Seedling competition between native cottonwood and exotic saltcedar: implications for restoration

Altered hydrology of southwestern United States rivers has led to a decline in native cottonwood (Populus deltoides). Areas historically dominated by cottonwood have been replaced by invasive saltcedar (Tamarix chinensis). Restoration of historic hydrology through periodic flooding of riparian areas has been a means of restoring native species. However, due to similarity in germination requirements of cottonwoods and saltcedars, flooding may create an unwanted increase in the number of saltcedar seedlings. Therefore, we evaluated competitive aspects of these co-occurring species in an extant riparian habitat in the arid southwestern US. We measured effects of competition between cottonwood and saltcedar seedlings and among cottonwood seedlings during the first growing season following seedling establishment in 360, 0.5 × 0.5-m plots at the Bosque del Apache National Wildlife Refuge, New Mexico. We used five interspecific density treatments and five intraspecific density treatments. Cottonwood seedling biomass and height were twice that of saltcedar seedlings across all density treatments. As density of cottonwood increased, intraspecific competition increased in severity and biomass of cottonwood seedlings decreased. At 4 plants/0.25 m², cottonwood seedlings had the greatest biomass; whereas, survival was highest at 10 plants/0.25 m². Our results support greenhouse studies and suggest that if favorable germination conditions are established for cottonwood in floodplains, saltcedar seedlings that cogerminate could be outcompeted by native cottonwood seedlings.     

Nest desertion by a cowbird host: an antiparasite behavior or a response to egg loss?

Natural selection can favor songbirds that desert nests containingeggs of the parasitic brown-headed cowbird (Molothrus ater).However, the high variability in desertion of parasitized nestswithin species is perplexing in light of the typically highcosts of parasitism. Because nest desertion can also be a responseto partial clutch predation, we first asked if Bell's vireos(Vireo bellii) deserted nests in response to the presence ofcowbird eggs (antiparasite response hypothesis) or to egg removalby predators and female cowbirds (egg predation hypothesis).Second, we asked whether variation in nest desertion was dueto intrinsic differences among individuals or to variation innest contents. We monitored a large number of nests (n = 494)and performed a clutch manipulation experiment to test thesehypotheses. The number of vireo eggs that remained in a nestwas a strong predictor of desertion both within and among pairs.Neither the presence of a single cowbird egg, which leads tonest failure for this host, nor the number of cowbird eggs receivedin a vireo nest influenced nest desertion. Furthermore, vireosdid not desert experimental nests when we immediately exchangedcowbird eggs for vireo eggs but deserted if we removed vireoeggs and replaced them with cowbird eggs the following morning.Desertion of parasitized nests by Bell's vireos can be almostentirely explained as a response to partial or complete clutchloss and does not appear to have been altered by selection frombrood parasitism.     

Avian Response to Bottomland Hardwood Reforestation: The First 10 Years

Bottomland hardwood forests were planted on agricultural fields in Mississippi and Louisiana predominantly using either Quercus species (oaks) or Populus deltoides (eastern cottonwood). We assessed avian colonization of these reforested sites between 2 and 10 years after planting. Rapid vertical growth of cottonwoods (circa 2–3 m/year) resulted in sites with forest structure that supported greater species richness of breeding birds, increased Shannon diversity indices, and supported greater territory densities than on sites planted with slower‐growing oak species. Grassland birds (Spiza americana[Dickcissel] and Sturnella magna[Eastern Meadowlark]) were indicative of species breeding on oak‐dominated reforestation no more than 10 years old. Agelaius phoeniceus (Red‐winged Blackbird) and Colinus virginianus (Northern Bobwhite) characterized cottonwood reforestation no more than 4 years old, whereas 14 species of shrub‐scrub birds (e.g., Passerina cyanea[Indigo Bunting]) and early‐successional forest birds (e.g., Vireo gilvus[Warbling vireo]) typified cottonwood reforestation 5 to 9 years after planting. Rates of daily nest survival did not differ between reforestation strategies. Nest parasitism increased markedly in older cottonwood stands but was overwhelmed by predation as a cause of nest failure. Based on Partners in Flight prioritization scores and territory densities, the value of cottonwood reforestation for avian conservation was significantly greater than that of oak reforestation during their first 10 years. Because of benefits conferred on breeding birds, we recommend reforestation of bottomland hardwoods should include a high proportion of fast‐growing early successional species such as cottonwood.     

