Passerine Densities Research Articles

Hindcasts of passerine density in arctic and subarctic Alaska suggest noncomplementary responses to shrub expansion by tundra- and shrub-adapted species

ABSTRACT Shrub expansion is among the most conspicuous of the warming-related phenomena occurring within tundra regions, but its effects on vertebrates are not well understood. Among passerines, the habitats of shrub- and tundra-adapted species are expected to increase and decrease, respectively, by roughly equivalent amounts. However, shrub expansion is highly discontinuous, and these predictions are typically based on coarse-scale models of vegetation that permit limited spatial heterogeneity. We used a hindcasting approach to investigate spatial and interspecific heterogeneity in passerine densities through time, as a function of changes in Normalized Difference Vegetation Index (NDVI). We conducted line transect surveys of passerines between 2015 and 2019 at four sites in arctic and subarctic Alaska. Collecting the encounter locations permitted fine-scale assessment of covariates of density. We fit spatial distance sampling models to these data and then used the predicted relationship between passerine density and NDVI to hindcast changes in density since 1999. Our hindcasts suggested limited areal changes in the habitat of tundra-adapted species over the last twenty years while the habitat corresponding to tall shrub–adapted species has likely expanded. Thus, our results contrast with previous predictions due partially to the finer spatial scale of our inference, which allowed for portions of the landscape to resist shrub expansion.

Spatial convergence of meadow passerine territory distribution with mowing delay: an experiment in lowland grasslands

We experimentally tested the idea that meadow passerines could tend to match grassland areas consistently managed for enhancing their reproductive success. In the Saone valley (eastern France), mowing was postponed until after July 15 in stable parts (meadows with controlled mowing MCM) of three study sites for 10 years. Bird territories were defined each year with the Territory Mapping method and hatching chronology was described after a systematic survey of prey carrying by adults. We arbitrarily considered that breeding was at the utmost risk of failure when at least two thirds of a territory area was mown earlier than the hatching date + 20 days. In study site 1 where mowing outside MCM was completed by July 15, the proportion of passerine territories within MCM increased throughout the study period from 15 to 60%, whereas MCM was only 29.1% of the total area. Meanwhile, the rate of theoretical failure due to mowers dropped to ca. 20%. In the two other study sites where significant surface areas outside MCM were still not mown by July 15, territorial birds tended to avoid the earliest mown grasslands: the theoretical risk of failure by mowers was usually ≤ 10% in site 2 where 74% of total area on average was mown after July 15, < 20% after 2014 in site 3 with 53% of the total area mown after July 15. Passerine density increased above 10 territories/10 ha in each study site after MCM implementation.

Increased songbird nest depredation due to Aleppo pine (Pinus halepensis) encroachment in Mediterranean shrubland

BackgroundIn recent decades, a decrease of passerine densities was documented in Mediterranean shrublands. At the same time, a widespread encroachment of Aleppo pines (Pinus halepensis) to Mediterranean shrubland occurred. Such changes in vegetation structure may affect passerine predator assemblage and densities, and in turn impact passerine densities. Depredation during the nesting season is an important factor to influence passerine population size. Understanding the effects of changes in vegetation structure (pine encroachment) on passerine nesting success is the main objective of this study. We do so by assessing the effects of Aleppo pine encroachment on Sardinian warbler (Sylvia melanocephala) nest depredation in Mediterranean shrublands. We examined direct and indirect predation pressures through a gradients of pine density, using four methods: (1) placing dummy nests; (2) acoustic monitoring of mobbing events; (3) direct observations on nest predation using cameras; and (4) observation of Eurasian jay (Garrulus glandarius) behaviour as indirect evidence of predation risk.ResultsWe found that Aleppo pine encroachment to Mediterranean shrublands increased nest predation by Eurasian jays. Nest predation was highest in mixed shrubland and pines. These areas are suitable for warblers but had high occurrence rate of Eurasian jays.ConclusionsEncroaching pines directly increase activity of Eurasian jays in shrubland habitats, which reduced the nesting success of Sardinian warblers. These findings are supported by multiple methodologies, illustrating different predation pressures along a gradient of pine densities in natural shrublands. Management of Aleppo pine seedlings and removal of unwanted trees in natural shrubland might mitigate arrival and expansion of predators and decrease the predation pressure on passerine nests.

