Gull (Laridae) species usually lay two or three eggs. We tested the hypothesis that clutch size is determined by incubation ability of parents in Slaty-backed Gulls (Larus schistisagus) whose normal clutch size is three on Teuri Island. Their clutch sizes were manipulated to be two-egg clutches where one egg was removed, three-egg clutches with no manipulation (control) and four-egg clutches where one egg was added. Hatching success and hatchling masses were not different between experimental and control groups. The mean incubation period of the four-egg clutches (29.2± 1.9 days) was one day longer than that of the two-egg clutches (28.1± 1.4days) and control (28.2 ± 1.6 days). The mean egg temperature of four-egg clutches (33.0 ± 2.8 °C) was lower than that of two-egg clutches (36.2 ± 1.8 °C) and control (35.9 ± 1.8 °C). These results suggest that the parents can not incubate experimental enlarged clutches effectively but this does not affect offspring production.
Vegetational characteristics of 20 nest stands and 35 nest trees of the Northern Goshawk (Accipiter gentilis) were investigated in the Tokachi plain, eastern Hokkaido. Of 20 nest stands, 12 were isolated woods, 4 were shelter belt and 4 were forests, respectively. In all cases, within circular plots of 5-km in radius centered at nest sites agricultural land accounted for 47.3 to 87.8%x. Tree densities (no. of trees/ha) in nest stands were significantly lower than those in 40 woods which selected at random and have no nest. However, no difference in tree diameter at breast height and basal area were found between nest stands and random plots. The relative dominance of larch (Larix leptolepis) within nest stands was significantly higher than broad-leaved trees and evergreen conifers. Larch was used more frequently as nest trees than other tree species. There was no corre-ation between distance from nest trees to forest edge and nest stands size. Jests were usually constructed at the base of horizontal branches in conf-fers, or in large primary crotches of deciduous broad-leaved trees and were usually at the approximately half of nest tree height. There was open. space around the nests.
Sexing without sacrificing or injuring birds is essential for field studies, though it is difficult to sex alcid species with no obvious sexual dimorphism or sex specific display. Rhinoceros Auklets Cerorhinca monocerata breeding on Teuri Island, Hokkaido, Japan had sexual difference in external measurements. A discriminant function was derived from external measurements of 34 male and 39 female breeding auklets. The sexes of the auklets could be separated by using the discriminate function D=114.22-3.25BD-0.64HL, where BD is bill depth and HL is head length. D was negative for males and positive for females. The efficiency of this discriminant function was 91.2% for males and 100% for females.
The habitat of 52 nest sites of the Grey-faced Buzzard Butastur indicus was examined in a low mountain region of central Japan. Most (90-100%)of nest trees were within 600 m of a stream and paddy field on the valley bottom. Therefore, the availability of wet areas which offer food (e.g.frogs and snakes) may be an important factor affecting the nest site selection. Eighty-eight percent of nest trees were on the middle or lower portions of steep slopes, irrespective of the direction of the slope aspect.Sixty-nine percent of nest trees were either trees at the forest edge or prominent trees within the forest. So, the accessibility for birds approaching the nest may also be an important factor.
The wintering flock of the Mandarin Duck Aix galericulata was observed from January to March 1999 at Talsodo near Cheju Island, Republic of Korea. The Mandarin Ducks are globally threatened and have been estimated about 50, 000 birds all over the world. They have been designated as a natural monument, No.327, in Republic of Korea. Mandarin Ducks were observed at the study area from 19 January to 15 March. The number of Mandarin Duck showed a peak, 2, 550 birds, on 27 January. This is the first record of a large wintering flock for the species in Cheju Island, Republic of Korea. This area is an important habitat for wintering Aix galericulata. We need to have a protective policy such as monitoring the population size and disturbance factors, etc. of the habitat for the conservation of this species.