Effects of Mesopredators on Nest Survival of Shrub-Nesting Songbirds

ABSTRACT Although nest predation is often the single largest source of mortality in avian populations, manipulative studies to determine predator impacts on nest survival are rare, particularly studies that examine impacts of mid-size mammalian predators (hereafter, mesopredators) on nest survival of shrub-nesting birds. We quantified nest survival and identified nest predators of shrub-nesting songbirds within 4 large (approx. 40-ha) exclosures and 4 control sites within a longleaf pine (Pinus palustris) ecosystem. During 2003–2006, we located and monitored 535 shrub nests (222 with videography) for 4,804 nest-days to quantify daily nest survival and document predation events. We found no support for a treatment effect, suggesting mesopredators had little impact on daily nest survival (0.9303 in controls and 0.9260 in exclosures) of shrub-nesting songbirds. For the 5 most commonly monitored species, daily nest survival within species was constant. Our analysis suggested that shrub nests were most vulnerable during the nestling stage and presence of cameras on nests increased survival with the increase in survival being more pronounced during the incubation stage. We filmed 107 nest predation events, identifying predators at 88 nests. Of these 88 nests, snakes caused 33%, red imported fire ants (hereafter fire ants, Solenopsis invicta) 28%, raptors 17%, corvids 8%, mesopredators 6%, and small mammals 8% of nest predations. Cause-specific nest predation in controls and exclosures did not differ from expectation, providing evidence that compensatory predation did not occur. Nest predators differed from expectation with regard to nest stage; fire ants and raptors only depredated nests during the nestling stage. Presence of cameras had no effect on nest abandonment. Fire ants were the most prevalent nest predator, and nest predation by fire ants was only observed on nestlings, potentially reducing likelihood of renesting. Magnitude and timing of fire ant predation suggests that fire ants may be the most influential nest predator of shrub-nesting birds within the longleaf pine ecosystem. Our data suggest that controlling mesopredators will have no effect on nest success of shrub-nesting birds within longleaf pine forests.     

Avian Nest Box Selection and Nest Success in Burned and Unburned Southwestern Riparian Forest

ABSTRACT Riparian forest communities in the southwestern United States were historically structured by a disturbance regime of annual flooding. In recent decades, however, frequency of flooding has decreased and frequency of wildfires has increased. Riparian forests provide important breeding habitat for a large variety of bird species, and the effects of this altered disturbance regime on birds and their breeding habitat is largely unknown. To evaluate effects of high-intensity spring and summer wildfire on the quality of breeding bird habitat in the Middle Rio Grande valley, we measured vegetation structure and composition, avian nest use, and nest success at 4 unburned plots and 4 wildfire plots over a 3-year period. We measured avian nest use and success at nest boxes located in unburned riparian forest plots and plots recently burned by wildfire. Recent wildfire plots (<7 yr after fire) had a much different vegetation structure than unburned plots; an older (>7 yr after fire) wildfire plot more closely resembled its paired unburned plot than did recently burned plots. Ash-throated flycatchers (Myiarchus cinerascens) and Bewick's wrens (Thryomanes bewickii; hereafter, flycatchers and wrens, respectively) used nest boxes in most of the plots. A model selection procedure applied to logistic regressions showed that frequency of nest box use by flycatchers was positively associated with wildfire, although flycatchers used boxes in unburned plots as well. Wrens showed a preferential use of nest boxes that were in unburned sites and in close proximity to vegetative cover. Growth rates, feeding rates, and fledging mass of flycatchers were similar in wildfire and unburned plots. Growth rates for wrens were slower in wildfire plots, while feeding rates and fledging mass were similar. Nest predation varied between years, was higher for flycatchers than for wrens, and was not directly influenced by wildfire. Model selection showed that predation increased with grass cover, an indicator of forest openness, and decreased with distance to habitat edge. Recovery of dense vegetation appears important in maintaining populations of Bewick's wrens, whereas ash-throated flycatchers were less sensitive to vegetative structure and composition of postfire succession. Postfire management that maintains nest sites in large forest strips would enhance nesting density and success of these cavity-nesting birds in riparian zones.     