Bird Use of Northern Alaska Oilfield Rehabilitation Sites

Breeding bird response to habitat rehabilitation after anthropogenic disturbance has received little attention in the Arctic. The North Slope of Alaska is an important breeding ground for many populations of migratory birds and has also supported major oilfields since the late 1960s. The most obvious impacts of industrial development to nesting birds are direct habitat loss and fragmentation resulting from the construction of infrastructure, along with increased mechanical noise, vehicle traffic, and other forms of anthropogenic disturbance. In response to state and federal requirements, efforts have been made to rehabilitate abandoned portions of the oilfields. We compared bird use at rehabilitation sites and at nearby paired reference sites. Densities of shorebirds and passerines varied between rehabilitation sites and reference sites, but waterfowl densities did not. Specifically, passerine and shorebird densities were higher at reference sites in the early or mid-season and lower at reference sites in the late season. Additionally, birds on rehabilitation sites were primarily observed foraging and resting, while behavior observed on paired reference sites was more diverse and included courtship displays, nesting, and aggression. Further, rehabilitation sites supported significantly fewer nests and fewer species than recorded at reference sites. Our findings suggest that sites 3 to 10 years post rehabilitation do not provide bird habitat comparable to nearby reference sites and, by extension, do not provide shorebird and passerine habitat comparable to that found prior to development. However, rehabilitation sites appear to provide adequate habitat for waterfowl and are important to shorebirds and passerines as foraging areas. Continued monitoring will be needed to establish the long-term suitability of rehabilitation sites, compared to reference sites, as breeding habitat for birds.

Variation in passerine use of burned and hayed planted grasslands

ABSTRACTCreation of grassland communities is a popular approach to conserve grassland‐nesting waterfowl and songbirds. Much uncertainty remains regarding the most appropriate seed mixtures and the type and frequency of management. We conducted line‐transect songbird surveys in grasslands converted from cropland to native and tame grass‐forb mixtures and managed by burning (native only) or haying (native and tame) in Saskatchewan and Manitoba, Canada. Fields were 1–9 years post‐management (YPM). Our objective was to improve our understanding of the effects of management regime (native hayed, native burned and tame hayed fields) and frequency (YPM) on passerine density and occurrence in planted grassland. We found limited support for an effect of management regime on passerine density and occurrence in both provinces. Vesper sparrow (Pooecetes gramineus) occurrence in Saskatchewan was greatest in tame hayed fields, and clay‐colored sparrow (Spizella pallida) density in Manitoba was greater in tame hayed fields than in native burned fields up to 5 YPM. Years post‐management most strongly influenced the occurrence of vesper sparrows in Saskatchewan and sedge wrens (Cistothorus platensis); common yellowthroats (Geothlypis trichas); clay‐colored, Savannah (Passerculus sandwichensis), Le Conte's (Ammodramus leconteii), and Nelson's (A. nelsoni) sparrows; and bobolinks (Dolichonyx oryzivorus) in Manitoba. Five of the 8 species influenced by YPM exhibited a year‐dependent non‐linear response such that they reached their greatest density 4–5 YPM. Our results indicate that the type of stand (native or tame) or management (burning or haying) matters little to the grassland songbird community occupying planted cover fields and that the frequency of management will likely depend upon local environmental conditions in a given region or year. Some form of management of planted grasslands, however, should occur every 4–6 years to maintain habitat for a number of generalist grassland species. © 2017 The Wildlife Society.

