Breeding Bird Populations Research Articles

Grand Teton National Park Breeding Bird Monitoring Project

Overview: GTNP Breeding Bird Monitoring Project. 1. Following initial independent work by M. L. Cody and 3y funding from NPS, we instigated a scheme for long-term monitoring of breeding land bird populations in a wide variety of habitats representative of the northern Rockies and the Greater Yellowstone Ecosystem (GYE). Census sites are located almost entirely within Grand Teton National Park, where a broad range of representative vegetation types is accessible within close geographic proximity. 2. 30 monitoring sites are established within and adjacent to the park in pristine habitat. Sites range from the Jackson Hole lowlands to subalpine and alpine sites, from meadow, sagebrush and marshland, through willow scrub, cottonwood and aspen woodlands, to lodgepole pine and spruce-fir forests. Some sites have a monitoring history of >30 y; others were established in the mid-1990's. 3. The location and accessibility of the study sites permits all to be regularly and repeatedly censused during the short (6-week) breeding season. Census sites are standardized in area (5-10 ha in size) and mapped in detail (topographic features, vegetation). Census schedules, timing, and methodological protocols are established, and allow for controlled inter-site and inter-year comparisons in breeding bird populations, species composition, and densities.

Forest area and distribution in the Mississippi alluvial valley: implications for breeding bird conservation

SummaryKnowing the current forest distribution and patch size characteristics is integral to the development of geographically defined, habitat‐based conservation objectives for breeding birds. Towards this end, we classified 2.6 million ha of forest cover within the Mississippi Alluvial Valley using 1992 thematic mapper satellite imagery. Although historically this area, from southern Illinois to southern Louisiana, was dominated by forested wetlands, forest cover remains on less than 25% of the floodplain. Remaining forest cover is comprised of > 38,000 discrete forest patches > 2 ha. Mean patch area (64.1±5.2 ha; x̄±SE) was highly skewed towards small fragment size. Larger patches had a higher proportion of more hydric forest cover classes than did smaller patches which had a higher proportion of less hydric forest cover classes. Public lands accounted for 16% of remaining forested wetlands. Fewer than 100 forest patches exceeded our hypothesized habitat objective (4000 ha minimum contiguous forest area) intended to support self‐sustaining populations of forest breeding birds. To increase the number of forest patches exceeding 4000 ha contiguous area, and thereby increase the likelihood of successful forest bird conservation, we recommend afforestation adjoining existing forest fragments ≥1012 ha and focused within designated Forest Bird Conservation Regions.

Impact of forest type and management strategy on avian densities in the Mississippi Alluvial Valley, USA

Avian territory densities were determined from 20 Breeding Bird Censuses in mature (>30 years) bottomland hardwood stands and 18 Breeding Bird Censuses in young (6–9 years old) cottonwood ( Populus deltoides) plantations in the Mississippi Alluvial Valley. Avian species richness, diversity, and territory density were greater ( p < 0.01) in bottomland hardwood stands than in intensively-managed cottonwood stands but these parameters were not impacted by selective timber harvest within bottomland hardwood stands nor by method of regeneration within cottonwood plantations ( p > 0.05). Even so, detrended correspondence analysis based on avian territory densities readily segregated forest types and silvicultural treatments. Timber harvest within bottomland hardwood stands resulted in a shift in bird communities toward those found in cottonwood stands by increasing the densities of early-successional species such as Indigo Bunting ( Passerina cyanea), Yellow-breasted Chat ( Icteria virens), and Common Yellowthroat ( Geothlypis trichas). Conversely, regenerating cottonwood stands from root sprouts, rather than planting stem cuttings, resulted in a shift in bird communities toward those found in bottomland hardwood stands by increasing densities of species such as White-eyed Vireo ( Vireo griseus) and Wood Thrush ( Hylocichla mustelina). Tree species diversity, angular canopy cover, and midstory density were positively associated with bird species assemblages in bottomland hardwood stands, whereas vegetation density at ground level was positively associated with bird communities in cottonwood plantations. Conversion of agricultural fields to short-rotation cottonwood plantations results in increased breeding bird populations by adding up to 140 additional territories 40 ha −1. Even so, relative conservation values, derived from indicator species analysis and Partners in Flight concern scores, suggest that mature bottomland hardwood forests are twice as `valuable' for bird conservation as are cottonwood plantations.

