Intraclutch egg‐size variation in the Barnacle Goose Branta leucopsis: an egg‐removal experiment

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Abstract
We carried out an egg‐removal experiment on Barnacle Geese Branta leucopsis to test the hypothesis that tactile contact between the brood patch and eggs accumulating in the nest during laying was causally related to intraclutch egg‐size variation, acting through the development of incubation behaviour. Egg removal had no effect on mean egg mass, relaying interval or clutch size: mean (±s.e.) clutch sizes for first clutches were 4.9 ± 0.9 and 4.7 ± 0.9 eggs for control and experimental females, respectively. We therefore conclude that Barnacle Geese have a determinate pattern of egg laying. Daily removal of all eggs from the first‐laid egg did not affect the within‐clutch pattern of egg‐size variation: in both control and experimental clutches the first egg was relatively small, the second or third egg was largest and there was a linear decline in egg size to the smallest, last‐laid egg. There was no significant difference in either the absolute or relative size of the last‐laid eggs in control and experimental clutches. Tactile contact with eggs in the nest, therefore, is not required for the expression of intraclutch egg‐size variation in Barnacle Geese. In this respect, determinate‐laying Barnacle Geese differ from species with indeterminate laying patterns (e.g. gulls) where egg removal does affect the normal pattern of within‐clutch egg‐size variation. If hormonal changes (e.g. prolactin) associated with onset of incubation are causally related to the intraclutch decrease in egg size, then the stimuli involved appear to differ between determinate and indeterminate layers.