Nest-site defense by competing honey bee (Apis mellifera) swarms during house-hunting

In cavity-nesting animals, competition for suitable cavities can be particularly strong when multiple groups of the same species migrate synchronously to found a new home. This may be the case for honey bees during the reproductive season, because neighboring colonies often cast swarms simultaneously, leading to potential competition for high-quality nests. To test the idea that honey bee swarms may compete for and defend potential cavities, we observed pairs of artificial swarms that were house-hunting concurrently. Workers from one swarm in each pair carried a gene influencing body color, so that the bees from the two swarms were easily distinguished. We set up a high-quality nest box and waited for nest-site scouts from each swarm to explore and recruit swarm-mates to it. We recorded all the interactions between competing scouts at the box and found that, when scouts from both swarms explored the box simultaneously, they behaved agonistically against bees from the other swarm. The level of aggression depended on the number of scouts from each swarm present at the box. When only 1-3 scouts from each swarm were at the box, they rarely fought. But when the scouts from one swarm outnumbered those from the other swarm (4-20 vs. 1-3 bees), those in the majority advertised their presence with a buzzing behavior at the entrance opening, and started to mob and kill those in the minority. When one swarm gained clear control of the box (20+ vs. 0-1 bees), some of its scouts guarded the box’s entrance, preventing entry by foreign scouts