The Effect of Weather on the Roosting Behaviour of the Little Egret Egretta garzetta in Winter

Abstract

The effects of weather on arrival and departure from the roost by Little Egrets Egretta garzetta, were investigated between December 1982 and May 1983, at a winter roost, at Suinin-tenno Mausoleum, Nara City, Japan.(1) Light intensity seems to be the most important factor affecting both arrival at and departure from the roost. Additional factors were temperature, foraging rate, and hunger level, of which temperature seems to be the most important.(2) The mean light intensity at the start of departure (14.3 lux, 16.1 min before sunrise) was lower than at the start of arrival at the roost (355.2 lux, 3.3 min after sunset)(3) The mean light intensity at the termination of departure (393.41 lux, 2.1 min before sunrise) was higher than that for arrival (4.7 lux, 19.6 min after sunset).(4) Both the mean time spent for arrival (24.4 min) and that for departure (18.2 min) were short; however, the former was significantly different from the latter. Time spent for a series of roosting behaviours (including pre-roosting assembly) was much longer than that for departure (including pre-flight activities).(5) The lower the temperature, the more widely egrets scattered over the foraging site.(6) (i) Gathering at pre-roosting assemblies started 125.3 min before sunset on average. The light intensity at the time of these gatherings was below 14, 000 lux (for 80% of the gatherings). (ii) Light intensity was 700 lux and below when 70% of pre-roosting birds had gathered, on three quarters of all observation days. (iii) Arrival at the roost began earlier on extremely cold days, and was delayed on days of high temperatures. In warm days, more individuals arrived in large flocks over a shorter time. (iv) Egrets chose sites where were not exposed to wind for pre-roosting assembly in the forest during the cold season. (v) No effect of rain, snow, or fog was observed on pre-roosting assembly.(7) (i) Pre-flight activity began with "goah" calls in the roost. (ii) Activity started 40.2 min before sunrise when the light intensity was below 0 lux, and lasted for 35.2 min. (iii) Departure was hastened by low temperature. (iv) Low temperature shortened the pre-flight activity period. (v) Some departing flocks returned to the forest on freezing morning, stayed in the forest for several hours then left again for foraging sites. (vi) Departure from the roost started with a single individual. Large flocks dispersed immediately after departure from the roost. (vii) Decreasing light intensity caused by rain delayed the termination of departure.