Empidonax Virescens Research Articles

EFFECTS OF GROUP-SELECTION OPENING SIZE ON BREEDING BIRD HABITAT USE IN A BOTTOMLAND FOREST

An increase in timber removals from southern bottomland forests of the United States has been predicted, warranting investigations of the effects of silvicultural alternatives on avian breeding habitat. We studied the effects of creating group-selection openings (man-made canopy gaps) of various sizes on breeding bird habitat use in a bottomland hardwood forest in the Upper Coastal Plain of South Carolina, USA. We used spot mapping and mist netting to estimate bird abundance at 0.06-, 0.13-, 0.26-, and 0.5-ha gaps and at uncut control areas during the 1996, 1997, and 1998 breeding seasons (1 May–1 August). There were significant increases in the number of species mapped (P = 0.0001) and netted (P = 0.0001) with successive increases in gap size. The greatest number of total spot-map detections (P = 0.0002) and mist net captures (P = 0.0004) also occurred in and around the large gaps. These patterns were the result of increased use of larger gaps by field-edge species, primarily Brown-headed Cowbird (Molothrus ater), Common Yellowthroat (Geothlypis trichas), and Indigo Bunting (Passerina cyanea), and some forest-edge species, such as White-eyed Vireo (Vireo griseus) and Northern Parula (Parula americana). Conversely, Acadian Flycatcher (Empidonax virescens) was less abundant in and adjacent to gaps. Because there were few differences in vegetation among gaps of different sizes, it is likely that birds that were detected more frequently in and adjacent to larger gaps selected those gaps based on other factors correlated with size. Creation of 0.5-ha group-selection openings in southern bottomland forests should provide breeding habitat for some field-edge species in gaps and habitat for forest-interior species and canopy-dwelling forest-edge species between gaps, provided that sufficient mature forest is maintained.

Songbird Abundance and Avian Nest Survival Rates in Forests Fragmented by Different Silvicultural Treatments

Abstract:Concerns over declining songbird populations have led to investigations of the effects of various silvicultural practices on breeding songbirds. Few studies published, however, have examined both songbird populations and avian nest success among harvesting treatments, particularly in forested landscapes. We conducted a study in the Monongahela National Forest of West Virginia during the summers of 1993 to 1996 to compare breeding‐bird abundance and daily nest survival rates among different sivicultural treatments: a two‐age treatment (a type of deferred removal in which 37–49 mature trees/ha remain after a harvest until the next rotation), clearcutting treatments 15 years after harvest, unharvested forest surrounding the harvested stands, and unharvested stands not adjacent to cuts. Abundance and daily nest‐survival rates did not differ among treatments ( p> 0.05) for four of the five species for which the most nests were found: Wood Thrush ( Hylocichla mustelina ), Rose‐breasted Grosbeak ( Pheucticus ludovicianus ), Acadian Flycatcher ( Empidonax virescens), Veery ( Catharus fuscescens ), Red‐eyed Vireo ( Vireo olivaceus ). Parasitism rates were low (6%), and most parasitized nests were associated with the two‐age harvest treatment. A source‐sink model for the Wood Thrush revealed that all treatments were likely population sources for this species. Thus, it appears that 15 years after harvest, cuts placed within otherwise extensively forested areas do not result in the type of edge effects (population sinks) observed in areas fragmented by agriculture in the midwestern United States. Further, because neither nest success nor avian abundance was lower in the two‐age than clearcut harvests, we conclude that two‐age management is a viable conservation alternative to clearcutting in large forested landscapes where Brown‐headed Cowbird ( Molothrus ater) parasitism is not a concern.

