Mixed-species flock sizes and compositions influence flock members' success in three field experiments with novel feeders

Abstract

Mixed-species groups and aggregations are quite common and may provide substantial fitness-related benefits to group members. Individuals may benefit from the overall size of the mixed-species group or from the diversity of species present, or both. Here we exposed mixed-species flocks of songbirds (Carolina chickadees, Poecile carolinensis, tufted titmice, Baeolophus bicolor, and the satellite species attracted to these two species) to three different novel feeder experiments to assess the influence of mixed-species flock size and composition on ability to solve the feeder tasks. We also assessed the potential role of habitat density and traffic noise on birds' ability to solve these tasks. We found that likelihood of solving a novel feeder task was associated with mixed-species flock size and composition, though the specific social factor involved depended on the particular species and on the novel feeder. We did not find an influence of habitat density or background traffic noise on likelihood of solving novel feeder tasks. Overall, our results reveal the importance of variation in mixed-species group size and diversity on foraging success in these songbirds.

Fig 1. The three novel feeder contexts assessed.

a = transparent tube feeder with regular seed mix, from Experiment 1. b = plastic blocks feeder with diverse seed mix, from Experiment 2. c = separate food source with diverse seed mix, from Experiment 3. The separate food source structure is 3 m closer to the photographer than the feeding station.

Fig 2. Carolina chickadee success at solving the transparent tube feeder related to mixed-species flock (MSF) diversity.

Data are medians (thick black lines), 25- and 75-percentiles (gray boxes), and ranges not including outliers (whiskers).

Fig 3. Tufted titmouse success at solving the transparent tube feeder compared to the number of titmice in the flock.

Data plotted as for Fig 2, with addition of outlier (circle).

Fig 4. The relationship between the number of chickadees and the latency to solve the plastic blocks feeder task for successful chickadee flocks (circles).

Fig 5

Tufted titmouse success at solving the plastic blocks feeder task compared to the diversity index of the flock (a) and to the number of titmice in the flock (b). Data plotted as for Figs 2 and 3.

Fig 6

Carolina chickadee success at solving the separate food task compared to mixed-species flock (MSF) size (a) and the relationship between the number of chickadees and the latency to solve the separate food task for successful chickadee flocks (b). Data plotted as in Figs 2–4.

Fig 7. The relationship between the interspecific flock size and the latency to solve the separate food task for successful titmouse flocks (circles).

Fig 8. White-breasted nuthatch success at solving the novel food source task compared to the mixed-species flock (MSF) size.

Data plotted as for Figs 2 and 3.