EGG SIZE, TEMPERATURE AND LAYING SEQUENCE - WHY DO SNOW GEESE LAY BIGEGGS WHEN ITS COLD

1. Several studies of altricial passerines ('income breeders') have sh own that females lay smaller eggs when they experience low temperature s during egg formation, This is usually explained in terms of a 'physi ological trade-off': birds having to divert more of their finite resou rces to maintenance and away from reproduction in cold weather, althou gh little is known about the proximate mechanism(s) underlying this ph enomenon. 2. We investigated the relationship between egg size and amb ient temperature during egg formation period in the Lesser Snow Goose (Anser caerulescens caerulescens), a precocial, 'capital-breeding' spe cies, using 10 years of data derived from a long-term. population stud y of this species at La Perouse Bay, Man., Canada. 3. Egg size was neg atively correlated with ambient temperature in the Snow Goose, i.e. fe males laid larger eggs when temperatures were lower during egg formati on, contrasting markedly with the positive correlation reported in alt ricial, income-breeders. 4. The effect of temperature on egg size was most marked for last-laid eggs within a clutch (b = 0.675 g degrees C- 1), and for temperatures during the last half of the rapid yolk develo pment (RYD) period (rather than for the whole or the middle of the RYD period), as predicted by an exponential growth curve for yolk develop ment. 5. The relationship between egg mass and temperature was time- a nd laying-sequence specific: for first and middle eggs the correlation was positive early in the period of RYD and negative closer to ovulat ion. For last-laid eggs the correlation between egg mass and temperatu re was negative throughout the 10 day RYD period. 6. We suggest that t he different relationship between egg size and temperature in the Snow Goose, compared to passerine species, may reflect different contribut ions of exogenous vs endogenous reserves, and different opportunities to increase energy availability through dietary intake. 7. Although st atistically significant, temperature has only a small effect on egg si ze in the Lesser Snow Goose: the average range of temperatures encount ered would only lead to a 3-5% difference in mean egg mass, This is co nsistent with high heritability of egg size.