Impacts of an invasive willow (Salix × rubens) on riparian bird assemblages in south‐eastern Australia

We explored how a woody plant invader affected riparian bird assemblages. We surveyed 15 200‐m‐long transects in riparian zones in a much‐changed landscape of eastern Victoria, Australia. Abundance, species‐richness, foraging‐guild richness and composition of birds were compared in transects in three habitat types: (i) riparian zones dominated by the invasive willow Salix × rubens; (ii) riparian zones lined with native woody species; and (iii) riparian zones cleared of almost all woody vegetation. We also measured abundance and richness of arthropods and habitat structure to explore further the effects of food resources and habitat on the avifauna. We observed 67 bird species from 14 foraging guilds. Native riparian transects had more birds, bird species and foraging guilds than willow‐invaded or cleared transects. Habitat complexity increased from cleared to willow‐invaded to native riparian transects, as did abundance of native and woodland‐dependent birds. Native shrub and trees species had more foliage and branch‐associated arthropods than did willows, consistent with a greater abundance and variety of foraging guilds of birds dependent on this resource. Willow spread into cleared areas is unlikely to facilitate greatly native bird abundance and diversity even though habitat complexity is increased. Willow invasion into the native riparian zone, by decreasing food resources and altering habitat, is likely to reduce native bird biodiversity and further disrupt connectivity of the riparian zone.     

Effects of proximity to riparian zones on avian species richness and abundance in montane aspen woodlands

Riparian zones often provide more food or nesting resources than surrounding ecosystems and thus support more species or a greater abundance of birds. However, the extent to which the positive effects of riparian zones extend into adjoining habitats has rarely been investigated. We examined bird species richness and abundance in aspen (Populus tremuloides) woodlands of the upper East River Valley in Colorado, a region with extensive stands of aspen located upslope from riparian zones dominated by willows (Salix spp.). Our objective was to determine how species richness and abundance in aspen stands located closer to riparian zones compared to that of aspen stands farther away. To control for elevation effects, we conducted point counts for birds and examined the effects of riparian proximity on avian species richness and abundance in three elevation belts (low, 2840–2875 m; middle, 2940–2960 m; and high, 3040–3060 m). We determined riparian proximity by measuring distances from each census site to the nearest body of water and to the nearest patch of willow. Proximity to major willow patches (≥6500 m²) had a stronger influence on species richness and abundance than did proximity to water or smaller patches of willows. Total species richness and abundance significantly increased with proximity to major willow patches at low elevation sites only. This relationship was driven by a greater prevalence of riparian‐nesting species at lower elevation sites, where aspens were generally closer to riparian zones. The positive effects of willows diminished with abrupt changes in elevation. Our results highlight the importance of protecting willows around riparian zones in valley bottoms where habitat destruction caused by human land use is most prominent.     

Breeding success and predation at nests of banded dotterel (

Egg and chick loss at banded dotterel (Charadrius bicinctus) nests was studied over the 1992/93 season on the Tekapo, Ohau and Ahuriri Rivers in the Central South Island. Egg loss at nests was higher on the Ohau and Tekapo Rivers than on the Ahuriri River, especially early in the season. Only 11% and 12% of nests fledged one or more chicks on the Tekapo and Ohau Rivers respectively, compared to 42% of nests on the Ahuriri River. Nests on islands within the braided riverbeds were more successful than nests on the mainland. Proximity of nests to potential predator cover did not influence hatching success although close proximity of nests to rabbit burrows increased the risk of predation. A lack of knowledge of the predator species involved in this study hampers assessment of the impact of various factors on the breeding success of banded dotterels. Further research to identify predators is necessary to target conservation management and better protect banded dotterel and endangered riverbed birds.     

Effects of Tamarix removal on the community dynamics of riparian birds in a semiarid grassland

