Living in a group
• Costs of group living
• Benefits of group living
– Predation avoidance
– Resource acquisition
• Optimal group size
Costs of group living
• Competition for resources
• Increased risk of parasitism or disease
• Increased opportunities for
reproductive interference or
Competition for food in fieldfares
Nestlings die primarily from starvation
Ectoparasitism in cliff swallows
Nests treated with insecticide
produce much larger chicks
Reproductive interference
Brood parasitism,
Extra-pair copulations
Reproductive suppression
Predation avoidance benefits
• Reduce encounter rate with predator
– Protected sites
– Selfish herd
• Reduce success of predator
Confusion, predator-predator interference
Mobbing, cooperative defense
Protected sites
The selfish herd
Predator dilution
More eyes and ears
Predator confusion
Predator confusion
As school size of prey increases, capture success decreases
Predator interference
Schreckstoff attracts other pike, which increases handling time
Cooperative defense:
Resource access benefits
• Passive attraction to limited resource
• Active attraction due to joint benefits
– Reduce path overlap
– Information transfer
• information center
• producers-scroungers
• acquire public information
– Group foraging
– Communal hunting
Passive attraction
Butterflies at a salt deposit
Information transfer?
Information transfer in osprey
Information transfer
in evening bats
Leaders “produce”, followers “scrounge”
Two pigeons produced, 14 scrounged
The producer-scrounger game
Spice finch were taught
to open lids and were
producers. Other birds
had to wait to scrounge
Public information
• Forager joins group to acquire information
about food availability and/or predator risk
• By observing foraging success of others in
the group, a naïve forager can estimate the
quality of a food patch
• Starlings left empty patches sooner when
foraging with a partner than when foraging
Public information
Reduce path overlap
Proposed for sparrow flocks in Mojave desert. Never been tested.
Group foraging
Permits capture of prey larger
than possible by singletons
Optimal group size
Group hunting in wild dogs
Larger packs kill larger prey, have higher capture success, and
travel shorter distances in a hunt, but must share kills with
more members of the group
Optimal size of wild dog foraging groups
Lion foraging success