Egg size variation was studied in a Great Tit Parus major population in eastern Spain during 13 years. Mean egg volume was 1.50 cm 3 (S.D.=0.12, n=521 clutches). Mean egg volume differed significantly between years, the differences being caused by a single year with an exceptionally large mean egg size. We found no relationship between mean egg volume of the year and mean air temperatures during the three months previous to the breeding season. Mean egg volume of the year was not related to mean laying date, but was positively related to mean clutch size of the year. Mean egg size was not related to laying date or clutch size in any of the 13 years. In conclusion, egg size showed a considerable constancy in the Great Tit population studied. A review of within-population trends of egg size variation in Great Tits showed that, though most studies reported no or little egg size variation with year, time of season or clutch size, all the possible trends were found in one or another population. Moreover, different trends could be found in the same population in different years. Studies lasting several years seem to be necessary to find out the potential responses of egg size to different environmental conditions.
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