Changes in breeding bird richness and abundance in Montreal parks over a period of 15 years

We compared the breeding bird population and vegetation structure of 201 randomly selected 1 ha plots in 49 urban parks in Montreal, 15 years apart (in 1979–1981 and 1994). The constancy of 17 bird species increased significantly, while that of four others decreased. Increased large-tree cover and a reduction in shrub cover do not explain more than a small proportion of this variation in species constancy. We contend that the installation of bird feeder stations in and around parks goes a long way toward explaining these changes, although the recent arrival in Quebec of the House Finch ( Carpodacus mexicanus) and the Northern Cardinal ( Cardinalis cardinalis) has definitely played a major role in the changes of the bird assemblage.

GTNP Breeding Bird Monitoring Project: The 1999 Season

Overview: GTNP Breeding Bird Monitoring Project. 1. Following initial independent work by M. L. Cody and 3y funding from NPS, we have instigated a scheme for long-term monitoring of breeding land bird populations in a wide variety of habitats representative of the northern Rockies and the Greater Yellowstone Ecosystem (GYE). Census sites are located almost entirely within Grand Teton National Park, where a broad range of representative vegetation types is accessible within close geographic proximity. 2. Some 30 monitoring sites are established within and adjacent to the park in pristine habitat. Sites range from the Jackson Hole lowlands to subalpine and alpine sites, from meadow, sagebrush and marshland, through willow scrub, cottonwood and aspen woodlands, to lodgepole pine and spruce-fir forests. Some sites have a monitoring history of &gt; 30 y; others were established in the mid-1990's. 3. The location and accessibility of the study sites permits all to be regularly and repeatedly censused during the short (6-week) breeding season. Census sites are standardized in area (5-10 ha in size) and mapped in detail (topographic features, vegetation). Census schedules, turung, and methodological protocols will be established and maintained, providing for strictly controlled inter-site and inter-year comparisons in breeding bird populations, species composition, and densities. 4. To evaluate the local versus more regional nature of inter-year variation in bird densities, one widespread habitat (willows) is replicated and censused at locations outside GTNP, in the northern Rockies (Glacier National Park) and central Rockies (Rocky Mountain National Park). 5. The project entails only modest costs (e.g. for transportation), but the projected benefits to science, specifically to resource management, will continue to accumulate as the data base is expanded in future years. As no comparable data base or monitoring scheme exists for the region, the value of the GTNP is apparent, and ensuring its continuance is of critical importance.

Effects of stream size on bird community structure in coastal temperate forests of the Pacific Northwest, U.S.A.

Abstract. Vegetation and spring breeding bird populations associated with riparian forests on high and low order rivers of the Olympic peninsula, Washington (USA), were censussed in 1990 and 1991. The objectives were to (1) provide a baseline description of bird communities associated with unmanaged riparian forests, (2) determine whether there is a difference between bird communities along large and small rivers, and (3) identify what biophysical aspects of the fluvial corridor are most important in determining bird community structure. The approach was to link riparian bird community structure to river order and discuss relationships to vegetation, geomorphology and associated hydrologic disturbance regimes.Large rivers tended to have more deciduous vegetation than small rivers (P = 0.07). Deciduous vegetation and patch types appeared to be linked to river size through stream valley shape and disturbance history. Bird species richness and abundance were significantly higher (P&lt;0.05) on larger rivers (67–140 m width) than on small rivers (12–21 m width) and were positively correlated with the percentage of deciduous trees present at a site. The data suggest that migrant birds, such as the black‐throated grey warbler (Dendroica nigrescens) and warbling vireo (Vireo gilvus), require large tracts of contiguous hardwoods for successful breeding. Additionally, birds such as the Hammond's flycatcher (Empidonax hammondii) and hairy woodpecker (Picoides villosus) appear to depend on the juxtaposition of hardwoods and conifers during the breeding season. We conclude that bird communities on large rivers differ from those on small rivers and that these differences occur predictably with stream order.