Invasion of riparian habitats by non‐native plants is a global problem that requires an understanding of community‐level responses by native plants and animals. In the Great Plains, resource managers have initiated efforts to control the eastward incursion of Tamarix as a non‐native bottomland plant (Tamarix ramosissima) along the Cimarron River in southwestern Kansas, United States. To understand how native avifauna interact with non‐native plants, we studied the effects of Tamarix removal on riparian bird communities. We compared avian site occupancy of three foraging guilds, abundance of four nesting guilds, and assessed community dynamics with dynamic, multiseason occupancy models across three replicated treatments. Community parameters were estimated for Tamarix‐dominated sites (untreated), Tamarix‐removal sites (treated), and reference sites with native cottonwood sites (Populus deltoides). Estimates of initial occupancy (ψ₂₀₀₆) for the ground‐to‐shrub foraging guild tended to be highest at Tamarix‐dominated sites, while initial occupancy of the upper‐canopy foraging and mid‐canopy foraging guilds were highest in the treated and reference sites, respectively. Estimates of relative abundance for four nesting guilds indicated that the reference habitat supported the highest relative abundance of birds overall, although the untreated habitat had higher abundance of shrub‐nesters than treated or reference habitats. Riparian sites where invasive Tamarix is dominant in the Great Plains can provide nesting habitat for some native bird species, with avian abundance and diversity that are comparable to remnant riparian sites with native vegetation. Moreover, presence of some native vegetation in Tamarix‐dominated and Tamarix‐removal sites may increase abundance of riparian birds such as cavity‐nesters. Overall, our study demonstrates that Tamarix may substitute for native flora in providing nesting habitat for riparian birds at the eastern edge of its North American range.     

Nest site selection of a primary hole‐nesting passerine reveals means to developing sustainable forestry

Anthropogenic habitat loss and fragmentation affect populations worldwide. For example, many bird populations of boreal forests have declined due to intensive forestry. To target conservation actions for such species, determining the key factors that affect their habitat selection is essential. Remote sensing methods provide highly potential means to measure habitat variables over large areas. We aim at identifying the key‐features of habitats by utilizing remote sensing data. As a case example, we study the nest site selection of a primary hole‐nesting passerine, the willow tit Poecile montanus, in a managed forest landscape. Using presence–absence data, we determine the most important habitat characteristics of the nest sites for three spatial scales by generalized linear mixed effect models. Our results highlight the importance of the availability of nesting sites – standing decaying deciduous trees – in the nest site selection of P. montanus. It seems to prefer moist habitats with high densities of deciduous trees and to avoid open areas, but does not require mature or intact habitats. Most of the nest site selection seems to occur within small scales. In this case, remote sensing data alone was insufficient for producing reliable models, but adding information of an ecologically important feature from direct field surveys greatly improved model performances. For the conservation and maintenance of dead wood dependent species, changes in forestry practices are necessary to keep the key characteristics of the habitat. Most importantly, continuous availability of standing decaying wood should be secured.     

Presence of other species may counter seasonal decline in breeding success: a field experiment with pied flycatchers Ficedula hypoleuca

Seasonal decline in breeding success limits fitness in many bird species nesting in the temperate, boreal and arctic zones. Factors affecting this decline, especially if the decline is reduced, can thus have significant ecological and evolutionary importance. In an experiment designed to investigate fitness consequences of heterospecific attraction, no seasonal decline in breeding success was observed for pied flycatchers Ficedula hypoleuca breeding in the presence of resident titmice Parus spp., whereas a pattern of steep decline was observed for birds breeding in areas where residents were removed. Randomisation of removal treatment and pied flycatcher territories with respect to arrival date leaves enhanced foraging that results from the presence of resident titmice during breeding as the best, albeit currently hypothetical, explanation for the observed absence of a seasonal decline. Among terrestrial vertebrates, reports of this kind of direct positive interactions are rare.     

Predictors of clutch predation of a globally significant avifauna in New Zealand's braided river ecosystems

Predation by introduced mammals is decimating New Zealand's indigenous fauna. Understanding factors that influence this process allows resources for predator control to be applied with maximum effect. This study examines how predation of a secondary prey species (a relatively common but declining native plover, the banded dotterel Charadrius bicinctus ) varied with reductions in abundance of a major prey source (rabbits), kill-trapping of predators, nest density and habitat complexity. Banded dotterels mostly nest in open braided riverbeds alongside a number of endemic threatened species. We measured the fate of 753 dotterel clutches exposed to predation by cats, ferrets and hedgehogs. We found key times and places of high predation risk. Immediately after widespread reduction in rabbit populations by rabbit haemorrhagic disease (RHD), clutch predation rates were almost as high (mean, 50%) as those recorded during past rabbit poisoning programmes (mean, 57%). Both rates were significantly higher than the mean predation rate of 22% without rabbit control, suggesting a shift in predator diet immediately after rabbit population declines. Unlike after rabbit poisoning, clutch predation rate remained high in the years after RHD. Other patterns observed included higher clutch predation rate where nest density was lower, suggesting that predation can potentially cause local extinction. Clutch predation was also higher along riverbed margins where vegetation was dense. There was equivocal evidence for an effect of predator kill-trapping on clutch predation rate. Management strategies that could potentially reduce clutch predation risks include focusing predator mitigation measures during periods of rabbit decline, maintaining them for more than one breeding season if the rabbit declines are widespread (e.g. RHD epidemics), and applying greater effort at sites with relatively low nest density and along riverbed margins where predator use is more frequent.     