Nest Survival of Forest Birds in the Mississippi Alluvial Valley

In the Mississippi Alluvial Valley, flood control has led to a drastic reduction in the area of forest habitat and altered the patchwork of forest cover types. Silvicultural management of the remaining fragmented forests has changed to reflect the altered hydrology of the forests, current economic conditions of the area, and demand for forest products. Because forest type and silvicultural management impact forest birds, differences in avian productivity within these forests directly impact bird conservation. To assist in conservation planning, we evaluated daily nest survival, nest predation rates, and brood parasitism rates of forest birds in relation to different forest cover types and silvicultural management strategies within this floodplain. Within bottomland hardwood forests, nest success of blue-gray gnatcatcher (Polioptila caerulea, 13%), eastern towhee (Pipilo erythrophthalmus, 28%), indigo bunting (Passerina cyanea, 18%), northern cardinal (Cardinalis cardinalis, 22%), and yellow-billed cuckoo (Coccyzus americanus, 18%) did not differ from that within intensively managed cottonwood plantations. However, average daily survival of 542 open-cup nests of 19 bird species in bottomland hardwoods (0.9516 ± 0.0028, ∼27% nest success) was greater than that of 543 nests of 18 species in cottonwood plantations (0.9298 ±0.0035, ∼15% nest success). Differences in daily nest survival rates likely resulted from a combination of differences in the predator community-particularly fire ants (Solenopsis invicta)-and a marked difference in species composition of birds breeding within these 2 forest types. At least 39% of nests in bottomland hardwood forests and 65% of nests in cottonwood plantations were depredated. Rates of parasitism by brown-headed cowbirds (Molothrus ater) were greater in managed cottonwoods (24%) than in bottomland hardwoods (9%). Nest success in planted cottonwood plantations for 18 species combined (∼14%), and for yellow-breasted chat (Icteria virens, 7%), eastern towhee (14%), indigo bunting (14%), and northern cardinal (17%) did not differ from nest success in cottonwood plantations that were coppiced from root sprouts following pulpwood harvest. Within bottomland hardwood forests, uneven-aged group-selection timber harvest reduced the combined daily nest survival of all species from 0.958 to 0.938, which reduced nest success by about 14%. Specifically, timber harvest reduced nest success of species that nest in the forest midstory and canopy, such as Acadian flycatcher (Empidonax virescens)-from 32% before harvest to 14% after harvest. Conversely, those species that nest primarily in the shrubby understory-such as northern cardinal-were not affected by timber harvest and maintained an overall nest success of about 33%. Thus, birds nesting in the understory of bottomland hardwood forests are not adversely impacted by selective timber harvest, but there is a short-term reduction in nest success for birds that nest in the canopy and midstory.

BIRD NESTING ECOLOGY IN A FOREST DEFOLIATED BY GYPSY MOTHS

Acadian Flycatcher (Empidonax virescens, n = 55), Indigo Bunting (Passerina cyanea, n = 60), Eastern Towhee (Pipilo erythrophthalmus, n = 41), and Wood Thrush (Hylocichla mustelina, n = 62) nests were monitored during 1995–1996 in the eastern panhandle of West Virginia, at the Sleepy Creek Wildlife Management Area. The objective of this study was to relate the outcomes of bird nests to surrounding habitat characteristics in an area that experienced heavy tree mortality from prior defoliation by the gypsy moth (Lymantria dispar). Large (> 22.9 cm dbh) standing snags in the nest patch were not associated with nest failure for any of the four bird species. Very large diameter (> 38.1 cm dbh) live trees and snags and reduced canopy cover increased the chances of Brown-headed Cowbird (Molothrus ater) parasitism only for Indigo Buntings. Nest patches of all four species differed in vegetation characteristics from random plots in similar habitat, typically by having greater densities of species' preferred nesting substrate in the nest patch. Gypsy moth defoliation, which can result in an increase in snags and opened canopy, is not likely to be a devastating ecological event for shrub and sub-canopy nesting avian species, and can create more nesting habitat for many species that use a dense forest understory.

Breeding Bird Abundance in Bottomland Hardwood Forests: Habitat, Edge, and Patch Size Effects

AbstractWe studied breeding bird communities in extensive bottomland hardwood forests along the lower Roanoke River in North Carolina during 1992 and 1993. We documented a rich avian community and recorded exceptionally high densities of two species (Prothonotary Warbler Protonotaria citrea, Acadian Flycatcher Empidonax virescens), as well as modest densities of three species rarely encountered elsewhere in the region (Cerulean Warbler Dendroica cerulea, Swainson's Warbler Limnothlypis swainsonii, American Redstart Setophaga ruticilla). The effects of patch size and edge on bird abundance were small in this forested landscape, but forest type had a large effect. We found half of the species analyzed to differ in abundance between the two primary habitat types, swamp forest and levee forest. In contrast, no species was consistently more abundant at patch interiors than near edges, and only two forest birds were more common in large compared with small patches. Species analyzed included permanent residents, short-distance migrants, Neotropical migrants, and those identified as forest-interior and area-sensitive species in other studies. Our results suggest that the Roanoke River bottomland forests may be functioning effectively as a reserve for a number of bird species.

