Effects of birds on the intensity of ant rain: a terrestrial form of invertebrate drift

Large numbers of wood ants, Formica aquiloniacrawl up trees to gather food (arthropods and honeydew), and most return to the ground the same way. A small but noticeable percentage, however, returns to the ground by falling. This continuous rain of free-falling ants, from trees to the ground, is analogous to the drift of invertebrates in freshwater streams, and provides ecologists with an easily replicated terrestrial system for studying the phenomenon of drift. In a field experiment in this system, when birds came to forage, the total numbers and percentages of ants entering the rain increased dramatically. Initial background levels of ant rain (before birds came to forage) ranged from 0 to 11.9% (X=4.0%). When birds came to forage in certain trees, the percentage of ants entering the rain in those trees ranged from 2.9 to 28.4% (X=9.1%). In trees not visited by birds, ant rain decreased slightly during the same period to a mean of 3.7%. As the total time birds spent foraging in a tree increased, ant rain there also increased.