On habitat characteristics driving meadow passerine densities in lowland hay-meadow systems in France

For meadow bird conservation in Europe, promoting specific farming models in their remaining strongholds could be a necessary complement to the ongoing agri-environmental policy. This study aims at understanding the main drivers of meadow passerine density in the objective of guiding conservation policies. The data were collected in 56 lowland hay-meadow regions important for grassland bird breeding in France. The objective was to explain, using GLMMs, the variation in meadow passerine density, measured annually in 490 12-ha plots during the period 2006–2010 by the count point method. Explanatory variables enabled us to describe farming management, sward structure and habitat size. Passerine density (all species except the skylark Alauda arvensis) depended on the extent of habitat available and on the percentage of meadow already mown on June 20. More specifically, the skylark was attracted to low, floristically rich and early mown swards, the whinchat Saxicola rubetra and the yellow wagtail Motacilla flava to large areas of favourable habitat, and the corn bunting Emberiza calandra to floristically rich meadows mown later than June 20. Extensive farming models based on late mowing in the largest habitat units should therefore be a target for meadow passerine conservation. The skylark was an exception. Because of its apparent tendency to use early mown grasslands, we recommend to rather invest on alternative habitats for sustaining the demography of this species.

Long‐term effects of agri‐environment schemes on breeding passerine populations in a lowland hay‐meadow system

Capsule Mowing postponement in 25% of a meadow system may lead to improved but instable population dynamics in meadow birds. Aim To monitor the long‐term effects of mowing postponement on the abundance and territory density of meadow passerines in 25% of a 3000‐ha hay‐meadow area in the Saône Valley (eastern France). Methods From 1993 to 2009, passerine abundance was measured annually in 78 plots using point counts and territory density was assessed in two study areas by Territory Mapping. Results The programme of mowing postponement led to substantially increased passerine abundance and territory density, with the highest increase detected in Whinchats Saxicola rubetra. No positive trend was observed in Corn Buntings Emberiza calandra. Immediately after the increase in abundance, bird distribution within the study area was not influenced by the timing of mowing. The hatching success, assessed by the systematic observation of food carrying, was negatively influenced by early mowing (<40% recorded when >60% of the meadow area was already harvested on 1 July) or, in late mown areas, by high meadow passerine territory density (<40% when territory density was >10 per 10 ha). Hatching rate was usually higher in Whinchats than in Corn Buntings. Conclusion By improving meadow passerine breeding outputs and density, mowing postponement led to instable population dynamics with dominance of certain species and density‐dependent breeding success.

Factors Influencing Habitat Use by Migratory Grassland Birds in the State of Chihuahua, Mexico

ABSTRACT. Breeding populations of grassland birds are declining across North America. Alteration of wintering-ground habitat in Mexico may be a significant causal component in these declines, considering the rapid changes in land use occurring there. We investigated the influence of precipitation, habitat structure, and plant community on the abundance and distribution of grassland passerines in the state of Chihuahua, Mexico. We conducted bird and vegetation surveys during 1998–2007 in 65 plots at two study areas 75 km apart, Sueco and Tinaja Verde. Grassland guild species were the dominant component of the wintering bird community at both study areas. Passerine density at Sueco during winter was directly related to cumulative rainfall from the preceding December to September, a factor known to be strongly correlated with grass production. Herbaceous and woody vegetation structure and habitat type influenced occurrence of migratory grassland birds at the plot level. Nine of 11 study species occurred in a...

Whinchat Saxicola rubetra reproductive success according to hay cutting schedule and meadow passerine density in alluvial and upland meadows in France