Plant species richness in ephemeral and perennial reaches of a dryland river

Ephemeral reaches are common along desert rivers but are less well studied than those with perennial stream flow. This study contrasted riparian plant species richness and composition (extant vegetation and soil seed bank) between stream reaches with different low-flow conditions (perennial vs. ephemeral flow) but similar flood patterns and similar watershed-derived species pools. Data were collected at Cienega Creek (Arizona, USA) over a 2 year period spanning drought conditions and wetter conditions. Consistent with expectations relating to water limitation effects on diversity, species richness in the riparian zone was lower at ephemeral-flow sites during a season with minimal precipitation and no overbank flooding; under these conditions, the more permanent water sources of the perennial-flow sites sustain the larger number of species. During seasons with greater precipitation and elevated stream flows, in contrast, species richness at ephemeral-flow sites increased to levels at or slightly above those of perennial-flow sites. For values pooled across two wet seasons of a calendar year, year-round richness was greater at the two ephemeral-flow sites (total of 92 vascular plant species) than at the two perennial-flow sites (68 species). This greater year-round richness was a combination of multiple factors: greater light, space, and bare ground, a diverse soil seed bank (with the seed banks equally species-rich among hydrologic types), and moderately abundant precipitation and flooding sufficient to stimulate establishment of opportunistic species (mainly annuals) during the bimodal wet seasons. These results indicate that long-term patterns of site water availability, by influencing woody plant cover, mediate the diversity response to episodic water pulses in dryland rivers. The results also have implications for riparian conservation efforts, which to date have focused primarily on perennial stream reaches: ephemeral reaches of spatially intermittent rivers harbor many riparian plant species, and warrant conservation efforts, as well.     

Genetic and environmental influences on leaf phenology and cold hardiness of native and introduced riparian trees

To explore the roles of plasticity and genetic variation in the response to spatial and temporal climate variation, we established a common garden consisting of paired collections of native and introduced riparian trees sampled along a latitudinal gradient. The garden in Fort Collins, Colorado (latitude 40.6°N), included 681 native plains cottonwood (Populus deltoides subsp. monilifera) and introduced saltcedar (Tamarix ramosissima, T. chinensis and hybrids) collected from 15 sites at 29.2–47.6°N in the central United States. In the common garden both species showed latitudinal variation in fall, but not spring, leaf phenology, suggesting that the latitudinal gradient in fall phenology observed in the field results at least in part from inherited variation in the critical photoperiod, while the latitudinal gradient in spring phenology observed in the field is largely a plastic response to the temperature gradient. Populations from higher latitudes exhibited earlier bud set and leaf senescence. Cold hardiness varied latitudinally in both fall and spring for both species. For cottonwood, cold hardiness began earlier and ended later in northern than in southern populations. For saltcedar northern populations were hardier throughout the cold season than southern populations. Although cottonwood was hardier than saltcedar in midwinter, the reverse was true in late fall and early spring. The latitudinal variation in fall phenology and cold hardiness of saltcedar appears to have developed as a result of multiple introductions of genetically distinct populations, hybridization and natural selection in the 150 years since introduction.     

Tamarix as Habitat for Birds: Implications for Riparian Restoration in the Southwestern United States

Exotic vegetation has become a major habitat component in many ecosystems around the world, sometimes dramatically changing the vegetation community structure and composition. In the southwestern United States, riparian ecosystems are undergoing major changes in part due to the establishment and spread of the exotic Tamarix (saltcedar, tamarisk). There are concerns about the suitability of Tamarix as habitat for birds. Although Tamarix habitats tend to support fewer species and individuals than native habitats, Arizona Breeding Bird Atlas data and Birds of North America accounts show that 49 species use Tamarix as breeding habitat. Importantly, the relative use of Tamarix and its quality as habitat vary substantially by geographic location and bird species. Few studies have examined how breeding in Tamarix actually affects bird survivorship and productivity; recent research on Southwestern Willow Flycatchers has found no negative effects from breeding in Tamarix habitats. Therefore, the ecological benefits and costs of Tamarix control are difficult to predict and are likely to be species specific and site specific. Given the likelihood that high‐quality native riparian vegetation will not develop at all Tamarix control sites, restoration projects that remove Tamarix but do not assure replacement by high‐quality native habitat have the potential to reduce the net riparian habitat value for some local or regional bird populations. Therefore, an assessment of potential negative impacts is important in deciding if exotic control should be conducted. In addition, measurable project objectives, appropriate control and restoration techniques, and robust monitoring are all critical to effective restoration planning and execution.     