Effects of fertilizer-induced reduction of invertebrates on reproductive success of Loggerhead Shrikes(Lanius ludovicianus)

We examined the effect of spraying the common fertilizer, sodium ammonium nitrate, on cattle pastures in central Florida. Shrikes are considered good indicators of habitat quality. Hence, eight pairs of shrikes whose territories were sprayed were considered as the treatment group, and four pairs in unsprayed territories as controls. The fertilizer was sprayed on 18 June 1992, when the resident shrikes were into their second and third nests, and within 24 h the dominant bahia grass and patches of blackberry bushes became chlorotic. The eight treatment pairs expanded their territories significantly by 138.5% on average, and the smaller their initial territory, the greater the change. The total number of insects collected in the sprayed pasture in the first three weeks was extremely low compared to the number in the unsprayed pasture. In the control pairs no loss of eggs, young, or adults owing to abiotic causes was observed. However, in the treatment pairs seven eggs, two nestlings, and eight fledglings disappeared, or died from causes attributed to the spraying. In addition, seven females and one male disappeared, and one territory was abandoned. Our observations suggest the need for additional studies to examine the effects of supposedly „environmentally friendly” fertilizers and show the value of monitoring breeding bird populations in agricultural systems.

Dynamics of North American breeding bird populations

Population biologists have long been interested in the variability of natural populations1,2,3,4,5,6. One approach to dealing with ecological complexity is to reduce the system to one or a few species, for which meaningful equations can be solved. Here we explore an alternative approach7,8 by studying the statistical properties of a data set containing over 600 species, namely the North American breeding bird survey9. The survey has recorded annual species abundances over a 31-year period along more than 3,000 observation routes10. We now analyse the dynamics of population variability using this data set, and find scaling features in common with inanimate systems composed of strongly interacting subunits11. Specifically, we find that the distribution of changes in population abundance over a one-year interval is remarkably symmetrical, with long tails extending over six orders of magnitude. The variance of the population over a time series increases as a power-law with increasing time lag, indicating long-range correlation in population size fluctuations12. We also find that the distribution of species lifetimes (the time between colonization and local extinction) within local patches is a power-law with an exponential cutoff imposed by the finite length of the time series. Our results provide a quantitative basis for modelling the dynamics of large species assemblages.

Effects of woodland structure on breeding bird populations in stands of coppiced lime (Tilia cordata) in western England over a 10-year period

Bird populations were estimated in three adjacent stands of small-leaved lime {Tilia cordata) at Shrawley Wood, Worcestershire, between 1984 and 1993. The stands were old coppice, recent coppice cut in the late 1970s and thinned coppice that had been singled probably in the 1970s. It is thought that the stands were all previously coppiced in the 1940s. The study area presented an unusual opportunity to (1) assess effects of woodland structure on bird populations in the absence of any major differences in tree species, (2) to compare the dynamics of bird populations, and hence assess habitat preferences, within large samples of each habitat. Overall densities of birds were consistently lowest in the old coppice and generally highest in the recent coppice. The singled coppice supported the least diverse bird communities as measured by a simple diversity index and the expected numbers of species within standardized samples of individuals. Warblers (Sylviidae) were strikingly more abundant in recent coppice than other habitats, being virtually absent from the old coppice. Wrens were by far the most abundant species in the singled coppice. There was strong evidence that the old coppice was the least preferred habitat of wren and robin. The most striking change during the study was the collapse of the warbler populations in the recent coppice after some 11 to 12 years of coppice growth. This was predictable from previous studies, but the timing of the warbler decline occurred later than expected and was probably a consequence of the relatively slow canopy closure of the coppice at Shrawley Wood. Differences between stands, and changes over the 10 years within stands, were interpreted in terms of habitat structure and effects of winter weather on survival of resident species. The findings are discussed in relation to management of abandoned coppice; it is suggested that for conservation purposes thinning should be regarded as a complementary approach, alongside coppicing and non-intervention.

Landscape composition models for breeding bird populations in lowland English farmland over a 20 year period