Summary In France, whinchat Saxicola rubetra populations now rarely breed at altitudes below 1,000 m except in flooded alluvial plains. Whinchat reproductive success, the timing of haymaking and meadow bird density were assessed in 2003 in six areas selected in three different flooded plains, in five areas from two massifs at altitudes ranging from 990 to 1,250 m, and in 2004 in six areas of three flooded plains, in seven areas from three massifs at altitudes ranging from 1,200 to 2,000 m. The percentage of whinchat territories in which juveniles were observed was negatively correlated with mown areas at the time when 80% of hatched broods (observation of prey carryings by adults) would theoretically be fledged. Additionally, assessed hatching success was negatively correlated with early haymaking in 2003 and with meadow passerine territory density in 2004. In upland meadows, whinchat populations exhibited either low reproductive success and low density (⩽1,200 m) or high reproductive success and high density (⩾1,200 m). In lowland flooded meadow however, high density could match low reproductive success and whinchat population then acts as a sink population. We conclude that adequate agri-environmental policy should not focus only on lowland meadows, even if the highest meadow bird diversity is often found there.

Impacts of Chronic Anthropogenic Noise from Energy‐Sector Activity on Abundance of Songbirds in the Boreal Forest

The effects of human activities in forests are often examined in the context of habitat conversion. Changes in habitat structure and composition are also associated with increases in the activity of people with vehicles and equipment, which results in increases in anthropogenic noise. Anthropogenic noise may reduce habitat quality for many species, particularly those that rely on acoustic signals for communication. We compared the density and occupancy rate of forest passerines close to versus far from noise-generating compressor stations and noiseless well pads in the boreal forest of Alberta, Canada. Using distance-based sampling, we found that areas near noiseless energy facilities had a total passerine density 1.5 times higher than areas near noise-producing energy sites. The White-throated Sparrow (Zonotrichia albicollis), Yellow-rumped Warbler (Dendroica coronata), and Red-eyed Vireo (Vireo olivaceus) were less dense in noisy areas. We used repeat sampling to estimate occupancy rate for 23 additional species. Seven had lower conditional or unconditional occupancy rates near noise-generating facilities. One-third of the species examined showed patterns that supported the hypothesis that abundance is influenced by anthropogenic noise. An additional 4 species responded negatively to edge effects. To mitigate existing noise impacts on birds would require approximately $175 million. The merits of such an effort relative to other reclamation actions are discussed. Nevertheless, given the $100 billion energy-sector investment planned for the boreal forest in the next 10 years, including noise suppression technology at the outset of construction, makes noise mitigation a cost-effective best-management practice that might help conserve high-quality habitat for boreal birds.

POPULATION TRENDS OF TUNDRA-NESTING BIRDS AT CAPE CHURCHILL, MANITOBA, IN RELATION TO INCREASING GOOSE POPULATIONS

To assess potential changes in tundra-nesting bird populations relative to increased herbivory by growing Lesser Snow Goose (Chen caerulescens caerulescens) and Ross's Goose (C. rossii) populations, we estimated avian population densities in coastal tundra near Cape Churchill, Manitoba, Canada in June 1984, 1999, and 2000. We also compared bird abundance in altered and intact freshwater sedge (Carex spp.) meadows to assess bird response at a smaller spatial scale, the habitat patch. At the landscape scale, Herring Gull (Larus argentatus) densities were significantly greater in both 1999 and 2000 than 1984, and Semipalmated Sandpiper (Calidris pusilla) and Savannah Sparrow (Passerculus sandwichensis) densities were greater in 2000 than 1984. As a group, tundra-nesting passerine densities were significantly greater in 1999 than 1984. Dunlin (Calidris alpina) densities were significantly lower in 1999 than 1984, and many larger, ground-nesting birds declined in abundance and occurrence along transects in later years. At the smaller spatial scale, passerines and shorebirds were less abundant in altered than intact freshwater sedge meadows. Our results indicate that most abundant breeding birds on the study area did not exhibit lower densities in later years despite increased effects of goose herbivory on vegetation. At the habitat patch scale, the same groups of species had lower abundances in altered versus largely intact habitats. Although goose herbivory can alter tundra vegetation and lead to lower abundance of several bird species, habitat alteration may have to be widespread and severe to elicit population declines over a large area.

