Sandwich terns Sterna sandvicensis feeding on shell fractions

During four evenings at the onset of egg laying, in late April and early May 1995, at least five Sandwich Terns Sterna sandvicensis were observed eating shell fragments on the island of Griend in the Dutch Wadden Sea. This behaviour was not observed before or after the egg-laying period. For fish-eating birds like terns, it has always been assumed that their regular diet would provide enough calcium for eggshell formation. Our observations on shell-eating Sandwich Terns may indicate that this assumption does not always hold.