Tritrophic interactions: willows, herbivorous insects and insectivorous birds

Insectivorous birds can increase plant growth by consuming herbivorous insects and reducing insect damage. However, plant traits such as the level of chemical defense may affect the quantity and quality of insects, and alter the foraging behavior of birds. Therefore, I predicted that plant traits can also modify the effect of birds on leaf damage and plant growth. This study compared the effect of insectivorous birds on the herbivory and growth of two chemically different willow species, weakly defended Salix phylicifolia and strongly defended S. myrsinifolia under two fertilization levels. Half of the willows were protected from birds using a translucent gill-net, which did not limit access by insects. The effect of birds on the densities of leaf-chewing insects and leaf damage was considerable on unfertilized S. phylicifolia but less obvious on fertilized ones. The effect of bird predation was negligible on S. myrsinifolia, which had very low insect densities in all treatments. Birds increased the growth of the experimental willows, but the effect was clear only in unfertilized S. phylicifolia. I suggest that birds avoided foraging on willows with low populations of insects and little visible damage. The study shows that bird predation can alter the patterns of insect densities we see on willows, emphasizing the importance of considering multitrophic effects when studying plant-insect interactions. Received: 25 May 1999 / Accepted: 9 August 1999     

Southwestern Willow Flycatcher Population and Habitat Response to Reservoir Inundation

ABSTRACT One of the largest known populations of the federally endangered southwestern willow flycatcher (Empidonax traillii extimus) occurs at Roosevelt Lake, Arizona, USA. Modifications to Roosevelt Dam, completed in 1996, raised the height of the dam and resulted in a high probability of willow flycatcher habitat inundation within the reservoir's conservation pool. We collected habitat measurements and monitored 922 willow flycatcher nests from 1996 to 2006 to investigate effects of inundation on willow flycatcher habitat and subsequent changes in nest success, productivity, and distribution. Inundation of willow flycatcher habitat at Roosevelt Lake occurred in 2005, changing the location and amount of suitable breeding habitat and significantly altering habitat structure (e.g., thinner vegetation, more canopy gaps) of formally occupied nest sites. The willow flycatcher population at Roosevelt Lake decreased 47% from 209 territories in 2004 to 111 territories in 2006 in response to habitat changes. Willow flycatchers made fewer nesting attempts and nest success rates were significantly lower during inundation (2005 and 2006: 45%) than preinundation (1996–2004: 57%). Combined, these factors negatively affected the population's productivity during inundation. Although inundation caused extensive vegetation die-off, we did observe regeneration of vegetation in some areas at Roosevelt Lake in 2006. The Roosevelt Lake population remains one of the largest willow flycatcher populations in the state and territory numbers remain high enough that the population may not suffer long-term effects if sufficient suitable habitat continues to exist during the cycle of inundation and regeneration. Reservoir managers may be able to develop dam management guidelines that reduce damage to habitat, encourage habitat growth, and mimic the dynamic nature of unaltered riparian habitat. These guidelines can be implemented, as appropriate, at reservoirs throughout the willow flycatcher's range.     

Diurnal and nocturnal ranging behaviour of Stone‐curlews Burhinus oedicnemus nesting in river habitat

Studying a species under a range of conditions is essential for fully understanding its ecology and for predicting its response to human impacts on the environment. We investigated the spatial behaviour and the habitat characteristics of foraging areas of Eurasian Stone‐curlews breeding in an important but poorly investigated habitat, gravel riverbed, throughout the full 24‐h cycle. The data collected for 17 radiotagged birds nesting in the Taro River Regional Park (Parma, Italy) showed a clear split between diurnal and nocturnal spatial behaviour. Almost all diurnal fixes and about two‐thirds of nocturnal ones were located in the gravel riverbed, which not only provided suitable breeding territories but probably part of the food resources needed for reproduction. Nocturnal excursions from breeding sites to feeding areas (mostly farmland), sometimes of a few kilometres, indicated that these resources do not cover all of the birds’ needs. Night spotlight counts of foraging birds in the agricultural area indicated that Stone‐curlews preferred recently harvested crops (mainly forage and wheat) and piles of farmyard manure. These habitat preferences are closely linked to the predominant agricultural activity of the study area, which is characterized by a high density of dairy farms for the production of Parmigiano Reggiano cheese. It seems likely that the proximity of gravel riverbed and farmland habitats is one of the main causes of the high breeding density recorded in the study area. We propose that the conservation of Stone‐curlews at this site could potentially be achieved only by a synergistic management of both natural and agricultural habitats.     