Landscape-dependent use of a seed-rich habitat by farmland passerines: relative importance of game cover crops in a grassland versus an arable region of Scotland

Capsule Game cover crops in a pastoral region of southwest Scotland supported passerines at more than twice the density found in similar crops in an arable region. Aims To investigate the use of seed-rich habitats (game cover crops) by farmland passerines in the agricultural grasslands of southwest Scotland relative to the arable east. Methods Birds were counted in game cover crops and conventional crops on nine farms in Dumfries and Galloway, and Ayrshire, during the winters of 2003/04 and 2004/05. Similar data from eastern Scotland were re-analysed to compare the relative use of the two crop types in the contrasting regions. Results Passerine densities in game cover crops in southwest Scotland were two orders of magnitude greater than in conventional crops. Furthermore, densities within grassland game cover crops were more than double those in arable areas and densities in grassland conventional crops were just 14% of those in equivalent arable habitats. Conclusion The significantly greater difference between passerine densities in game cover crops and conventional crops in a grassland versus an arable region of Scotland could not be explained by differences in the bird communities, crop types, boundary habitats, weather, observer effects or survey methods used. We suggest that game cover crops may be more attractive in grassland regions because alternative food sources are rarer than in arable regions.

The use of winter bird crops by farmland birds in lowland England

Winter food for farmland birds can be enhanced by planting seed-rich ‘winter bird crops’ (WBCs), the content of which is highly variable. Data from three winters (October to March in 1998/1999, 1999/2000 and 2000/2001) were used to assess the value of different WBCs for lowland farmland birds in England, based on around 100 plots per winter. Densities of gamebirds, insectivorous and seed-eating passerines were generally higher on WBCs than on conventional crops, stubbles or grassland (except for skylark ( Alauda arvensis) and rook ( Corvus frugilegus). Kale supported high densities of the widest range of bird species (including insectivorous and seed-eating species), quinoa was used by finches and tree sparrows ( Passer montanus), and (unharvested) seeding cereals supported high densities of buntings. Sunflowers, phacelia and buckwheat supported low densities of most bird species. The differences between WBCs were more marked in late winter, owing to better seed retention by kale and quinoa compared with other crops. A combination of kale, quinoa and seeding cereals will support the highest densities of the greatest variety of bird species, but WBC content can also be varied to suit circumstance or species. This study demonstrates the value of WBCs for a wide range of birds, and discusses this with respect to existing ‘agri-environment schemes’, highlighting the need for similar options in pastoral landscapes where seed resources are particularly rare.

The impact of roads on birds: Does song frequency play a role in determining susceptibility to noise pollution?

Traffic noise is known to have a negative impact on bird populations in general, but little is known about the mechanisms by which sound pollution affects bird communities. However, a knowledge of these mechanisms is imperative if we want to account for the differences in susceptibility to traffic noise that exist between species, and may thus be critical for conservation action. To address this issue, population assessments were carried out in a contiguous area of oak-beech forest at differing distances from a much frequented motorway to determine the road effect on the whole bird community. As expected, species richness and diversity decreased towards the motorway, and bird abundance was significantly lower along the motorway than in the control area. However, a few species defied the negative impact of the motorway. The songs of the more abundant passerines were analysed with regard to three frequency parameters to determine whether or not a relationship exists between the song pitch of a species and its sensitivity to noise pollution. A significant relationship was found between dominant frequency and decline in abundance towards the motorway, which indicates that having a higher-pitched song with frequencies well above those of traffic noise makes a bird less susceptible to noise pollution. These results suggest that acoustic masking is one of the mechanisms by which traffic noise negatively affects passerine density along roads.

