Breeding Bird Populations in Relation to Vegetational Change in a Grassland in Hokkaido

Transcription

1 Jap. J. Ornithol. 35: 67-73, 1986 Breeding Bird Populations in Relation to Vegetational Change in a Grassland in Hokkaido Yuzo FUIIMAKI and Mariko TAKAMI Laboratory of Wildlife Resource Ecology, Obihiro University of Agriculture and Veterinary Medicine, Inada, Obihiro, 080 Agricultural lands of Hokkaido consist of mosaic of pastures, paddy, cultivated and fallow fields, farmstead shelterbelts, groves and so on. In these situations of decreasing native grasslands open areas such as pasture and fields provide important breeding habitats to grassland birds. In agricultural lands, however, vagetational changes by human-related activities such as cultivation, mowing and grazing by livestock would affect the birds exploiting the vegetation cover as nesting and foraging sites. We have obtained some information on breeding forest bird communities in relation to vegetational alteration (FOJIMAKI 1970, 1986, KOBAYASHI & FUJIMAKI 1985), but knowledge of how the habitat change might affect bird populations in open country is almost lacking. During the 3-years study of bird populations in a grassland in Obihiro, eastern Hokkaido, this study plot was used for grazing by cattle after the second summer, and was transformed from an undisturbed tall-grass field to a short-grass one. This paper describes the changes inn habitat structure and in breeding bird population that accompanied this environmental alteration in a grassland. STUDY AREA AND METHODS This study was conducted from 1977 to 1979 in a grassland located at the farm of the Obihiro University of Agriculture and Veterinary Medicine (42*52'N, 143*11'E), a typical inland agricultural land of eastern Hokkaido. The study plot of 5.6 ha was bounded by a road on the south side and is surrounded by mowing fields for hay on other sides. The plot was a fallow field which experienced several full growing seasons without disturbance and mainly consisted of Dactylis glomerata, Phreum pratense, Poa pratensis, Agrostis alba, Phalaris arundinacea, Artemisia montana and Breea setosa when the study started. A small stream runs along the west boundary of the plot. There are shrubs, predominantly Alnus japonica, along the stream and Pinus banksiana of 2 m high at 5 m intervals along the road. Vegetation measurements were made from mid-may through mid-july every year. The vegetation cover was recorded at the hights of 0, 0.15, 0.3 m, and thereafter 0.3 m intervals up to 2.1 m, then at 1 m interval from 3 m upward at 110 points spaced 25 m intervals each other (standing dead vegetation was excluded). The

2 68 Yuzo FUJIMAKI and Mariko TAKAMI [Jap. Vol. J. 35 ornithol. No. 2/3 vegetation structure was constructed by determining the percentage of points with foliage at each height (Fig. 1). Grasses and forbs began to grow in early May. Of the dominant plant species P. arundinacea, A. montana and B. setosa attained 1.5 to 2 m high in July. The vegetation structure in 1978 was similar to that in 1977 by mid-july until the plot was exposed to grazing by cattle. After that forbs were eliminated except for shortened stems and grasses remained short. In 1979 forbs did not grow due to the intensive grazing in the previous year (Fig. 2). The vegetation cover was composed evenly of grasses and continued to remain 40 to 60 cm high during the growing season because grazing was resumed in mid-june (Fig. 1). In 1979 twigs of shrubs were almost eaten by cattle up to 1.5 m high from the ground. The mean monthly temperatures for April, May, June and July were 4.1, 10.5, 13.8 and 17.5*, with mean precipitation of 50, 53, 74 and 75 mm for the respective months (meteorological data from the University Farm). Censusing of birds on the plot was conducted by the spot-mapping method described by KENDEIGH (1944). Stakes of 2 m high were placed at 50 m intervals to enhance the accurate mapping of territories and nests. The location and activity of every bird seen on each visit were recorded on a scale map of the plot. Intensive nest searches were conducted simultaneously with or immediately after the census. The number of pairs was calculated for each species from the number of males occupying a territory and/or the number of nests. When the limits of the certain territory reach outside the plot, it was designated as 0.5 pair. Most of birds occurring on the plot were captured in mist-nets and were marked with a combination of celluloid coloured rings on the left leg and a numbered aluminum ring on the right leg. For marked males the return rate was determined as the percentage of the number of birds which returned to the number of birds marked in the previous year. The plot was censused 58, 34 and 15 times respectively from late April to late July from 1977 to Birs species diversity was calculated using the SHANNON-WEAVER index (H'). RESULTS In 1977 and 1978, undisturbed period, 13 and 11 birds species established their breeding territories on the plot respectively (Table 1). These species were summer visitors in Hokkaido. Of these species the evidence of breeding (occupied nests or feeding of nestlings) were obtained for Erithacus calliope, Saxicola torquata, Locustella lanceolata, Acrocephalus bistrigiceps, Emberiza fucata, Emberiza aureola, Emberiza spodocephala and Uragus sibiricus. A. bistrigiceps arrived on the plot in early June and occupied the area covered with tall forbs or grasses which attained approximately 1 m high at its arrival (Figs. 1 & 2). In undisturbed period it was abundant species, representing 9.0 and 8.0 pairs, or 25% of the whole pairs respectively. Of 9 males marked in 1977, 3 birds (OB, BY and GO) returned on the plot in 1978 (Fig. 2), the return rate being 33%. However, it disappeared immediately after grazing by cattle after mid-june in 1978 due to the removal of tall grasses and forbs. E. aureola was also abundant species, representing 8.0 pairs, or 23 to 25% of the total pairs. They were followed by S. torquata, with 5.0 to 5.5 pairs, or 15 to 16% of the total pairs. The latter 2 species occupied a wider range of the plot. The other 8 to 10 species establishing territories on the plot represented less than 3.0 pairs (Table 1). In 1979, grazing period, A. bistrigiceps and Locustella ochotensis did not occur on the plot. No U. sibiricus established territory, although one male (OO) which nested

