House-hunting by honey bee swarms: collective decisions and individual behaviors
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Thousands of individuals in a house-hunting honey bee swarm make a collective decision for one among many nest sites discovered. We recorded the dances on swarms in a forested area, where one swarm's search encompassed about 150 km2 and many different sites. We then analyzed swarms in a desert area with only nest sites that we provided and monitored, to study how the swarm winnows multiple finds to a single site over the course of a few days. Most bees did not visit any site, very few visited more than one. Apparently choices were made with little or no direct comparison, through the interaction of two mechanisms: positive feedback through recruitment leading to growth in the number of scouts visiting good nest sites, and attrition reducing activity and recruitment for non-chosen sites. Individual differences between bees substantially affected these dynamics. Scouts varied considerably in amount of dancing and persistence, but most that danced did so vigorously after their first few visits, and then dropped out, ceasing their dancing though continuing to visit the nest site. Dances were nearly twice as long as reported for nectar and pollen. Scouts followed dances of others, and occasionally visited alternative sites, but rarely switched their dancing. When unanimity is reached, the bees must recognize that a decision has been made, break up the swarm cluster, and fly to the nest site. Buzz-running (Schwirrlaufen) probably plays a role here, but we observed less buzz-running than previously reported, and this occurred even early in the process; it might function as a chain-reaction effect triggering the end of the house-hunting process. Our results suggest that the choice among nest sites relies less on direct comparison of nest sites, and more on inherent processes of positive feedback and attrition by dancers dropping out.
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