Effectiveness of nest defence in the Acadian Flycatcher Empidonax virescens

We used presentations of models to determine the effectiveness of nest defence in the Acadian Flycatcher Empidonax virescens against a nest predator (Blue Jay Cyanocitta cristata) and a brood parasite (Brown‐headed Cowbird Molothrus ater). Principal components analysis (PCA) of four component variables of nest defence (call rate, swoop rate, closest approach and number of adults) generated a measure of overall nest defence (aggression). We determined effectiveness of defence by looking for correlations between measures of defence and measures of nest success (nest predation and brood parasitism). We also determined whether nest defence increased with clutch size, nestling age and time in the breeding season. Defence against model Brown‐headed Cowbirds did not correlate with levels of parasitism, clutch size, age of young or time of breeding. There was, however, a strong, but insignificant, trend for nests with high levels of all measures of defence to suffer less from brood parasitism. Aggression, vocalization rate, closest approach and number of adults defending against models of predatory Blue Jays correlated positively with nesting success during the egg stage but not the nestling stage of the nesting cycle. Aggression, vocalization rate, closest approach correlated with clutch size and age of the brood. These results suggest that nest defence can effectively deter nest predators, but may be less effective against brood parasites. Different behavioural components of nest defence may work at different stages of the nest cycle and against different nest predators. The components of nest defence that correlated with nest success also correlated with clutch value, a result consistent with hypotheses on the evolution of nest defence.

Acadian Flycatcher, Empidonax virescens, nest site characteristics at the northern edge of its range

Acadian Flycatcher, Empidonax virescens, nest site characteristics at the northern edge of its range

BREEDING BIRD ABUNDANCE IN BOTTOMLAND HARDWOOD FORESTS: HABITAT, EDGE, AND PATCH SIZE EFFECTS

Abstract We studied breeding bird communities in extensive bottomland hardwood forests along the lower Roanoke River in North Carolina during 1992 and 1993. We documented a rich avian community and recorded exceptionally high densities of two species (Prothonotary Warbler Protonotaria citrea, Acadian Flycatcher Empidonax virescens), as well as modest densities of three species rarely encountered elsewhere in the region (Cerulean Warbler Dendroica cerulea, Swainson's Warbler Limnothlypis swainsonii, American Redstart Setophaga ruticilla). The effects of patch size and edge on bird abundance were small in this forested landscape, but forest type had a large effect. We found half of the species analyzed to differ in abundance between the two primary habitat types, swamp forest and levee forest. In contrast, no species was consistently more abundant at patch interiors than near edges, and only two forest birds were more common in large compared with small patches. Species analyzed included permanent residents...

Does Nonrandom Nest Placement Imply Nonrandom Nest Predation?: A Reply

In response to the critique by Schmidt and Whelan (1999), we find that the relationship between nest success and tree selectivity is dependent upon inclusion or exclusion of particular tree species, whether or not years are pooled, and the selectivity index used. We question their use of point estimates of nest success with extremely high variances, defend our index, question the application of the Chesson (1983) index to our data, and explain the need to analyze years separately. Bottomland hardwood forest systems are extremely variable; hydroperiods alter the suitability of nesting substrates, availability of alternative food, and behavior of predators and their prey. Given these features, actively searching for Acadian Flycatcher (Empidonax virescens) nests is seldom an efficient predator foraging strategy. Therefore, these predation events are best described as random; nests are principally encountered opportunistically by generalist predators while searching for other prey.

Nest Placement and Mortality: Is Nest Predation a Random Event in Space and Time?

In their recent article, Wilson and Cooper (1998) reported that Acadian Flycatchers (Empidonax virescens) selected nest trees nonrandomly, but that their choice of nest tree did not influence nest success. They concluded that nest predation appears random in time and space. However, we show their metric for nest tree use (% use - % available) is a poor indicator of nest tree selection. We reanalyzed their data using an appropriate selectivity index (from Chesson 1983), and found that nest tree selectivity was greater for nest tree species with higher daily nest survival rates. In general, nest predation, while it may appear unpredictable, may often be related to nest site characteristics.