Use of Restored Riparian Habitat by the Endangered Least Bell's Vireo (Vireo bellii pusillus)

A primary objective of riparian restoration in California is the creation of habitat for endangered species. Four restoration sites in San Diego County were monitored between 1989 and 1993 and evaluated for their suitability as nesting habitat for Vireo bellii pusillus (Least Bell's Vireo), a state and federally endangered obligate riparian breeder. Vegetation structure at each site was quantified annually and compared to a model of canopy architecture derived from Least Bell's Vireo territories in natural habitat. Vireo use of restored habitat was documented through systematic surveys and nest monitoring. By 1993, only one site in its entirety met the habitat suitability criteria of the model, but portions of each site during all years did so. Differences between sites in the time required to develop suitable habitat—well-developed layered vegetation from the ground to under 8m in height)—were attributable largely to variation in annual rainfall. Vireos visited restoration sites to forage as early as the first growing season, but they did not establish territories or nest there until at least part of the site supported suitable habitat as determined from the model. Placement of territories and nests coincided with patches of dense vegetation characteristic of natural nesting areas. Occupation of restored sites was accelerated by the presence of adjacent mature riparian habitat, which afforded birds nest sites and/or foraging habitat lacking in the planted vegetation. Vireos nesting in restored habitat achieved success comparable to that of vireos nesting in surrounding natural habitat, and there was no evidence that productivity was reduced in created areas. These findings indicate that creating nesting habitat for this target species is feasible and suggest that the critical components of vireo nesting habitat have been captured in both the design and quantitative assessment of restoration sites.     

C : N : P stoichiometry of dominant riparian trees and arthropods along the Middle Rio Grande

1. We examined the role of flooding on the leaf nutrient content of riparian trees by comparing the carbon : nitrogen : phosphorus (C : N : P) ratio of leaves and litter of Rio Grande cottonwood (Populus deltoides ssp. wislizenii) in flood and non‐flood sites along the Middle Rio Grande, NM, U.S.A. The leaf C : N : P ratio was also examined for two non‐native trees, saltcedar (Tamarix chinensis) and Russian olive (Elaeagnus angustifolia), and six species of dominant riparian arthropods. 2. Living leaves and leaf litter of cottonwoods at flood sites had a significantly lower leaf N : P ratio and higher %P compared with leaves and litter at non‐flood sites. A non‐flood site downstream from wastewater effluent had a significantly lower litter C : N ratio than all other sites, suggesting N fertilisation through ground water. The non‐native trees, saltcedar and Russian olive, had higher mean leaf N content, N : P ratio, and lower C : N ratio compared with cottonwoods across study sites. 3. Riparian arthropods ranged from 5.2 to 7.1 for C : N ratio, 56–216 for C : P ratio, and 8.9–34 for N : P ratio. C content ranged from 25 to 52% of dry mass, N content from 4.7 to 10.8%, and P content from 0.59 to 1.2%. Differences in stoichiometry between high C : nutrient leaf litter and low C : nutrient invertebrates suggests possible food‐quality constraints for detritivores. 4. These results suggest that spatial and temporal variation in the C : N : P ratio of cottonwood leaves and leaf litter is influenced by surface and subsurface hydrologic connection within the floodplain. Reach‐scale variation in the elemental composition of riparian organic matter inputs may have important implications for decomposition, nutrient cycling, and food webs in river floodplain systems.     

Parameters of brown-headed cowbird Molothrus ater egg discrimination in warbling vireos Vireo gilvus

We examined which egg parameters warbling vireos Vireo gilvus use to discriminate brown-headed cowbird Molothrus ater eggs and, by comparing our results to other studies, tested the prediction that ejecter species with eggs more similar in appearance to cowbird eggs will be less tolerant of foreign eggs. In addition, we tested the hypothesis that egg characteristics influence the cost of ejection and probability of committing ejection errors. Warbling vireos ejected 100% of eggs with a cowbird spot pattern and only spot pattern significantly influenced the probability of ejecting a foreign egg, whereas size and nest stage did not. Foreign eggs that differed in two parameters were not ejected significantly more than those that differed in one parameter. Thus, warbling vireos appear to be less tolerant of foreign eggs than species with eggs more divergent from cowbird eggs. There was no significant difference in the number of vireo eggs that were damaged when foreign eggs of different sizes and spot patterns were ejected, which is counter to the assumptions of the evolutionary equilibrium hypothesis. Similarly, foreign egg characteristics did not significantly influence the probability of ejection errors. Finally, egg discrimination in warbling vireos appears to have evolved directly to counter cowbird parasitism because all conspecific eggs switched into their nests were accepted.     

