Call rate as an index of nest count in wading bird colonies

Traditional monitoring methods for colonial waterbirds, such as nest counts, can be expensive, time-intensive, disruptive to nesting birds and limited by colony accessibility. Our study tested the use of acoustic sampling as a colony monitoring technique for three species of colonially nesting herons (Ardeidae) by quantifying the relationship between chick call rate and nest count in colonies. Because acoustic detection can be influenced by soundscape variables, we measured day since colony initiation, body condition index of small heron chicks, nest height, vegetation density and wind speed and evaluated their effects on call rate detection. Generalized linear mixed-effects models showed that call rate detection increased with daily nest count for all species. Snowy Egret Egretta thula and Tricoloured Heron Egretta tricolour detected call rate was also dependent on environmental, temporal and physiological variables, whereas Great Egret Ardea alba call detection was affected by vegetation density around nests and wind speed. Our study showed that wading bird nest counts can be estimated using acoustic sampling, reducing the need for manual colony surveys, and facilitating long-term monitoring of wading bird nesting trends, which are important measures of wetland restoration worldwide.