The aim of this paper was to assess which components of landscape structure in lowland English farmland could best predict patterns m bird community composition, total bird density and densities of 12 bird species As far as possible, predictors were sought that were consistent m their effects over a 20 yr period The data were derived from mapping censuses in 1966, 1970, 1974, 1978, 1982 and 1986 with yearly sample sizes ranging from 41 to 51 plots, of minimum size 50 ha Landscape composition was quantified in terms of 10 landscape variables representing broad structural (e g density of woodland) and crop attributes (e g proportion of area under cereals) plus altitude Gradients in bird community composition were defined as plot scores on axis 1 of a detrended correspondence analysis (AXIS1) This gradient consistently represented a gradient from plots rich in woodland bird species to ones rich in field‐feeding and wetland species Multiple regression was used to build models containing 2‐, 3‐ and 4‐vanables Two approaches were used to combine data from the six years and models based on the single best subset in each year were found to be most consistent with respect to predictor selection Predictors were tested by building models based on each combination of five years and assessing how well the predictors explained variation in the bird variables for the sixth, independent year The predictors were relatively successful in explaining variations in densities of wren, robin, willow warbler, blue tit, great tit and chaffinch but especially AXIS1 which was explained in terms of woodland density, field size and altitude The predictors were unsuccessful in explaining total bird density, song thrush density and whitethroat density Structural attributes of landscape (especially density of hedgerow and woodland, but also rivers and ponds) were frequent predictors of species densities, but variables relating to farming system were not, with the exception of skylark and yellowhammer which were positively associated with extent of cereal crops Implications for landscape design and management, and the relative contribution of woodland and hedgerows to bird communities m farmland landscapes are discussed There was considerable variation between individual years which suggests that models of bird‐habitat relationships based on single years may have limited general application

What Characteristics of Shelterbelts are Important to Breeding Success and Return Rate of Birds?

-My field assistants and I studied breeding bird populations in 16 sites made up of shelterbelts and four sites including sections of woody draws in S-central North Dakota from 1985-1989. We documented a total of 42 species of birds nesting in wooded habitat on the study area. The composition of breeding bird species in shelterbelts was similar but not identical to that in the woody draws (23 of 42 species nested in both habitats). Breeding bird communities in these shelterbelt sites were more similar to those of local woody draws than they were to communities of previously studied woody draws in western North Dakota and of shelterbelts in Minnesota. I used logistic regression to examine the relationships between landscape-level and macrohabitat characteristics of shelterbelt sites such as isolation, area and grazing status of a site and the nesting success and return rates of three common breeding species of migratory songbirds (American robin, Turdus migratorius; brown thrasher, Toxostoma rufum, and loggerhead shrike, Lanius ludovicianus). Sites ranged in size from 0.3 to 3.4 ha and ranged in isolation from other wooded habitat from <0.1 to 1.6 km. Five of the 15 intensively studied sites were regularly grazed by livestock. Nesting success of brown thrashers was significantly higher in more isolated sites (P < 0.001). Thrashers were also significantly more likely to return to breed again to more isolated sites (P < 0.04) and to grazed sites (P < 0.002). These results should caution against uncritical support for construction of dispersal corridors.

Changes in Breeding Bird Populations in North Dakota: 1967 to 1992-93

We compared breeding bird populations in North Dakota using surveys conducted in 1967 and 1992-93. In decreasing order, the five most frequently occurring species were Horned Lark (Eremophila alpestris), Brown-headed Cowbird (Molothrus ater), Western Meadowlark (Sturnella neglecta), Red-winged Blackbird (Agelaius phoeniceus), and Eastern Kingbird (Tyrannus tyrannus). The five most abundant species-Horned Lark, Chestnut-collared Longspur (Calcarius ornatus), Red-winged Blackbird, Western Meadowlark, and Brown-headed Cowbird-accounted for 31-41% of the estimated statewide breeding bird population in the three years. Although species composition remained relatively similar among years, between-year patterns in abundance and frequency varied considerably among species. Data from this survey and the North American Breeding Bird Survey indicated that species exhibiting significant declines were primarily grassland- and wetland-breeding birds, whereas species exhibiting significant increases primarily were those associated with human structures and woody vegetation. Population declines and increases for species with similar habitat associations paralleled breeding habitat changes, providing evidence that factors on the breeding grounds are having a detectable effect on breeding birds in the northern Great Plains.

