Most birds sense when their clutch is complete via tactile stimulation of their brood patch, the featherless area on their bellies that warms the eggs. But there is evidence that some species count their eggs by sight.
American coots (pictured above), for example, are rather good at spotting (and removing) eggs left sneakily in their nests by neighbours during the laying period. If the parasitic egg is a very close match to their own, however, not only do they fail to spot it, but they apparently lay one less egg, suggesting that they are deciding how many eggs to lay based on the total number in the nest.
The theory is controversial, though - it's possible that the parasitic eggs are added once the laying period is drawing to a close.
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