Short-Term Effects of Understory and Overstory Management on Breeding Birds in Arkansas Oak-Hickory Forests

Relatively little is known about the effects of uneven-aged forest management practices on eastern forest birds, despite the fact that such methods are now commonly practiced. In 1993-94, we studied the short-term effects of uneven-aged forest management on bird communities in oak-hickory forests of north-western Arkansas. We estimated bird abundance in mature forests and on managed plots receiving either a heavy cutting of understory vegetation (understory treatment) or a combination of both understory cutting and selective cutting in the forest overstory (full treatment). Two nesting guilds and 7 of 14 species with adequate sample size showed significant treatment effects. Ovenbirds (Seiurus aurocapillus), worm-eating warblers (Helmitheros vermivorus), Acadian flycatchers (Empidonax virescens), and the understory-nesting guild were most abundant in mature forest. Indigo buntings (Passerina cyanea), white-breasted nuthatches (Sitta carolinensis), and eastern wood-pewees (Contopus virens) were more abundant on full treatment plots. Tufted titmice (Baeolophus bicolor) were most abundant on mature forest and understory treatment plots. The canopy-nesting guild was most abundant on understory and full treatment plots. Our results suggest that if removal of understory vegetation was practiced widely in the Arkansas Ozarks as part of uneven-aged management, populations of some ground- and shrub-nesting forest interior species of birds could be negatively affected, whereas a few forest canopy and edge species may respond positively. Future research on this type of uneven-aged management should examine effects of removing varying amounts of understory vegetation on both forest interior bird populations and forest regeneration.

Acadian Flycatcher Nest Placement: Does Placement Influence Reproductive Success?

We located 511 Acadian Flycatcher (Empidonax virescens) nests in bottomland hardwood forest of eastern Arkansas. Microhabitat characteristics were measured and their relationship with nest success evaluated. Fifty-two percent of all nesting attempts resulted in predation. Attributes of nest placement were similar between successful and unsuccessful nests, although successful nests were placed higher. Similarly, nonparasitized nests were typically higher than parasitized nests. Nests initiated late in the breeding season were placed in larger trees with higher canopy bases resulting in increased vegetation around the nest. Fifteen different tree species were used for nesting. Acadian Flycatchers chose nest trees in a nonrandom fashion, selecting Nuttall oak (Quercus nuttallii) and possumhaw (Ilex decidua) in greater proportions than their availability. However, there was no relationship between tree species used for nesting and nest success. Nest height was positively correlated with concealment at the nest site, supporting the predator-avoidance theory. No other attribute of nest placement differentiated successful nest sites, suggesting that nest predation is likely a function of random events in space and time.

Effect of Stand Width and Adjacent Habitat on Breeding Bird Communities in Bottomland Hardwoods

Bottomland hardwood forests support an abundant and diverse avifauna, but area of this forest type has been reduced, and current projections indicate continued declines. We compared breeding bird abundance indices and species richness among bottomland hardwood stands ranging in width from 1,000 m and enclosed by forested habitat. We also compared avian abundance indices and richness among stands enclosed by pine (Pinus spp.) forest and stands enclosed by field-scrub habitats. Total species richness and species richness of Neotropical migrants were associated positively (P 1,000 m. Counts of Neotropical migrants differed (P < 0.05) among width classes in 1993 and 1995 and followed the same general trend as total bird count. Acadian flycatcher (Empidonax virescens), blue-gray gnatcatcher (Polioptila caerulea), and red-eyed vireo (Vireo olivaceous) were more abundant in smaller width classes (P < 0.05), whereas the opposite was true for white-eyed vireo (Vireo griseus) and northern parula (Parula americana). Probability of occurrence was associated positively (P < 0.05) with stand width for 12 species and negatively with stand width for 1 species. Total bird count and the counts of blue-gray gnatcatcher in 1995 and of northern cardinal (Cardinalis cardinalis) in both years were higher in field-enclosed stands (FES) than in pine-enclosed stands (PES). No species analyzed was more abundant in PES than in FES. We conclude that even narrow riparian zones can support an abundant and diverse avifauna, but that conservation of wide (≥500 m) riparian zones is necessary to maintain the complete avian community characteristic of bottomland hardwood forests in South Carolina.

