Predicting the optimal prey group size from predator hunting behaviour.

Area of Science:

• Behavioral Ecology
• Population Dynamics
• Predator-Prey Interactions

Background:

• Group size influences prey vulnerability and non-lethal predation consequences.
• Theory predicts optimal group sizes balancing anti-predation benefits against costs.
• Predator attack success and dilution effects are key factors in group dynamics.

Purpose of the Study:

• To investigate flock size risk thresholds for attack rate, success rate, and dilution in redshanks (Tringa totanus) preyed upon by Eurasian sparrowhawks (Accipiter nisus).
• To determine how these factors influence selection for optimal group size.
• To identify which thresholds best predict commonly observed redshank flock sizes.

Main Methods:

• Analysis of redshank flock sizes in relation to Eurasian sparrowhawk predation.
• Quantification of sparrowhawk attack rate and success rate as a function of redshank flock size.
• Calculation of individual risk reduction, incorporating dilution effects and attack probabilities.

Main Results:

• Sparrowhawk attack rate peaked at 55 redshanks, while attack success rate declined exponentially with flock size, showing a 95% decrease by 70 birds.
• Individual risk of death decreased by 95% by flock size 30, with dilution alone accounting for a 20% reduction.
• Redshanks commonly formed groups that maximized individual predation risk reduction, often exceeding benefits from dilution or reduced attack rates.

Conclusions:

• Redshank flocking behavior is primarily driven by the reduction in predator success rate, leading to group sizes larger than optimal for dilution or attack rate benefits alone.
• Individual variation in risk-taking behavior may constrain the formation of the largest possible groups.
• These findings have significant implications for understanding animal population dynamics and the non-lethal effects of predation risk.