How selfish is a cowbird nestling?

Brood-parasitic young are reared in the nests of different species and can derive no genetic benefit from the survival of host offspring. However, although the nestlings of many parasitic cuckoo and honeyguide species routinely kill host young soon after hatching, nestling brown-headed cowbirds, Molothrus ater, tolerate host offspring and are commonly reared alongside them for at least part of the nestling period. I used comparative analyses of data from the literature to investigate whether brown-headed cowbird nestlings gain direct benefits by allowing host young to live. The brown-headed cowbird (44 g) parasitizes many passerines (adult mass range about 5-90 g) and the likelihood that host young survive to fledge from parasitized nests varies between species. In common with previous work, I found that host offspring mortality was highest in species whose offspring were relatively small compared with the cowbird nestling. Furthermore, cowbird nestlings were most likely to fledge when reared alongside host young of intermediate size. In these nests, one or two host young typically fledged as well. I suggest that cowbirds, and other host-tolerant brood parasites, could benefit from the presence of host nestlings through the assistance that host chicks offer in soliciting a higher provisioning rate, and that such benefits might outweigh the costs of having competition for food at each nest visit. Variation in this cost-benefit ratio could explain differences between brood parasite species in their tolerance of host young.     

Management for oak regeneration: Short-term effects on the bird community and suitability of shelterwood harvests for canopy songbirds

Interest in regenerating oaks (Quercus spp.) has promoted use of partial harvesting techniques that create an open forest structure. From 2007 to 2009, we studied songbirds in mixed-oak forests in southeastern Ohio, comparing shelterwoods recently harvested to 50% stocking and closed-canopy mature second-growth. We surveyed birds using distance-based methods (56 line transects in 18 stands at 4 forests). We intensively investigated suitability of shelterwoods for canopy-nesting species by examining habitat preferences, as measured by settlement patterns, age distributions, and site fidelity; we also examined nesting success. Several midstory and ground-nesting species were 26–73% less abundant in shelterwood than unharvested stands, whereas shrub-nesting species increased >100% several years post-harvesting. Canopy-nesting species were 31–98% more abundant in shelterwoods, but cerulean warbler (Setophaga cerulea) responses varied by forest. Patterns of settlement and site fidelity were generally similar among stands. Proportions of young males were actually greater for several species in shelterwood than unharvested stands, which may have been a consequence of young birds colonizing newly created (or improved) habitat. Even in our predominantly forested study system, nesting success (>700 nests) was low, ranging from 15% to 19% for yellow-throated vireos (Vireo flavifrons) and cerulean warblers, to 27–36% for scarlet tanagers, blue-gray gnatcatchers (Polioptila caerulea) and eastern wood-pewees (Contopus virens). However, nest survival did not differ between shelterwood and unharvested stands, possibly because numbers of avian predators did not change with harvesting. Despite increased numbers of brown-headed cowbirds (Molothrus ater) in shelterwoods, only 2% of canopy nests in which young could be identified were parasitized. Although these results suggest shelterwood harvests containing abundant overstory trees can provide short-term breeding habitat for canopy songbirds, long-term responses of birds to partial harvesting may differ from those documented here depending on different management options employed. Management for oak regeneration will typically remove all overstory trees later in the cutting cycle, initially resulting in loss of nesting substrates and hence breeding habitat for canopy songbirds. © 2011 The Wildlife Society.     

