AbstractA ground‐nesting gull or tern (Laridae) parent will attack a foreign conspecific chick that is substituted for the parent's chick at about the time the parent's chick is mobile. Presumably, parents discriminate among chicks using some combination of vocal, morphologic, and behavior cues. It is not known which cues are used. Morphologic variation in chicks' down potentially provides discrimination cues in some species of Laridae, such as the Caspian tern (Sterna caspia). To test discrimination based on the down color of chicks, a parent's chick was replaced sequentially with foreign conspecific chicks that were similar and dissimilar to the down color of the parent's chick. This cue‐isolation experiment indicated that parents rejected dissimilar chicks more frequently. Thus, parents learn the down color of their chicks and then use this information as a basis for aggressive rejection of foreign chicks. A recognition system based on morphology allows parents to detect and reject foreign chicks with a probability greater than cha
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