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Aggressive head-up displays in great-tailed grackles (Quiscalus mexicanus).

Article Excerpt
In great-tailed grackles (Quiscalus mexicanus) the head-up display is used by both sexes as an aggressive signal to compete for resources such as food and mates (Johnson et al. 2000). This display is silent and typically consists of an individual pointing its head back while keeping its feathers sleeked against its body (Johnson & Peer 2001). This display has been described qualitatively (Johnson & Peer 2001), but no one has collected quantitative data for the behavior. Both male and female great-tailed grackles mate with multiple partners (Johnson & Peer 2001). These birds are sexually dimorphic, with male grackles weighing approximately twice as much as females (Jaramillo & Burke 1999).

This study provides the first quantitative data describing head-up displays in great-tailed grackles in relationship to sex and social context. Both male and female grackles are known to give the head-up display and the first objective of this study was to see if there was a difference between the sexes in the likelihood of using head-up displays. The second objective of this study was to examine in which social context head-up displays were most frequently given. Three types of social groups were defined for this study: (1) a group composed entirely of same-sex individuals, (2) a group in which...

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