UV reflectance of eggs of brown-headed cowbirds (Molothrus ater) and accepter and rejecter hosts

Many hosts of obligate brood parasites accept parasitic eggs despite the high costs of parasitism. Acceptance is particularly perplexing in brown-headed cowbird (Molothrus ater) (hereafter “cowbird”) hosts because the eggs of cowbirds and most hosts do not appear to match closely in visual characteristics detectable by humans. However, recent evidence suggests that parasite and host eggs may match in their ultraviolet (UV) reflectance, undetectable by humans, and that birds may use UV signals for egg discrimination. We determined whether egg colour matching in UV reflectance separates accepters and rejecters of cowbird parasitism by comparing the total UV (300–400 nm) reflectance of the eggs of 11 host species to cowbird eggs. Eggs of three of five accepter species and five of five rejecter species differed significantly from cowbird eggs in UV reflectance. We found no significant difference in the UV reflectance of the eggs of three closely related pairs of accepter and rejecter species. There also was no significant difference in the UV reflectance of cowbird eggs laid in nests of five host species, and the UV reflectance of cowbird eggs was not significantly correlated with that of host clutches. Thus, we found no support for the UV-matching hypothesis in brown-headed cowbirds and UV reflectance does not appear to separate accepters and rejecters of parasitism. Differences in UV reflectance between cowbird and host eggs, however, provide potential cues for use in egg discrimination. Experimental testing is needed to determine the relative importance of UV reflectance compared to other visual cues.     

Black-capped vireo nest predator assemblage and predictors for nest predation

Nest predation is a major limiting factor for songbird productivity, including the federally endangered black-capped vireo (Vireo atricapilla). However, nest predator information is limited across the range of the black-capped vireo in central and southwest Texas. We monitored nests in 3 counties within the breeding range of black-capped vireos in Texas in 2008 and 2009 and used continuous recording digital video cameras to record predation events. We video-monitored 115 nests and documented 39 predation events by at least 9 predator species. Overall, we observed avian species (51%, n = 39), specifically brown-headed cowbirds (Molothrus ater; n = 12), and snakes (26%, n = 39) as the most frequent nest predators. The estimated daily nest survival rate during the laying and incubation stage was 0.985 (95% CI = 0.967–0.993) and 0.944 (95% CI = 0.921–0.961) during the nestling stage. In addition, we analyzed models of predator-specific nest predation using multinomial logistic regression. Effect of nest height on predation rate was significant for snakes; nest stage was significant for nests depredated by avian predators. By identifying and increasing our knowledge of nest predators and vegetation characteristics associated with greater risk of predation in multiple locations within the black-capped vireo's range, we can effectively manage habitat to benefit recovery efforts of the species. © 2012 The Wildlife Society.     

Brood parasitism of Black‐capped Vireos: frontline and post‐laying behavioral responses and effects on productivity

The cost of brood parasitism favors the evolution of host behaviors that reduce the risk or expense of being parasitized. Endangered Black‐capped Vireos (Vireo atricapilla) have likely coexisted with brood‐parasitic Brown‐headed Cowbirds (Molothrus ater) for more than 10,000 yr, so it is likely that they have evolved anti‐parasitic behaviors. We monitored naturally parasitized and non‐parasitized vireo nests to evaluate factors that might explain parasitism risk and nest desertion behavior and also assessed whether behaviors that occurred after being parasitized improved reproductive output. Vireos reduced the risk of parasitism by initiating breeding early and nesting farther from open grasslands and edges of woody thickets. Post‐laying, nest desertion was common (70% of parasitized nests) and increased with both the presence of at least one cowbird egg in nests and clutch reduction by cowbirds. After accounting for these cues, desertion was also more likely at nests located closer to cowbird foraging habitat and below potential cowbird vantage points. Despite its regularity, desertion did not appear to provide reproductive benefits to vireos. Instead, accepting cowbird eggs was a more effective strategy because 42% of cowbird eggs did not hatch. Furthermore, cowbird eggs were somehow ejected from at least three vireo nests. Our results suggest that Black‐capped Vireos can behave in a variety of ways that reduce the impact of brood parasitism, with frontline behaviors appearing to provide the greatest benefit. Our results also suggest that habitat management should focus on providing Black‐capped Vireos with adequate breeding habitat that provides access to safe nesting sites, and with high‐quality wintering habitat that allows vireos to migrate and initiate nesting early.     