Multi-scale habitat selection and foraging ecology of the eurasian hoopoe (<Emphasis Type="Italic">Upupa epops</Emphasis>) in pine plantations

Bird conservation can be challenging in landscapes with high habitat turnover such as planted forests, especially for species that require large home ranges and juxtaposition of different habitats to complete their life cycle. The eurasian hoopoe (Upupa epops) has declined severely in western Europe but is still abundant in south-western France. We studied habitat selection of hoopoes in pine plantation forests using a multi-scale survey, including point-counts at the landscape level and radio-tracking at the home-range scale. We quantified habitat use by systematically observing bird behaviour and characterized foraging sites according to micro-habitat variables and abundance of the main prey in the study area, the pine processionary moth (Thaumetopoea pityocampa). At the landscape scale, hoopoes selected habitat mosaics of high diversity, including deciduous woods and hedgerows as main nesting sites. At the home-range scale, hoopoes showed strong selection for short grassland vegetation along sand tracks as main foraging habitats. Vegetation was significantly shorter and sparser at foraging sites than random, and foraging intensity appeared to be significantly correlated with moth winter nest abundance. Hoopoe nesting success decreased during the three study years in line with processionary moth abundance. Thus, we suggest that hoopoes need complementation between foraging and breeding habitats to establish successfully in pine plantations. Hoopoe conservation requires the maintenance of adjacent breeding (deciduous woods) and foraging habitats (short swards adjacent to plantation edges), and consequently depends on the maintenance of habitat diversity at the landscape scale.     

Spatial pattern in the occurrence of the lichen Lobaria pulmonaria in managed and virgin boreal forests

Two populations of the lichen Lobaria pulmonaria. growing on aspens and goat willows in 12 and 20 km² study areas of boreal forest in Finland, were surveyed thoroughly to investigate the factors influencing the spatial distribution of the lichen. In one study area, where forestry has been intensive and old-growth forest is highly fragmented, L, pulmonaria was sparse and grew mostly on willows. In contrast, a large and continuous virgin forest area supported a higher incidence of L. pulmonaria. with the lichen being common on both aspens and willows. In both study areas, the distributions of aspen and willow were clumped over the scales of 100-1000 m. The spatial pattern of L pulmonaria was more clumped in the managed forest than in the virgin forest. The reduced incidence of the lichen on aspens in the managed area was attributed to a disruption of habitat continuity and small average tree size. There was no comparable reduction in the incidence on willows, probably because the willow had a very aggregated distribution in the managed area, which probably facililated local colonization of the lichen. Presence of the lichen was significantly related to size-corrected local density of aspen and willow trees as well as to spatial connectivity to neighboring lichen-occupied trees.     

Relationships between bird community and habitat structure in shelterbelts of Hokkaido,Japan

Summary The relationship between the bird community and habitat structure in wind shelterbelts of Ishikari district was examined.Breeding birds were classified into nesting guilds (hole, canopy, and bush) and also into foraging guilds (outside, canopy, and bush) Both density and species richness in the outside-foraging guild were positively related to forest age variables. On the other hand, bird density in all three nesting guilds and the other two foraging guilds (canopy and bush) was correlated with the vegetation cover of their nesting or foraging sites, and species richness was positively correlated with tree species complexity variables.Bird species diversity (BSD) was closely related to tree species complexity, but not to foliage height diversity (FHD) within woods. However, if woods were re-classified into two groups such as natural and artificial and these were separately analysed, the correlation between BSD and FHD was significant.From these results, it is concluded that both FHD and tree species complexity should be considered together in order to predict BSD within forests. A new index is proposed for this purpose, and its usefulness discussed.     