Monitoring Breeding Bird Populations in Grand Teton National Park: 1995-1997

In summer 1997 our NPS-funded project # CA-1460-5-0010, covering a 3-y period from summer 1995 through summer 1997, was completed. The immediate goals of the project were to instigate a system for monitoring the densities of breeding bird species, by establishment of flxed sites as a basis for a long term monitoring plan and of census protocols that can detect changes of breeding species and their densities over successive years. The monitoring scheme is conducted largely within Grand Teton National Park (GTNP), but covers habitats and an avifauna representative of the Greater Yellowstone Ecosystem (GYE) and the central-northern Rocky Mountains in general. The project emphasizes the need for long­term and on-going studies on breeding bird species and densities and their importance as a tool for evaluating the impact of both local and distant influences on breeding bird populations. For residents, species that remain all year in or near the breeding habitat, local effects include those operating on-site during the non-breeding season as well as during the breeding season. For migrant species, those that breed on-site but leave to spend the non-breeding season in other locations, often distant and usually of quite different habitat composition, there are both on-site influences on breeding population densities, such as inter-year changes in vegetation structure and productivity, and off-site or distant influences, including factors that affect over-wintering success in the non­breeding habitat and others that influence a successful transit between wintering and breeding grounds. The assessment of long-term trends in bird densities may be used as a form of bioassay of the state of the local environments. Information from such studies can provide region-wide indicators that, given a sufficiently comprehensive data base, can segregate local from distant influences on populations. Such indicators can be incorporated into management strategies to aid in determining which local strategies may be necessary (and feasible) to help maintain the biota.

Research on breeding birds in Dutch dune areas

Dune areas, including salt marshes, comprise only 1% of the land area of The Netherlands. Yet almost 80% of the 180 bird species that breed in The Netherlands, are known to breed or to have bred in dunes, which also represent valuable habitat for migratory and wintering birds. Being aware of the high natural value of the dunes and of the expected impact of a variety of threatening activities, nature conservation studies were set up by concerned volunteers and universities. Breeding birds played an important role from the beginning. It certainly contributed to the rather high nature protection status that most dunes areas currently enjoy. Furthermore these old studies provide a picture of the avifaunal value of these areas in the more pristine days. During the last decades intensive long-term monitoring of breeding bird populations has become more and more important in the Dutch dunes. In 1958 the first inventory of dune breeding birds in relation to management issues started. Its aim was to monitor the changes in species composition and numbers after the artificial infiltration of river water for drinking water production. Since then many projects of the same kind evolved in different dune areas, almost exclusively organised by volunteer bird watchers, yielding a data set which offers promising analytical possibilities. This article provides an overview of breeding bird research projects in dune habitats in relation to important actual management issues. The following issues will be dealt with: the impact of drinking water production; the impact of tall grass dominance and the ongoing growth of shrubs; the impact of the reappearance of the red fox Vulpes vulpes. The review will show that much insight has been gathered over the years concerning breeding birds and dune management but that many gaps in our knowledge still exist.

The effects of traffic on the density of breeding birds in Dutch agricultural grasslands

The effect of traffic on the breeding density of grassland birds was studied in 1989 in 15 transects along main roads in The Netherlands. Out of 12 species that could be analysed, 7 showed a reduced density adjacent to the road. There was also a strong effect on the summed densities of all species. Disturbance distances varied between species, ranging from 20 to 1700 m from the road at 5000 cars a day and from 65 to 3530 m at 50,000 cars a day (car speed 120 km/h). At 5000 cars a day most species had an estimated population loss of 12–56% within 100 m of roads, but beyond 100 m > 10% loss only occurred in black-tailed godwit Limosa limosa (22% for 0–500 m zone) and oystercatcher Haematopus ostralegus (44% up to 500 m and 36% for 0–1500 m zone). At 50,000 cars a day all species had estimated losses of 12–52% up to 500 m while lapwing Vanellus vanellus, shoveler Anas clypeata, skylark Alauda arvensis, blacktailed godwit and oystercatcher populations were reduced by 14–44% up to 1500 m. In The Netherlands, with a dense network of extremely crowded motorways, traffic should be considered a serious threat to breeding bird populations in grasslands. Greater care should be taken in planning new roads, and it is important to explore how the present effects can be reduced.

Monitoring Breeding Bird Populations in Grand Teton National Park

This report covers year 2 of a three year project, 1995-1997 inclusive, to instigate a permanent program of monitoring landbird species composition and densities in a variety of representative habitats within Grand Teton National Park (GTNP). Habitats range from grassland and sagebrush on the valley floor of Jackson Hole (around 1900 m) through a range of scrub, woodland, and tall foothill forest vegetation types to montane sites of subalpine fir and tundra (ca. 3000 m). The monitoring program is intended to provide data on year-to-year fluctuations in breeding bird species and densities, and document longer-term changes (if any) in the local avifauna of resident and migratory species. The data base will document variability in size of breeding populations among years, local shifts in distribution and abundance over habitat types, and potentially form an information source on which management and conservation decisions might be based.