Temporal patterns of host availability, brown-headed cowbird brood parasitism, and parasite egg mass

I studied relationships between temporal patterns of host availability, brood parasitism, and egg mass for the parasitic brown-headed cowbird (Molothrus ater). At a study site consisting largely of edge habitat in north-eastern Illinois, I found 834 bird nests from 27 species. A total of 407 cowbird eggs and nestlings were found in these nests over three laying seasons. Nearly all (n= 379; 93%) were found in the nests of seven host species. For these species and all taken together, weekly nest availability generally decreased whereas parasitism frequency generally increased throughout the cowbird laying season, but the proportions of nests parasitized and the mean number of cowbird eggs in them did not. Additionally, no correlation was found between the proportions of nests parasitized and nest availability. Cowbird egg mass generally increased throughout the laying season, indicating that foraging conditions improved and that, early in the laying season, egg mass and quality may be less important than quantity. Consistently high weekly levels of parasitism indicate that cowbird reproduction was less limited by resources needed for egg production and more by the availability of suitable host nests. Fluctuating weekly host availabilities suggest that previously established, constant rates of cowbird egg laying would produce an excess of eggs during periods of low host availability. Further, the low frequency of parasitism (1%) of nests in stages too advanced for successful parasitism, and of abandoned nests, is consistent with the hypothesis that cowbirds' consistently high rate of egg production helps assure an egg is available when an appropriate nest is found. Frequently, nests were parasitized multiple times, raising the possibility that cowbirds were interfering with their own reproduction. A diverse host community increases the possibility that a decline of any one host species is unlikely to affect cowbird reproduction significantly. Received 11 July 1997 / Accepted: 31 March 1998     

Assessing the extent and diversity of riparian ecosystems in Sonora, Mexico

Conservation of forested riparian ecosystems is of international concern. Relatively little is known of the structure, composition, diversity, and extent of riparian ecosystems in Mexico. We used high- and low-resolution satellite imagery from 2000 to 2006, and ground-based sampling in 2006, to assess the spatial pattern, extent, and woody plant composition of riparian forests across a range of spatial scales for the state of Sonora, Mexico. For all 3rd and higher order streams, river bottomlands with riparian forests occupied a total area of 2,301 km². Where forested bottomlands remained, on average, 34% of the area had been converted to agriculture while 39% remained forested. We estimated that the total area of riparian forest along the principal streams was 897 km². Including fencerow trees, the total forested riparian area was 944 km², or 0.5% of the total land area of Sonora. Ground-based sampling of woody riparian vegetation consisted of 92, 50 m radius circular plots. About 79 woody plant species were noted. The most important tree species, based on cover and frequency, were willow species Salix spp. (primarily S. goodingii and S. bonplandiana), mesquite species Prosopis spp. (primarily P. velutina), and Fremont cottonwood Populus fremontii. Woody riparian taxa at the reach scale showed a trend of increasing diversity from north to south within Sonora. Species richness was greatest in the willow-bald cypress Taxodium distichum var. mexicanum—Mexican cottonwood P. mexicana subsp. dimorphia ecosystem. The non-native tamarisk Tamarix spp. was rare, occurring at just three study reaches. Relatively natural stream flow patterns and fluvial disturbance regimes likely limit its establishment and spread.     

Western Yellow-Billed Cuckoo Nest-Site Selection and Success in Restored and Natural Riparian Forests

The western distinct population segment of yellow-billed cuckoo (Coccyzus americanus; western cuckoo) has been extirpated from most of its former breeding range in the United States because of widespread loss and degradation of riparian cottonwood (Populus spp.)-willow (Salix spp.) forests. Restoration and management of breeding habitat is important to the recovery of this federally threatened species, and identification of high-quality breeding habitat can help improve the success of recovery. In 2005, the Lower Colorado River Multi-Species Conservation Program, a long-term, multi-agency effort, was initiated to maintain and create wildlife habitat within the historical floodplain of the lower Colorado River (LCR) for federally endangered and threatened species, including western cuckoos. We conducted an empirical, multi-scale field investigation from 2008–2012 to identify habitat characteristics selected by nesting western cuckoos along the LCR. Multiple logistic regression models revealed that western cuckoos selected nest sites characterized by increased densities of small, native, early successional trees measuring 8–23 cm diameter at breast height, and lower diurnal temperature compared to available habitat in restoration and natural forests. Nesting cuckoos selected sites with increased percent canopy closure, which was also important for nest success in restoration sites along the LCR. Our results show habitat components selected by nesting western cuckoos in restoration and natural riparian forests and can help guide the creation, enhancement, and management of riparian forests with habitat conditions necessary to promote nesting of western cuckoos. © 2021 The Wildlife Society.