Effects of nest-site availability and distribution on density-dependent clutch size and laying date in the Chough Pyrrhocorax pyrrhocorax

We compared the breeding phenology and clutch size of Choughs Pyrrhocorax pyrrhocorax in three Spanish areas that differed in feeding habitat, breeding environmental conditions and the distribution and availability of different nesting sites (cliffs or artefacts). The variation in the timing of laying seemed to be related to differences in climatic conditions determined by the altitude. The variation in clutch size was associated with the different use that Choughs made of the feeding areas because of the contrasting availability and distribution of their nesting sites. Within southeastern Madrid, clutch size of the Chough decreased and its variance increased at high nest densities on cliff sites, suggesting that density dependence in fecundity arises from habitat heterogeneity rather than by interference. At high densities, individuals, territories, nest sites or a combination of these may differ in quality, thus promoting differences and increasing the variance in the initial investment of reproductive effort. The syncronization of the onset of laying at increased breeding densities suggests that social influences arising from communal foraging or avoidance of predation by early warning and predator swamping may be acting also. We suggest that nest-site availability and distribution have a major influence on the social organization of Choughs through their breeding and foraging strategies.     

Habitat selection in reintroduced bird populations: a case study of Stewart Island robins and South Island saddlebacks on Ulva Island

An understanding of the mechanisms influencing habitat selection in reintroduced bird populations is fundamental for successful translocation programmes. Plant species composition, abundance, structure and food availability are likely to influence animal movement and habitat choice, but few studies have evaluated their combined effect on habitat selection of translocated birds. Stewart Island robins (Petroica australis rakiura) and South Island saddlebacks (Philesturnus carunculatus carunculatus) are two threatened New?Zealand bird species that have been reintroduced to Ulva Island (Stewart Island). We hypothesised that their initial settlement patterns were driven by habitat quality. We tested this hypothesis by comparing habitat components between occupied and unoccupied habitats as the population grew after initial tanslocation. We also modelled probabilities of site selection as a function of the composition and structure of vegetation, availability of food (invertebrate composition) and nesting resources (cavity type). Founding pairs of both species first established territories in coastal habitat in the western part of the island, which is characterised by structurally complex broadleaved vegetation. Birds also selected sites with a greater abundance and diversity of food resources. Thus in the early stages of population establishment robins and saddlebacks appear to select high quality habitat that offers enhanced cover and foraging opportunities.     

Numerical and behavioural responses of migrant passerines to experimental manipulation of resident tits (Parus spp.): heterospecific attraction in northern breeding bird communites?

Summary We studied experimentally interspecific competition among foliage-gleaning passerine birds by manipulating the density of resident tits. In 1988 tit density was experimentally increased on three small islands in a central Finnish lake, and decreased on three other islands by tit removal. In order to avoid the effects of between-island differences in habitat quality, the role of the islands was reversed when the experiment was repeated in the following year. Censuses and observations on foraging and feeding behaviour were conducted to assess the numerical and behavioural responses of migrant conguilders (mainly chaffinches and willow warblers) with respect to the manipulated abundance of the tits. We also measured whether variation in food consumption of tits affected the frequency with which the migrants found food by calculating average intervals between successful prey captures, time lags to prey-capture and giving-up times. Our results indicate that interspecific competition is of minor importance in structuring breeding bird assemblages and species feeding ecologies on the study islands. No consistent difference in foraging or feeding niches of chaffinches and willow warblers was found between low and high tit density conditions. Niche overlap analysis showed no avoidance by chaffinches and willow warblers of the microhabitats which tits used. Tit abundance had no significant effect on feeding success or behaviour. Experimentally increased abundance of resident birds was associated with increased abundance of breeding migrants, however. This pattern was found not only in the foliage gleaning guild but also with all passerine birds, indicating that food was not an important contributor to this pattern. We elaborate a hypothesis suggesting heterospecific attraction in northern breeding bird assemblages. Habitat generalist migrants may use the presence of residents as an indicator of safe and/or productive breeding sites in northern unpredictable circumstances.     

The effects of habitat structure on predation risk of birds in agricultural landscapes

It has been suggested that increased predation rates may rival habitat alteration as a causal agent in farmland bird population declines. Such a view may be over-simplistic, however, as changes in habitat structure may influence habitat selection and foraging efficiency through their influence on perceived and actual predation risk. We review evidence from the literature on the effects of habitat structure on predation risk of foraging and nesting birds and apply these principles to investigate the likely effects on the 20 species that comprise the UK Government's 'Farmland Bird Index'. Shorter vegetation is likely to enhance foraging efficiency and reduce predation risk (when ground foraging) for 15 of the 20 species. However, within grassland systems longer vegetation is known to enhance food supplies (e.g. Tipulid larvae and voles) of several farmland bird species and so mosaics of short and long vegetation may provide the optimum conditions for most species (e.g. Lapwing Vanellus vanellus , Starling Sturnus vulgaris , Barn Owl Tyto alba ). Agricultural intensification has encouraged uniform dense swards, thus reducing habitat diversity, and agri-environment schemes that provide heterogeneous sward structure may thus facilitate farmland bird conservation. Intensification has also resulted in less dense hedgerows; although a reversal of this trend may improve foraging efficiency for many species, it may be detrimental to a smaller number of species that prefer shorter, less dense hedges for nesting. Before these tentative conclusions can be confirmed, more research is required that considers how the effects of habitat structure on individuals is likely to translate into population-level impacts.     