3 December 1986] Breeding Bird Populations in a Grassland in Hokkaido 69 Vegetation cover Fig. 1. Vegetation structures in mid-may (A), mid-june (B) and mid-july (C) in 1977 (solid line) and 1979 (broken line). Fig. 2. Vegetation maps in 1977 and 1979 (1: tall forb, 2: tall grass, 3: short grass and 4: shrub), and distribution of territories for Saxicola torquata (St), Acrocephalus bistrigiceps (Ab) and Emberiza aureola during the study. Black circles indicate nest. on the plot in undisturbed period held the territory in the adjoining area in 1979 (Fig. 3). Other 8 species continued to occupy their territories. Of these species E. spodocephala decreased slightly in number of pairs, and S. torquata and E. aureola remained fairly abundant, representing 4.5 and 7.0 pairs respectively. Only one species, Alauda arvensis, slightly increased in density. Gallinago hardwickii, E.

4 70 Yuzo FUJIMAKI and Mariko TAKAMI [Jap. J. Ornithol. Vol.35 No. 2/3 Table 1. The number of pairs of birds on a grassland from late April to late July, Plus marks indicate the presence, but establishing no territory on the study plot. calliope, L. lanceolata and E. fucata remained low densities, showing no apparent changes in density. No other species preferring open habitats established territory in spite of enlargement of short grass area. Figs. 2 and 3 show distribution of territories and their shift for males marked with rings. One E. calliope male (RP) marked at the west boundary of the plot in 1977 established the territory on the plot both in 1978 and 1979 (Fig. 3) and the return rate was 33% and 50%

5 December 1986] Breeding Bird Populations in a Grassland in Hokkaido 71 Fig. 3. Distribution of territories for Erithacus calliope(ec), Emberiza spodocephala (Es) and Uragus sibiricus (Us) during the study. respectively. Of nine S. torquata males marked in 1977 and 1978, 2 birds (GG and ) established territoty in the adjoining mowing field and 3 birds (RW, YY and OO BB) returned on the plot in the following breeding season (Fig. 2), the return rate being 60% in Of 12 E. aureola males marked in 1977 and 1978, 5 birds (OO, YO, BG, YB and OG) returned in 1978 and 3 birds (OO, RW and GY) returned in 1979 (Fig. 2), the return rate being 63% and 33% respectively. Thus, the return rates for birds nesting on the ground were fairly high even in grazed period. On the other hand, the return rates for birds preferring the area with trees or shrubs such as E. spodocephala and U. sibiricus were low in grazed period. Comparing the results of the first 2 years with that of the third year, the total bird density drastically declined from 35.5 and 32.5 in undisturbed situation to 17.5 in grazed period with decrease in the number of species. Bird species diversity also decreased from in 1977 and in 1978 to in In addition to these bird species, 10 to 21 species of non-nesting birds were observed. They included migrants and birds nesting in a nearby forest island, farm facilities and riparian habitats. Of them Dendroropos minor fairly frequently occurred on the plot, foraging on standing dead forbs during undisturbed period. Motacilla alba was observed more frequently in 1979 than in the previous period perhaps because of the enlargement of more open habitat suitable for feeding. Thus, a total of 39 bird species was recorded during the study. Factors affecting bird populations other than habitat changes included the trample by cattle. In 1978 and 1978, 3 nests (each one for E. calliope, E. aureola and E. ocephala. respectively) were destroyed by cattle-trampling. spod

