Changes In Bird Populations Research Articles

Forty-Year Changes in the Avifauna of Grand Teton National Park, Wyoming, with Emphasis on Neotropical Migrants

Ornithologists and wildlife biologists have always been interested in documenting long-term changes in bird populations (e.g., Temple and Temple 1976, Kendeigh 1982). Long-term comparisons can assist in identifying patterns of change. These patterns, in turn, provide assistance in defining human impacts that may lead to some species or groups declining towards extirpation. One study specifically (Wilcove 1988) has had a major impact upon current resource issues within the Fish and Wildlife Service. That study used 40-year comparisons to confirm that fragmentation of the eastern deciduous forest had led to declines of neotropical migrants. The Wilcove (1988) study along with an essay (Hutto 1988) and an ecological study in New Hampshire (Holmes and Sherry 1988) were fundamental in identifying that declines in this group of birds cannot be blamed solely upon changes on Latin American wintering grounds. Missing from the neotropical migrant story, however, is an image of how neotropical migrants from western North America have changed in recent decades. An historical data set for comparison is available for seven vegetative associations in the vicinity of Jackson, Wyoming (Salt 1957). This project was begun in 1993 to replicate Salt's 1957 study in Grand Teton National Park. The work in 1993 was a pre-study to evaluate the potential for replicating Salt's study in the mid 1990's, 40 years after the original work.

Status and population changes of farmland birds in southern Sweden

Since 1950 there have been considerable changes in the use of Swedish farmland which have led to a more large-scale, homogeneous and intensively managed agricultural landscape. These changes in Swedish farmland have affected the populations of many farmland birds. We give the densities of 29 selected farmland birds and 15 selected species of forest birds at eight large study sites (farmland landscapes of 14.5–21.5 km2 size; total of 140 km2) in southern Sweden and review the population changes of 48 species of farmland birds in southern Sweden. The number of breeding farmland bird species at the different sites was positively correlated with total farmland area, area of scrub and trivial deciduous forest, as well as with the fragmentation of the forest landscape; there were more farmland species in fine-grained than in coarse-grained landscapes. Species found in forest edges and dry pastures were the most widespread ones (found at most sites), and included several species with high densities. Most of these species have not decreased as dramatically as other species. Several species of arable land were also widespread, but most of them have declined in numbers since 1950, possibly as a consequence of changes in farming practices. Meadow is the farmland habitat that has declined most, and this is reflected in the low densities and scattered distribution of many species found on cut or grazed meadows. Several of these species are now largely confined to marginal habitats and areas where farmland still has a traditional structure. However, on abandoned meadows overgrown with bushes and trees a few successful colonists are increasing in numbers. Of the 48 farmland birds breeding in Sweden the populations of only 10 have remained relatively stable since 1950; seventeen have increased (8 markedly) while 21 have decreased (12 markedly). Of the 21 species that primarily breed on farmland, 9 (43%) have declined markedly compared with only 3 (11%) of the 27 secondary farmland birds, which have a large proportion of the population in other habitats. Similarly, only 5% of the primary species have increased markedly compared with 26% of the secondary species. More primary than secondary farmland species are also extinct, or categorized as endangered, vulnerable or requiring consideration on Swedish Red Data lists. Large parts of the Swedish farmland are now being taken out of production, which probably will have negative effects on several farmland bird populations.

Impacts of a Severe Drought on Grassland Birds in Western North Dakota

We studied the effect of a severe drought on the population dynamics and community structure of grassland birds in western North Dakota. During the spring and summer of 1988 the northern Great Plains suffered one of the warmest, driest periods in its recorded history. We compared the changes in bird populations and nesting productivity over a 3-yr period before, during, and after the drought. Total grassland bird density declined 61% (P < .05) between June 1987 and June 1988. Densities of six of eight common species declined significantly during the drought. Populations of all but two species recovered in 1989 and total bird density in June 1989 did not differ significantly from June 1987. Species richness and species diversity both declined significantly during the drought and recovered to predrought levels in 1989. Species richness declined more on fair condition than on good condition range during the drought. Vesper Sparrow (Pooecetes gramineus) hatching success, number of young fledged per successful nest, and nesting success were significantly lower in 1988 than either 1987 or 1989. Clutch size did not differ among the three years. The decline in nesting success in 1988 was primarily due to nest abandonment during incubation. Nesting of Vesper Sparrows, Horned Larks (Eremophila alpestris), and Western Meadowlarks (Sturnella neglecta) ended abruptly in mid-June 1988 during a period of extremely hot weather. In 1987 and 1989, nesting continued into July. Despite substantial reductions in bird density and productivity during the drought, many species recovered to predrought levels 1 yr following the drought. This suggests that year-to-year fluctuations in densities of some of these species may not be tightly linked to short-term changes in local productivity. However, sequential years of low productivity may have more substantial effects on these short-lived species. Thus, if drought conditions in North American grasslands become more frequent, as some climate models predict, there could be related changes in the avifauna of the region.