Causes of reintroduction failure of the brown treecreeper: Implications for ecosystem restoration

Reintroductions are conducted to re‐establish a self‐sustaining population of a species and contribute to ecosystem restoration. The brown treecreeper (Climacteris picumnus) reintroduction into two nature reserves in the Australian Capital Territory in south‐eastern Australia failed to meet its predetermined criteria for success. This occurred despite prior habitat restoration within the reserves where reintroduction occurred. Low survival of reintroduced brown treecreepers, particularly due to predation by native predators, has previously been highlighted as a key factor in the failure of the programme. We compared bird behaviour and habitat characteristics between the reintroduction reserves and the sites where brown treecreepers were sourced (which support stable brown treecreeper populations). We did not identify an indication of significantly higher predation pressure in the reintroduction reserves in comparison with the source sites. However, our results revealed that reintroduced individuals may be more vulnerable to predation because of an increased flight time to reach a refuge area. This was a result of a significantly lower number of refuge areas in logs and trees and a higher number of shrubs (which may obstruct escape paths and hinder detection of predators) in the reintroduction reserves compared with the source sites. We identified a lower ground foraging habitat quality in the reintroduction reserves because of lower numbers of ant mounds and lower areas of forageable ground. However, brown treecreepers were able to disperse extensively throughout the reserves and settle in areas with generally higher‐quality foraging habitat. Therefore, the negative effect of low ground foraging habitat quality would have been most pronounced immediately after release. This study emphasizes the inherent complexities of species reintroductions and ecosystem restoration. Despite experimental restoration activities within the reintroduction reserves, there were still deficiencies in habitat quality. We emphasize that further habitat restoration is required within these reserves to achieve more complete restoration.     

Trade-offs in nest site selection in coastal populations of Lapwings Vanellus vanellus

In coastal populations of Lapwings Vanellus vanellus in southwestern Sweden, arable fields predominated as foraging habitat before laying. Females caught more large prey items on arable fields and shores than on pastures. Close to egg laying, females foraged mainly near their future nest sites. Arable land and pastures were used to a similar extent for nesting. We found no difference in nest predation between habitats. Egg volume varied among females and was correlated with wing-length, body mass and condition. Mean egg volume also was positively correlated with feeding time on arable land before laying. Pairs nesting on arable fields therefore generally produced larger eggs than those on pastures. The distances between nests and chick foraging areas, however, were significantly longer for birds nesting on arable land than for those on pastures. Moreover, in 2 of 3 years, the proportion of hatched chicks that survived until fledging was negatively correlated with this distance. There was no difference in chick survival between broods hatched on arable fields and pastures. We suggest that nest site selection and offspring production involve a trade-off between the benefits of nesting close to rich feeding grounds for adults and the costs of moving long distances between nest sites and chick-rearing areas.     

Nest-site selection by Common Black-Hawks in southwestern New Mexico

ABSTRACT.   Despite the interest of resource managers and conservationists in the status of Common Black-Hawk ( Buteogallus anthracinus ) populations in the southwestern United States, little is known about their nesting success and habitat requirements. Because such information is essential for effective population and habitat management, I examined the nesting success and nest-site selection of Common Black-Hawks in southwestern New Mexico during 2000 and 2001. Of the 37 nesting attempts in 21 territories, ≥1 young fledged from 25 nests (68%). Comparison of nest-sites and nonused sites suggested that breeding Common Black-Hawks selected nest-sites in areas with a sparser and shorter subcanopy tree layer and in trees with a smaller trunk diameter and a greater minimum crown diameter. These differences appear to be related to variation in forest ages within territories, with nonused sites having fewer, but older, canopy trees than nest-sites. Sites with younger, smaller subcanopy trees may provide forest structure for more effective foraging, whereas the characteristics of younger nest tree canopies may reduce the risk of nest predation or offer more protection from inclement weather. Due to the limited range of this species in the southwestern United States, efforts to encourage the establishment and maturation of riparian forests in Common Black-Hawk breeding areas could be important in sustaining available nesting habitat and, in turn, maintaining or expanding current population levels.