Influence of Topography on Density of Grassland Passerines in Pastures

Pastures provide substantial habitat for grassland birds of management concern in the Driftless Area of southwestern Wisconsin. The rolling topography in this region is characterized by lowland valleys surrounded by relatively steep and often wooded slopes which are set apart from more expansive treeless uplands. We hypothesized that there would be lower densities of area sensitive grassland passerines in lowland grasslands compared to upland grasslands because of their preference for larger more open grasslands. To test this hypothesis and assess how well pasture area and vegetation structure predicted grassland passerine density compared to upland/lowland status, we conducted point counts of birds in 60 pastures in May–June 1997 and 1998. Upland pastures generally supported greater densities of grassland passerines than lowland pastures. Densities of Savannah sparrow (Passerculus sandwichensis) and bobolink (Dolichonyx oryzivorus) were significantly higher in upland pastures than in lowland pastures. Grasshopper sparrow (Ammodramus savannarum) density was significantly higher on uplands in one of the study years. The density of eastern meadowlark (Sturnella magna), western meadowlark (S. neglecta) and sedge wren (Cistothorus platensis) did not differ significantly between uplands and lowlands. Grassland passerine density was also predicted by pasture size and vegetation structure. Densities of bobolink and grasshopper sparrow were higher in larger pastures. Bobolink and Savannah sparrow occurred on pastures with greater vegetation height-density and less bare ground; bobolink also preferred shallower litter depths. Lowland pastures supported grassland bird species of management concern and should not be neglected. However, we recommend that pasture management for grassland passerines in areas of variable topography favor relatively large upland pastures that will contain higher densities of species of management concern.

Passerine Densities in Riparian Forests of Southeast Alaska: Potential Effects of Anadromous Spawning Salmon

AbstractSpawning salmon (Oncorhynchus spp.) greatly influence many biotic processes in both the aquatic and terrestrial components of riparian systems, possibly including the community of breeding birds. In southeastern Alaska in 1996–1997, the riparian forests bordering salmon streams supported, on average, higher densities, but not diversity, of forest passerines compared to non-salmon streams. The presence of salmon in the fall increases the abundance of terrestrial and aquatic invertebrates and possibly enhances food resources to riparian birds in the spring.Densidades de Aves Paserinas en Bosques Riparios del Sureste de Alaska: Efectos Potenciales del Desove de Salmones AnádromosResumen. El desove de salmones anádromos (Oncorhynchus spp.) ejerce una importante influencia sobre muchos procesos bióticos en los componentes acuáticos y terrestres de los sistemas ribereños, incluyendo posiblemente las comunidades de aves. Entre 1996 y 1997, los bosques riparios del sureste de Alaska ubicados alrededor de arroyos con salmones tuvieron en promedio mayores densidades (pero no mayor diversidad) de aves paserinas de bosque que bosques situados alrededor de arroyos sin salmones. La presencia de los salmones en el otoño incrementa la abundancia de invertebrados terrestres y acuáticos y posiblemente causa aumentos en la cantidad de recursos alimenticios disponibles para las aves ribereñas en la primavera.

Effects of wetland excavation on avian communities in eastern Washington

Despite having an arid climate, the Columbia River Basin of eastern Washington is one of the most important areas for breeding, migrating, and wintering wetland birds in the Pacific Northwest. Extensive farmland irrigation near the O’Sullivan Reservoir and other areas has raised water tables, resulting in closed canopies of emergent vegetation in what are now permanent, shallow wetlands. During 1994–95, we found that avian communities on vegetation-filled wetlands excavated to create open water had greater mean densities of dabbling and diving ducks (Anatidae), shorebirds, American coots (Fulica americana), and terns (Sterna spp.) than untreated control wetlands. Densities of yellow-headed blackbirds (Xanthocephalus xanthocephalus) were greater on excavated wetlands, while the densities of red-winged blackbirds (Agelaius phoeniceus), Sora (Porzana carolina), and Virginia rails (Rallus limicola) were greater on unaltered sites. Densities of marsh wrens (Cistothorus palustris) and other passerines were similar among excavated and control wetlands. Zooplankton biomass was greater on altered wetlands and generally greater in older excavations. We suggest that managers may design constructed wetland basins to improve population management of diverse wetland bird species by creating an appropriate mix of wetland habitats at the landscape level.