Effect of Water Level and Time of day on Black-Crowned Night-Heron (Nycticorax nycticorax) Foraging Behavior at an Artificial Weir: 2013–2014

Abstract

Black-crowned Night-Herons (Nycticorax nycticorax) feed primarily at night throughout the year but have been reported to also feed diurnally during the breeding season. Little previous research has compared daytime and nighttime foraging activity in this species. To better understand how light level affects foraging behavior, a pilot study was conducted in 2013 at an artificial weir located in Wichita, Kansas, used by Black-crowned Night-Herons and other wading birds. Based on 24 h of observation (14 h day, 10 h night), it was determined that many activities (strike rates, capture rates, capture efficiencies, relocation rates) did not differ between day and nighttime, but birds captured significantly larger fish during the day and also experienced significantly higher rates of aggression. That study was expanded in 2014, during which water levels at the weir differed significantly from 2013. An additional 63 h of observations recorded almost four times as many birds per hour in 2014 than in 2013, both during day and night. Strike rates and capture rates both were significantly lower in 2014 than in 2013, but the aggression rate was significantly higher. Neither capture efficiency (successful strikes/total strikes) nor prey size differed between 2013 and 2014. Since many measures of foraging behavior and social interactions differed between years, it is likely that water level influenced prey detection and availability, and high levels of aggression further limited prey capture. Despite lower rates of energy intake in 2014, diurnal foraging by Black-crowned Night-Herons appears to be important in meeting increased demands of reproduction.