The Effects of Car Traffic on Breeding Bird Populations in Woodland. IV. Influence of Population Size on the Reduction of Density Close to a Highway

1. This study tested the assumption that in years with a low overall population size the density-depressing effect of roads on breeding birds will be more important than in years with a high overall population size. 2. The effect on the density was investigated in 1984, 1986 and 1988. Of the 23 species that could be analysed each year, 17 species had a lowered density close to the road in at least 1 year. In 1984 and 1986 the number of affected species was much higher (10 and 14, respectively) than in 1988 (4). 3. The number of affected species, as well as the effect size for all species combined and for most individual species, was negatively correlated with the overall population size. 4. It is concluded that in years with a high-overall population size, the use of density as a response variable will cause a considerable underestimation of the reduction of the habitat quality close to roads. This has not been recognized well in impact studies concerning road traffic, nor in other impact studies.

Effects of Grasshopper Control Programs on Rangeland Breeding Bird Populations

We investigated the effects of grasshopper control methods on breeding bird populations in western rangelands. We estimated bird densities on 13 treated and 11 untreated sites before and after grasshopper control operations. Four different treatments were used in these applications: malathion, sevin-4-oil, carbaryl bait and Nosema locustae bait. There were few differences among the 4 treatments in their effects on bird community parameters (total bird density, and species richness, diversity, or evenness). Bird community parameters did not differ between spray and bait applications but sample sizes were small for bait treatments. When data from all treatments were combined for analysis, there was no difference in any of the bird community parameters between pre- and post-treatment samples. Densities of western meadowlarks (Sturnella neglecta), however, were significantly lower on treated than untreated sites 10 and 21 days after treatment. We found no relationship between changes in bird numbers 21 days post-treatment and either the size of the area treated or the date of treatment (i.e., early or late in nesting season). Malathion is an acetylcholinesterase (AChE) inhibitor, but brain AChE levels in birds collected on sites treated with malathion showed no significant inhibition. These results suggest that grasshopper integrated pest management treatments generally have little effect on breeding bird communities but some insectivorous bird species may decline on rangeland treated with broad-spectrum insecticides because of reduction in food base. Pesticide applications that have adverse impacts on birds and other nontarget wildlife that prey on grasshoppers may be counterproductive to long-term integrated pest management goals.

An Analysis of Midwestern Breeding Bird Population Trends: 1966-1993

-Life-history traits associated with midwestern breeding bird population trends were assessed using data from the North American Breeding Bird Survey (BBS). Analyses of BBS population trends by bird-habitat association, nest location and migratory strategy groups showed that grassland bird species had exhibited more extensive population declines between 1966 and 1993 than other groups of midwestern breeding bird species. Analysis of species representation in the midwestern BBS data, however, revealed significant biases in species representation based on habitat associations. Wetland bird species are significantly underrepresented on midwestern BBS routes, whereas urban and shrubland/early successional bird species are overrepresented relative to other groups of bird species. Because of these differences in species representation, additional surveys and monitoring programs are needed in order to compile a comprehensive pattern of population trends of midwestern

The Effects of Car Traffic on Breeding Bird Populations in Woodland. III. Reduction of Density in Relation to the Proximity of Main Roads

1. This study investigated the effect of car traffic on the breeding density of birds in deciduous and coniferous woodland, and the importance of noise and visibility of cars as possible factors affecting density. 2. Of the 43 species analysed in both woodland types, 26 species (60%) showed evidence of reduced density adjacent to roads (based on analysis with Wilcoxon signed-ranks test and regression). 3. Regression models with noise load as the only independent variable gave the best overall results. Calculated «effect distances» (the distance from the road up to where a reduced density was present) based on these regressions varied between species from 40-1500 m for a road with 10 000 cars per day to 70-2800 m for a road with 60 000 cars per day (120 km h −1 and 70% amount of woodland along the road). For a zone of 250 m from the road the reduction of the density varied from 20 to 98%. 4. When visibility of cars was controlled for, the number of species showing density reductions was much higher on plots with a high noise load than on ones with a low noise load. When noise conditions were held constant, however, there was no difference in bird densities between plots with high and low visibility of cars. 5. It is argued that noise load is probably the most important cause of the reduced densities. Visibility of cars, direct mortality and pollution are considered unimportant. 6. The results of this study stress the importance of considering the effect of car traffic on the breeding density of birds in planning and constructing main roads