Comparison of Ship and Aerial Surveys of Birds at Sea

Field tests were made off California to compare estimates of seabird density and species composition resulting from simultaneous ship and aerial observations. In controlled survey experiments, significantly higher mean densities were calculated from aerial observations than from the ship. Under variable field conditions, however, densities derived from vessel surveys for five species groups were statistically indistinguishable from corresponding aerial figures. On ship surveys, 95-97% of all birds were identified to species, whereas from the air, specific identifications were made for 77-96% of the birds seen. About the same total numbers of species were noted from the two platforms, and reported species composition was similar. J. WILDL. MANAGE. 49(2):405-411 During the mid-1970's, the needs of managers for basic population inventories of seabirds led to several large scale, multi-year studies involving both shipand aircraft-based observations at sea (Briggs et al. 1981, Wahl et al. 1981, Gould et al. 1982, Vermeer et al. 1983, Blake et al. 1984). Survey methodology in openwater habitats previously had received little critical examination, so investigators devised a variety of field procedures and accumulated vast data bases, with only modest standardization between research groups. It has become apparent that aerial and shipboard methodologies should be evaluated and compared. Qualitative comparisons of strip censuses from vessels and aircraft have been made by Briggs et al. (1981), Wahl et al. (1981), and Gould et al. (1982), whereas limited quantitative comparisons, emphasizing littoral habitats, have been reported by King and Conant (1982) and Savard (1982). Generally, aerial surveys cover wide areas (102-104 km2) in a few hours' to a few days' time and have been reported to yield relatively low counts along coastlines when compared to counts from shore or a boat. Due to short sighting times, aerial counts are reported to produce relatively imprecise information concerning species composition. Because ship surveys cover comparable areas more slowly, they introduce the possibility of changes in bird populations during a single survey. However, they yield more precise identifications and have the advantage of facilitating collection of correlative oceanographic information. Both types of survey are thought to have i herent biases leading to error in determination of bird density: (1) birds may be overlooked or their numbers overor underestimated; (2) on ship transects inclusion of counts of flying birds may exaggerate apparent densities (Tasker et al. 1984); (3) ship-followers (e.g., gulls [Larus spp.] and fulmars [Fulmarus spp.]) or avoiders (e.g., loons [Gavia spp.]) may be represented differently from those that are neutral to the ship's presence. Glare, altitude, and the width of transect corridors can affect density and species composition estimates from airplanes (Briggs et al. 1985). In this paper we compare some aspects of strip transects made during ship and aerial surveys. We also examine survey data taken off southern California during 1975-78 to compare density estimates under normal (variable) field conditions. Though limited, these appear to be the first direct comparisons reported for the two types of survey in offshore areas. D. H. Dettman, K. F. Dettman, E. W. Chu, G. D. Farrens, M. O. Pierson, and M. L. Bonnell This content downloaded from 157.55.39.177 on Wed, 16 Nov 2016 04:33:30 UTC All use subject to http://about.jstor.org/terms 406 SHIP/AERIAL BIRD SURVEYS * Briggs et al. J. Wildl. Manage. 49(2):1985 assisted with fieldwork. Analyses were discussed with G. L. Hunt, Jr., M. Tasker, and S. M. Speich. D. W. Anderson, J-P. L. Savard, and K. Vermeer provided helpful comments on earlier drafts; and C. Hitchcock patiently prepared the manuscript. Financial support was provided by the U.S. Dep. of Inter., Miner. Manage. Serv. (formerly Bur. of Land Manage.) under contracts AA550-CT7-36 and 14-12-0001-29090 with the Univ. of California.

