Mixed species flocks of birds as a strategy: behavior and seasonality in yungas foothill of north-west, Argentina

It has been long recognized that bird mixed-species flocks are seasonal in temperate forests and annual in tropical forests, however, seasonality of mixed flocks in the subtropical forests remains poorly studied despite their potential role to clarify how intermediate climatic conditions affect flocking behavior. We surveyed the number of mixed flocks, number of species and number of individuals in mixed flocks in Yungas-foothill of Northwest Argentina, to examine seasonality of mixed flocks and flocking behavior in response to climatic elements (temperature, humidity, wind speed) and fragmentation, to test the hypothesis that birds flock more frequently under harsh conditions. In both, continuous forest and forest fragments, mixed flock formation was seasonal (they occurred more frequently and had more species and individuals during the harsh dry season, than during the more benign wet season), and harsh climatic conditions lead to flocking (the number of mixed flocks, and number of species and individuals in them increased as temperature and humidity decreased within and between seasons). Although forest fragments were climatically harsher (drier and windier) than continuous forest, fragments had fewer mixed flocks with fewer species and individuals. Since flock formation was a function of the number of species and individuals, either reduced connectivity has limited the number of individuals reaching the forest fragments or forest fragments were so harsh that birds could not thrive in them. The benefits gained by flocking remain to be elucidated, but whichever those benefits are they should be understood in the context of seasonal variation in life-history traits.