6 72 Yuzo FUJIMAKI and Mariko TAKAMI [Jan. Vol. J. 35 Ornithol. No. 2/3 DISCUSSION In agricultural lands of Hokkaido the dominant bird species preferring open habitats include G. hardwickii, A. arvensis, E. calliope, S. torquata, A. bistrigiceps, E. fucata, E. aureola and E. spodocephala (FUJIMAKI 1984), all of which occurred on the study plot. The number of species and dominant-species composition in undisturbed situation are similar to those in a native grassland in the Isikari district, central Hokkaido (NAKAMURA et al. 1964), although there are a little difference in dominance order of each species between the 2 study areas. These facts may suggest that the present study plot in undisturbed situation supports one of representative bird communities of open country in Hokkaido. In the tall grass field the grazing by cattle produced immediate changes in habitat structure and plant-species composition. Each bird species holding territories on the plot showed the different response to the vegetational alteration. A. bistrigiceps which closely related with tall grasses and forbs (NAKAMURA 1963) and L. ochotensis disappeared. E. spodocephala decreased slightly and U. sibiricus was not stationaly in grazed period. On the other hand, S. torquata and E. aureola continued to be fairly abundant, and A. arvensis slightly increased. The other species were not abundant enough to discuss the population trends. The grazing obviously affected A. bistrigiceps and L. ochotensis because these species nest above the ground in tall forbs, which provide not only the cover but the structural support to the nests. The loss of structural support and cover for nests would make the habitat unsuitable for A. bistrigiceps and L. ochotensis. Similar trends in bird populations in relation to vegetational change were reported for certain birds whose habitat requirements include tall grasses and forbs (BEST 1972, OWENS & MYRES 1973). The grazing also affected E. spodocephala and U. sibiricus perhaps due to decrease of concealed sites for nesting in shrubs. On the other hand, the effect of grazing on birds nesting on the ground was not so much, although the total population density decreased with changes in habitat structure. Possible reason for this is the existence of some resources such as nest cover for these birds despite elimination of tall grasses and forbs. Also, humanrelated activities appeared to result in destruction of nests. In the present study plot there were cases, in which nests containing eggs or nestlings were trampled by cattle. In the Nemuro district, easternmost part of Hokkaido, 22 of 42 grasslandbird nests observed were unsuccessful by cutting of grasses (SHU 1957). In agricultural lands bird speciess nesting or foraging on tall grasses or forbs are affected intensively through changes in habitat structure caused by grazing or grasscutting, and also human-related activities influncing directly breeding success. SUMMARY Breeding bird populations in relation to vegetational changes by grazing were studied in a grassland in eastern Hokkaido from 1977 to During the study 39 bird species were recorded. Of them 13 species established their territories on the plot. After the grazing by cattle A. bistrigiceps and L. ochotensis were diasppeared, E. spodocephala and U. sibiricus decreased slightly, S. torquata and E. aureola did not change so much in densities, and A. arvensis increased slightly. Densities of other species remained low before and after the grazing. As a whole the total density decreased with the decrease in number of species. Open-country birds are affected through both changes in habitat and human-related activities influencing directly breeding succes.

7 December 1986] Breeding Bird Populations in a Grassland in Hokkaido 73 LITERATURE CITED BEST, L. B First-year effects of sagebrush control on two sparrow. J. Wildl. Mgt. 36: FUJIMAKI, Y An avifaunal study in different types of forest in the central part of Hokkaido, Japan. Bull. Hokkaido For. Exp. Sta. 8: (In Japanese with English summary.) Birds of Tokachi District, Hokkaido. 4. Birds of agricultural land. J. - Yamashina Inst. Ornithol. 16: (In Japanese with English summary.) Breeding bird communities in a deciduous broad-leaved forest in southern Hokkaido Jap. J. Ornithol. 35: KENDEIGH, S. C Measurement of bird populations. Ecol. Monogr. 14: KOBAYASHI, S., & Y. FUJIMAKI, Breeding bird communities in a deciduous broad-leaved wood and a larch plantation. Tori 34: (In Japanese with English summary.) NAKAMURA, T A survey of an upland grassland bird community during breeding season. Misc. Rep Yamashina Inst. Ornithol. 3: (In Japanese with English summary.) NAKAMURA, T., S. YAMAGUCHI, K. IIJIMA, & T. KAGAWA, A comparative study on the habitat preference and home range of four species of the genus Emberiza on peat grassland. Misc. Rep. Yamashina Inst. Ornithol. 5: (In Japanese with English summary.) OWENS, R. A., & M. T. MYRES, Effects of agriculture upon populations of native passerine birds of an Alberta fescus grassland. Can. J. Zool. 51: SHU, H Factors affecting the breeding of birds in Nemuro, Hokkaido. Tori 14 (68): (In Japanese with English summary.) (Received 25 July 1985)