Predicting Avian Community Response to Lakeshore Cottage Development

Breeding songbird species were studied in both developed and undeveloped lakeshore habitats of central Ontario. Information is presented for managing lake shorelines for human recreational development without significant loss of wildlife values. The disturbed ground vegetation layer was significantly correlated with a number of habitat variables deemed important in describing avian habitats. Avian species were placed in one of three groups: the ovenbird (Seiurus aurocapillus), an example from the group found primarily in undeveloped habitats; the eastern phoebe (Sayornis phoebe), one of the group found in highly developed habitats; and the veery (Catharus fuscescens), from the group that was not significantly affected by development. These findings are presented in a simple model of breeding bird response to disturbance that can be used to assess the negative impact of cottage development. J. WILDL. MANAGE. 48(4):1239-1247 Human encroachment on undeveloped areas often impacts the plant and animal communities located there. Avian species respond as their breeding habitat is altered or destroyed. Neotropical migrants may be particularly affected by development because not only is their breeding habitat being altered, but wintering grounds are being converted to pasture and farmland (Terborgh 1974). In the last 50 years, many lakes in south-central Ontario have become popular for recreational activity. Cottages have been built along many lakeshores with associated roads and transmission line corridors nearby. Unlike successional sequences, cottage construction tends to impose permanent change in the vegetation and associated environment. Resource managers need information to predict how changes imposed by this development affect wildlife. Robertson and Flood (1980) found an increase in diversity of the songbird community along portions of cottage lakesh re in the nearby Rideau Lakes. Martin (1960) investigated changes in bird populations in relation to forest succession in the Algonquin Park area. James and Wamer (1982) and others have demonstrated that bird species diversity depends on vegetation structure and composition. The mportance of large blocks of unbroken for st was reviewed by Robbins (1979). We have added to these studies to provide resource managers with a tool to predict how future cottage development m y affect bird communities along lakeshores. Songbird populations are notoriously diffic lt to measure, and the populations themselves fluctuate widely. Preston (1960) noted that for 57 breeding species in Ohio, based on 10 years of survey data, nly a out half (28 species) bred every year. James and Wamer (1982) showed that study plots of less than 10 ha often have less than the total species breeding in an area. In our work, based on 2 years of data and small sample plots, we cannot hope to show the long-term comprehensive impact of cottage construction on songbird populations. However a general model of cottage development and the re'Present address: Ontario Ministry of Environment, Box 21, Dorset, Ontario POA 1EO, Canada. 2 Present address: Ontario Ministry of Natural Resources, Cochrane, Ontario, POL 1CO, Canada. J. Wildl. Manage. 48(4):1984 1239 This content downloaded from 157.55.39.70 on Tue, 06 Sep 2016 05:51:31 UTC All use subject to http://about.jstor.org/terms 1240 AVIAN COMMUNITY RESPONSE * Clark et al. suiting impact on the wildlife community is needed to provide planners with guidance in approving or disapproving plans for cottage subdivisions. A concurrent study described the habitat associations of 19 species of breeding birds at varying levels of cottage development and determined that tree density, shrub coverage, conifer composition, and canopy volume were important in describing the habitat associations (Clark et al. 1983). The purpose of this study was to develop a method to predict changes in avian species composition caused by cottage development that could be used to model the impact of habitat disturbance on wildlife commu-

