, Volume 13, Issue 3, pp 211-219

Sociality of grassland birds during winter

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Summary

Sociality of granivorous passerine birds occupying open grassland habitats of the southcentral United States was evaluated during winter in relation to habitat cover, seed density and bird density. Habitat height and habitat density, measures of exposure to potential predation, are associated with the major distinction of social pattern between the two granivore subgroups; sparrows, which tend to be more solitary, occupy the taller and denser grasslands, while horned larks (Eremophila alpestris) and longspurs (Calcarius spp.), which tend to be gregarious, occupy sparse and open habitats.

Group size increased with increasing seed density for sparrows and for longspurs, but not for both horned larks and longspurs or for all granivores taken together. For sparrows, group size increased less in relation to seed density than for longspurs. For granivores in total and for sparrows, bird density increased with increasing seed density. However, this association did not exist for longspurs.

The interaction of ecological variables may synergistically influence granivore group sizes. Habitat cover, in combination with other variables, appears to polarize the two granivore subtypes towards primarily gregarious or solitary strategies. Potential mechanisms leading to gregarious or solitary behavior are discussed in relation to hypotheses of predator avoidance and risk-sensitivity.