Summary (AGH): How much energy parent birds invest in producing young is obviously a critical part of breeding behaviour. We often think of this investment as feeding the young and defending them against predators, but one of the first steps in investment is how much effort to put into eggs in the first place. Given its limited resources, how many eggs a bird should lay in each nest, how many nests should it attempt each year, and how big should each egg be? Trade-offs are inescapable; for instance, the more eggs a bird lays, the more offspring are possible, but then the eggs will have to be smaller, and if eggs are too small the offspring might not survive (smaller eggs have less of the yolk that feeds the embryo before hatching). Most birds vary widely in all these respects, depending on environmental conditions. But sandpipers are surprisingly consistent, almost always laying just one clutch of four eggs per year. The Least Sandpipers on Sable Island nest much farther south than most of this mainly arctic species, yet this study showed similar patterns shown in other Least Sandpipers and shorebirds in general. Egg size was slightly bigger for bigger females, and increased slightly from first- to last-laid eggs, perhaps to help late-hatching chicks catch up to their earlier-hatching, but lighter, siblings. Otherwise, all birds laid single nests of four eggs each, even on Sable Island, so far south of the bulk of this species’ breeding grounds. Why shorebird clutches are locked in at four eggs is a puzzle, but that they do so on Sable Island suggests it isn’t because of characteristics of the arctic environment, such as its short breeding season only allowing limited investment. Instead, the explanation might have more to do with a trait all shorebirds share: their eggs are pyramid-shaped, rather than ovoid (literally, egg-shaped), so heat transfer during incubation is best achieved with just four eggs.
