Age-specific diving and foraging behavior of the great grebe (Podicephorus major)

Abstract

The foraging and diving behavior of the Great Grebe (Podicephorus major) was studied in southern Brazil, from June to October 2005. A total of 547 dives performed by adult birds and 578 dives of juveniles were recorded. There was no age-related difference in dive duration (adults = 19.43 s; juveniles = 20.00 s) but the time elapsed between two successive dives (pause or recovery time) differed among age-classes (adults = 16.1 s; juveniles = 15.0 s). Duration of dive and pause times were weakly correlated in adults and not correlated in juveniles. Both age classes had feeding bouts of similar durations. Adults were more efficient in capturing prey (0.2 prey/min) in comparison with juveniles (0.1 prey/min) and took significantly larger prey. The main food item was the White Croaker (Micropogonias furnieri), an abundant demersal fish in the area. Juveniles had higher prey handling time in comparison with adults (78.2 s vs. 20.1 s), making them more prone than adults to being cleptoparasitized by other seabirds. Diving duration varied during daytime hours, with juveniles diving longer during early morning and late afternoon, while adults avoided foraging during midday hours. Feeding throughout the day could be a mechanism used by juvenile grebes to compensate for low foraging efficiency.