Late Friday afternoon, I shot a video of a Semipalmated Plover behaving agonistically toward a conspecific on the beach at the mouth of the river Jiboa, on the Pacific coast of El Salvador. Although originally nearly two minutes in length, I edited the video for 'postability' on line.
In itself, aggression between Semipalmated Plovers is not that unusual. However, according to the literature, conspecific agonistic behavior is rare in Semipalmated Plover on the wintering grounds. Adam Smith and Erica Nol studied winter foraging behavior and prey selection of the Semipalmated Plover in coastal Venezuela and found that "neither sex exhibited territorial behavior" (Smith & Nol, 2000). Two other researchers also studied the Semipalmated Plover on the wintering grounds in Venezuela, and found that "time devoted to aggression was very low" (Morrier & McNeil, 1991). They did find a significantly higher 'time-aggression index' in October, as compared to the rest of the winter season, and cite other sources that mention agonistic behavior at migration stop-over sites.
If true, why would Semipalmated Plover exhibit such behavior more in migration, and less on the wintering grounds?
Well, assuming that perceived food scarcity drives agonistic behavior, perhaps a wintering plover, having had more time to familiarize itself with local food resources, might be less territorial than a less well-informed migrant passing through, or a recent arrival?
Morrier, A. & McNeil, R. (1991) "Time activity budget of Wilson's and Semipalmated Plovers in a tropical environment", Wilson Bulletin 103 (4): pp. 598 - 620.
Smith, A.C. & Nol, E. (2000) "Winter foraging behavior and prey selection of the Semipalmated Plover in coastal Venezuela", Wilson Bulletin 112 (4): pp. 467 - 472.