THE NORFOLK BROADLAND: EXPERIMENTS IN THE RESTORATION OF A COMPLEX WETLAND

Summary1. The Norfolk Broadland comprises wide river valleys, floored with deep deposits of peat and clay. Over forty mediaeval peat pits (the Broads) became flooded after the fourteenth century and were mostly connected with the rivers by navigation channels. Between about 1400 A.D. and 1800 A.D. the valleys supported a diverse wetland ecosystem, partly maintained by deliberate cropping of wetland plants. Some of the wetland was gravity‐drained, but extensive aquatic habitats held diverse fens and submerged vegetation dominated by short‐growing aquatic plants in very clear water.2. The Enclosure Acts passed around 1800 encouraged more intensive agriculture in the catchments and this, coupled with pumped drainage, particularly of the wetlands in the lower valleys, caused them to sink and so they were embanked against the rivers. The drained land, intersected by channels (dykes), was grazed.3. In the late nineteenth century, migration of human populations to the towns led to pressures for improved sewage disposal; consequently, increasing amounts of raw sewage, and later, treated effluent, both major sources of phosphorus, were discharged to the rivers and Broads, which were thus progressively eutrophicated. Agricultural changes led to an increased nitrogen supply. A tourist boating industry also began then.4. Increasing eutrophication caused replacement of low‐growing submerged aquatic plants together with ranker species, and then, particularly after the Second World War, submerged plants were lost from much of the waterway and replaced by phytoplankton‐dominated communities. The mechanism by which this change took place concerns growth of epiphytic and filamentous algal communities and probably interactions with phytoplankton‐grazing animals associated with the plants. The loss of submerged plants is linked with decreased invertebrate diversity, and changes in fish and water bird populations. Eutrophication is also associated with fish‐kills caused by a toxic flagellate, Prymnesium parvum, oubreaks of avian botulism and decreased fen diversity through flooding with enriched water. Increased boat activity has led to severe bank erosion. Erosion is exacerbated by loss of bank protection, because of loss of submerged plants and of fringing reedswamp through damage by coypu, an exotic rodent.5. Lack of labour‐intensive management of the undrained fens has led to a decrease in their diversity, through natural succession to alder swamp.6. Restoration of an aquatic‐plant‐dominated waterway is desirable and has been attempted through various means of reducing phosphorus input. Complete isolation of Broads or dykes has proved generally useful, but complications with release of phosphate from sediments, though such release eventually declines, are delaying success of an attempt to reduce phosphorus levels by precipitation of phosphorus from sewage effluent. Such a technique is the only one widely practicable in Broadland.7. Simple nutrient reduction may not always be sufficient to cause a return from phytoplankton‐dominated, turbid water to clear water with aquatic plants. It may be necessary to reinstate a higher intensity of zooplankton grazing than at present possible because of predation of grazers by fish in a habitat lacking refuges for the zooplankton from such predation.8. The drained grazing marshes contain, in their dykes, a rich aquatic plant and invertebrate community, and provide nesting sites and grazing for birds. Changed political and economic factors may lead to future diminution of these communities through intensified drainage for arable cultivation.

Bird Populations before and after Wildfire in a Great Lakes Pine Forest

-Birds in a 6.25-ha quadrat in a 73-year-old jack pine-black spruce forest (Pinus banksiana-Picea mariana) in Cook Co., Minnesota were intensively studied in June 1976. A wildfire burned through the area in August. The following spring we resurveyed the same quadrat to determine the first-year changes in bird populations. Species and guilds were compared by density, territorial space, existence energy, and importance values. Twelve species had territories in the study grid before the fire; six were not there the following spring, but eight additional species had established territories. Tree-foliage searchers had the greatest importance value before the fire and ground-brush foragers the greatest value afterwards. Density, total biomass, and combined existence energy of birds decreased after the fire by 50, 23, and 41%, respectively, but species using the area after the fire were 63% heavier on the average. Average energy consumption per unit of body weight was calculated to be 23% less after the fire. Fire apparently reduced the total food available for birds, but increased the kinds of food, especially at or near

Changes in bird populations as criteria of environmental changes

Järvinen, O. and Väisänen, R. A. 1979. Changes in bird populations as criteria of environmental changes. – Holarct. Ecol. 2: 75‐80.Birds are a powerful tool of environmental monitoring, on account of their ecological diversity. Because of this, bird populations seem best suited for monitoring biological, possibly non‐linear effects of specified environmental changes, such as habitat modification, and for “general monitoring” aimed at detecting unexpected environmental changes as they occur. Because many population changes have multiple causes, monitoring specific environmental changes is most rewarding if birds are grouped by e.g. habitat, major strategy (e.g. resident vs. migrant species), or feeding guild. A blueprint for a Nordic monitoring system based on breeding land birds is presented. Local trends may be atypical, and representative coverage of the major habitats in large areas should thus be ensured. As annual population fluctuations usually give little information in environmental monitoring, long‐term projects are necessary. The line transect method seems applicable to many monitoring purposes, as it is rapid, inexpensive, relatively accurate, and suitable for sampling the northern terrestrial biota of the Holarctic region, probably including the temperate deciduous forests.

