Breeding Phenology and Reproductive Success of Little Blue Herons (Egretta caerulea) in an Urban Mixed-Species Colony

Abstract

As human populations continue to expand into suburban and rural areas, conflicts with wildlife are likely to increase. Many populations of long-legged wading birds (Family Ardeidae) face the challenge of human disturbance both at feeding sites and breeding colonies. Little Blue Herons (Egretta caerulea) are medium-sized wading birds that often join egrets and other herons in mixed-species breeding colonies. In June and July 2018, a random sample of Little Blue Heron nests located in an urban colony were monitored from incubation to fledging. The goals of this study were to (1) characterize adult nest-activity patterns over the course of the breeding cycle, and (2) document Little Blue Heron reproductive success in an urban colony and identify sources of human disturbance. Instantaneous samples with 5-min intervals were used to record adult activity during 50 h of observation at 24 nests. An additional 77 h of continuous observation at two different nests documented fine-scale activities and changes in nesting status. Observations made at random times during morning, midday, and evening periods yielded 636 nest-hours across all breeding stages. During incubation, attending parents mostly sat (63 %) or stood (23 %) in or near their nests, spent less time in nest maintenance (5 %) and self-maintenance (scratching, preening; 5 %), and seldom (< 1 %) left the nest unprotected. By the end of the breeding cycle, nests were unattended 69 % of the time. The 26 monitored nests fledged a total 39 chicks (1.4 ± 0.3 chicks/nest attempt). The 16 successful nests (61 % of all nests) fledged 2.4 ± 0.2 chicks. Known causes of nest failure included destruction or usurpation by Cattle Egrets (Bubulbus ibis) and predation by Black-crowned Night-herons (Nycticorax nycticorax). Sources of disturbance included dogs allowed in the colony, intentional loud noises, and human presence. We compare reproductive success at this urban colony to that in less disturbed colonies reported in the literature.