspections to ascertain the egg laying sequence, the egg mass in 5-egg clutches with full and partial hatching success was found to be higher than in deserted clutches. The eggs in fully hatched clutches were broader than “infertile” eggs.