Semipalmated Sandpipers (Calidris pusilla) and Lapland Longspurs (Calcarius lapponicus) share similar nesting habitat preferences on small islands in the Mackenzie River Delta, Northwest Territories, Canada. A Semipalmated Sandpiper was observed incubating a mixed clutch of its own eggs with two smaller eggs presumed to be from a Lapland Longspur. This is a unique combination of a precocial and an altricial species' eggs in the same nest. Possible explanations for the occurrence of this interspecific mixed clutch include nest parasitism, accidental laying, opportunistic egg dumping, or nest usurpation.