Reexamination of Cowbird Parasitism and Edge Effects in Wisconsin Forests

We investigated the incidence of parasitism by brown-headed cowbirds (Molothrus ater) on the nests of Acadian flycatchers (Empidonax virescens) in relation to distance of hosts' nests to non-forest openings in the Kettle Moraine State Forest (KMSF) in southeastern Wisconsin during 1991-94. We also reexamined the results of this relation in these (and other) species of songbirds in an oft-cited paper by Brittingham and Temple (1983), who reported a significant increase of parasitism with the proximity of host nests to non-forest openings on another southern Wisconsin study area. Parasitism in the KMSF was calculated at 7-12% of 69 flycatcher nests; we found no correlation (P > 0.05) between parasitism and nests' nearness to non-forest openings. Extraction of Acadian flycatcher nests from the composite data for 13 host species of songbirds in Brittingham and Temple (1983) eliminated the significant relation between parasitism rates and non-forest proximity for the combined nests of the 12 remaining hosts (P = 0.18), and for the flycatcher evaluated separately (P = 0.16). The Wisconsin data reported and reexamined here do not support the proposition that the rate of cowbird parasitism on forest-interior birds diminishes with increasing distance from forest edges and openings over distances of 300 m for openings ≥ 0.2 ha in size.

Nongame Bird Use of Habitat in Central Appalachian Riparian Forests

Most studies that have shown higher bird species richness and abundance in riparian zones have been conducted where there is contrast between vegetation in riparian and upland habitats. In central Appalachian hardwood forests, substantial riparian habitat is potentially subject to habitat alterations. However, little information is available on the importance of riparian habitats to songbirds where the riparian-upland vegetation gradient is relatively uniform. We sampled relative bird abundance and habitat characteristics along a gradient from a stream to adjacent upland habitat at 16 sites in southwest Virginia. We evaluated total relative bird abundance, species richness, and relative abundance of 28 bird species to determine whether riparian habitats influenced bird assemblages. Total bird relative abundance and species richness showed no (P > 0.10) riparian influence; however, Acadian flycatchers (Empidonax virescens) and Louisiana waterthrushes (Seiurus motacilla) were most associated (P < 0.001) with streams. American robins (Turdus migratorius) showed a weaker but positive association (P = 0.068) with streams. Eastern woodpewees (Contopus virens) (P = 0.01), black-and-white warblers (Mniotilta varia) (P < 0.001), pine warblers (Dendroica pinus) (P < 0.001), worm-eating warblers (Helmintheros vermivorus) (P < 0.001), and scarlet tanagers (Piranga olivacea) (P = 0.041) were negatively associated with streams. We used cluster analysis to group the 28 bird species into 5 assemblages on the basis of their distributions at 69 sampling stations distributed among the 16 sites. Species were classified as belonging to riparian (n = 2 species), upland forest (n = 6 species), mesic forest (n = 3 species), xeric upland forest (n = 6 species), and mature hardwoods generalist (n = 11 species) assemblages. Management or protection of riparian zones often is justified on the basis of higher bird species richness and abundance found in these sites. Our data did not support this. However, Acadian flycatchers and Louisiana waterthrushes were dependent on riparian habitats, and such habitats should be protected for these species

Distribution of Summer Birds Along a Forest Moisture Gradient in an Ozark Watershed

Vegetational and physical habitat factors were measured in 110.0.05—ha circular plots around singing ♂ ♂ of eight bird species in a small watershed in northwestern Arkansan during June and July 1973. Sixty birdless plots were similarly sampled in the driest and most moist areas of the watershed to determine a forest moisture gradient. Principal component analysis of the combined avian community habitat data determined that the forest moisture gradient is important in the distribution of the birds, although multiple analysis of variance showed that no one factor can account for the significant distribution. Principal component analysis of the individual species determined the unique characteristics of the realized niches of each species. An ordination of the species using linear discriminant function analysis of the 60 forest samples was similar to the ordination based on the bird distribution, but suggests that other factors are determining the precise distribution of the birds. No species shows a great affinity for the drier forest, although Tufted Titmice seem able to utilize the moist and dry forests equally. The Hooded Warbler, Ovenbird, and Acadian Flycatcher have shorter niche widths than early arriving migrants and permanent residents and can be considered obligatory moist forest species. The results show that the moist forests is of great importance to the avian community and reasons for the lack of birds in the dry forest are discussed.