Climatic Changes, Habitat Changes, and Competition: Dynamics of Geographical Overlap in Two Pairs of Congeneric Bird Species in Finland

The relative densities and overlap zones of Parus cinctus (northern) and P. cristatus (southern), and of Fringilla montifringilla (northern) and F. coelebs (southern) were studied in censuses made in Finland in 1910-29, 1936-49, 1952-63 and 1973-77. The population changes of the species are described. P. cinctus greatly decreased, first in the southern and later in the middle parts of its range. P. cristatus first expanded both northwards and to the Aland Islands, but then decreased, particularly in the north. In their overlap zone, both species decreased drastically (by 90-95%). F. montifringilla was stable, except for a temporary decrease in the 1940's. F. coelebs increased, but the 1:1 zone of the Fringilla species has fluctuated mainly owing to fluctuations in the numbers and range of F. montifringilla. Two major theories purport to account for the dynamics of North European biota: either the climatic amelioration of the past 100 yr is of major importance, or recent habitat changes (often due to man or his livestock) are more effective. Recent changes in climate and habitats are compared with bird population changes in order to test the two biogeographic theories. Climatic amelioration may account for some, but certainly not all population changes. Habitat changes (including the abandonment of forest grazing, the increase of spruce, increased edge effect, and extensive clear-cuttings particularly in N Finland) may explain all long-term trends. A few short-term population changes were probably due to exceptional winter or spring weather. The presumed interspecific competition between the congeners studied is examined critically. Standard competition equations do not adequately model the dynamics of the species pairs, but at least three additional points must be taken into account: environmental changes, population changes in relation to ecological saturation, and the evolutionary time scale.

Bird population changes after clearance of deciduous scrub

Clearance of deciduous scrub by cutting in Valdres, South Norway, or by spraying with 2,4,5-T, was followed by a 30% reduction in the number of bird territories in the following spring. The recorded changes were related to differing preferences of the bird species for successional stages of vegetation, i.e. the pioneer species increased while those which prefer old forest were heavily reduced. Four years later the bird population as a whole had increased in the cut area, but not in the sprayed area. The results are discussed in relation to changes in the invertebrate fauna and the vegetation of the habitat. A total of 314 single surveys (1970–73 and 1976) were made in five census plots which covered a total area of 113 ha comprising both clearings and old spruce ( Picea abies) forest. The analysis is based on counts of singing males and estimates of territories held.

Bird Population Changes for the Years 1975–76

The year 1976 saw decreases in many resident birds, particularly the smaller species. Both partridges made substantial increases and the Whitethroat continued to recover from the 1969 population crash, but has a long way to go yet.

Bird Population Changes for the Years 1974–75

Many resident birds are now maintaining higher population levels than was the case during the first ten years of the CBC, but some summer visitors are much less common. However, the decline in Redstart and Sedge Warbler seems to have halted.

Bird Population Changes for the Years 1973–74

The population trends apparent in the last few years continue, with further increases in many of our small resident birds. In contrast, several of the summer visitors remain at a very low level while some, such as the Swallow, have suffered further marked decreases.

Bird Population and Distribution Changes and the Impact of Man

Changes in the fortunes of bird populations may have complex causes. Climate may play a part, but in many cases the important factors inducing status changes arise from man's modifications to the environment. The author outlines some ways in which such ‘non-natural’ influences can work.

Bird Population Changes for the Years 1972–73

The Wren and other small resident birds continue to increase, and the Goldcrest is now ten times commoner than it was a decade ago. Some summer visitors, however, have now reached their lowest ‘index’ since the Common Birds Census began.

Bird Population Changes for the Years 1971–72

Notable increases in the population levels of Wren, Goldcrest and Redpoll were recorded by the Common Birds Census in 1971–72; however, Corn Bunting and Jackdaw showed a decline. Among summer visitors several West African wintering birds remain at a much reduced level.

Bird Population Changes for the Years 1970–71

(1972). Bird Population Changes for the Years 1970–71. Bird Study: Vol. 19, No. 4, pp. 241-248.

Bird Population Changes on Farmland and in Woodland for the Years 1968–69

Bird Population Changes on Farmland and in Woodland for the Years 1968–69

Bird population changes on farmland and in woodland for the years 1967–68

Bird population changes on farmland and in woodland for the years 1967–68

An index of bird population changes in woodland

An index of bird population changes in woodland