Molt and Age Determination in Western and Yellowish Flycatchers

TEN years ago I presented a review of research on molts in tyrannid flycatchers of the genus Empidonax, in a paper that included original data for five species (Johnson 1963b). Since that time very little has been published on this aspect of the biology of these flycatchers, although Mumford (1964: 46) and Traylor (1968) have added new details to data available for the Acadian Flycatcher (Empidonax virescens). In the present paper I describe molt patterns and criteria for determining age in two close relatives in the genus Empidonax, the Western Flycatcher (E. difficilis) and the Yellowish Flycatcher (E. flavescens). The molts of these species have not been studied previously in any detail. This work was undertaken to gather background information necessary for the proper interpretation of size variation in a systematic analysis of this difficult group (Johnson MS). Finally I consider patterns and progress of cranial development in these flycatchers and offer a new hypothesis to explain retarded pneumatization of the skull roof in birds.

Breeding range extension of the Acadian Flycatcher

Breeding range extension of the Acadian Flycatcher

Feeding Ecology of the Red‐eyed Vireo (Vireo olivaceus) and Associated Foliage‐Gleaning Birds

The sexes of the Red—eyed Vireo, an abundant insectivorous bird in eastern deciduous woodlands, differed in their foraging heights, with an overlap of only about 35%. The males foraged higher than the females, and non—random sequences of movements were employed to maintain this separation. The foraging level and the nest height of female Red—eyes was similar, whereas the males foraged closer to their singing perches. Thus the foraging efficiency of both sexes was likely greater in accordance with their differing behavior roles. The "small" territory (1.3—1.7 acres) of this species actually consists of a cylinder extending from the forest canopy to the low understory. The associated White—eyed Vireo (Vireo griseus) was generally separated from Red— eyed Vireo by habitat, and the Yellow—throated Vireo (Vireo flavifrons) overlaps in habitat but possesses structural and behavioral differences which indicate different prey preferences. Foraging behavior of migrating Red—eyed Vireos differs markedly from that of breeding birds. Differences were also noted between summer and winter foraging behavior patterns in the White—eyed Vireo. All three vireo species exhibit species—specific patterns of foraging movements. Two other foliage—gleaning insectivorous birds occupying the same forests, the American Redstart (Setophaga ruticilla) and the Acadian Flycatcher (Empidonax virescens), also possess species—specific foraging behaviors (niche—exploitation patterns).

Identification of the Flycatchers of Eastern North America, with Special Emphasis on the Genus Empidonax

Among eastern birds, the flycatchers are notoriously difficult to identify. Vague identifications or unqualified errors occur not infrequently in the genus Empidonax, and we have personally seen banders identify wood pewees as Empidonaces and vice versa. Part of the problem lies in the fact that most of us are more familiar with birds in the field than we are with birds in the hand, where they give none of the behavioral clues that aid us in field identification. For this reason, banders have a special need for useful keys. But unfortunately the available texts and keys often lack vital information on when and where to expect a given species, and they commonly neglect juvenal plumages and plumage sequences. Some keys in common usage place undue emphasis upon single characters which, because of their variability, should only be used as corroborative evidence. A case in point is the often-quoted distinction between two groups of Empidonaces based solely on back coloration, which has inadvertently led to the misidentification of some green-backed Traill's Flycatchers as Acadian Flycatchers. Banders should thus find useful a new key and summary of pertinent information which covers all plumages of all eastern flycatchers, i.e. those species normally found anywhere east of the Great Plains and north of southernmost Texas, and including those extralimital species for which there are a number of specimens taken in the east. The nomenclature used is that of the A. 0. U. Check-list of North American Birds (1957). The key emphasizes characters visible in the living bird but not often mentioned in the literature, such as color of mouth lining, and also considers the times and places where certain species are not to be expected. Its use requires no more than an accurate caliper and a millimeter ruler. With these aids, and a careful reading of all the characters, we believe that 98 per cent or more of all the fullgrown birds netted can be readily keyed out, even in the difficult genus Empidonax.