Veery (Catharus fuscescens) nest architecture and the use of alien plant parts.
The effect of alien plant invasions on the ecology of North American forest songbirds has received widespread attention (Borgman and Rodewald, 2004; Heckscher, 2004; McCusker et al., 2010; Gleditsch and Carlo, 2011). Alien plants have invaded much of the temperate broadleaf forests of the United States often replacing native species and altering forest structure (Hunter and Mattice, 2002). However, whether alien plant species in North American temperate broadleaf forests are detrimental to songbird populations has been largely equivocal (e.g., Schmidt and Whelan, 1999; Heckscher, 2004; Gleditsch and Carlo, 2011). For example, Schmidt and Whelan (1999) reported that American Robin (Turdus migratorius) nests had higher predation in alien shrubs (Lonicera maachii and Rhamnus cathartica) than in comparable native shrubs and Heckscher (2004) demonstrated alien plants provided the additive foliage density necessary to stimulate Veery (Catharusfuscescens) nest placement. However, the potential for alien plant species to alter the availability and quality of plant litter required for the construction of open-cup leaf nests has not been investigated. Forest songbirds in temperate North America have evolved in concert with native plant parts that comprise the plant litter available for nest construction. Avian nest architecture is thought to reflect evolutionary history at the family, genus, or species level (e.g., Collias, 1997; Sheldon and Winkler, 1999; Zyskowski and Prum, 1999). This implies a behavioral predisposition for birds to: (1) use specific materials that occur naturally within their environment and (2) arrange those materials in specific ways that result in a structure architecturally similar among closely related taxa. Tamialojc (1992) demonstrated the distribution of the Song Thrush (Turdus philomelos) is likely limited by the specific material needed for the construction of its open-cup nests. If nest construction is indeed a largely derived behavior, and if the availability of specific nest material can limit open-cup nesting bird populations, then the replacement of forest litter originating from native plants with that from alien plants could affect the nest construction process for multiple species.
As a preliminary investigation of this topic, we examined Veery nest construction in a forest infested with nonnative plant species (see below; Heckscher, 2004). The Veery is a suitable subject for such an investigation because it is a territorial, declining, forest-dependent, Nearctic-Neotropical migrant that uses forest plant litter to construct open-cup leaf nests and, in the Middle-Atlantic region, often occupies habitat with invasive alien shrub species (Heckscher, 2004; Schmidt et al., 2005). Our goals were: (1) document the architectural approach used by Veeries to build nests in part to more thoroughly understand the importance of specific nest materials; (2) determine whether Veeries used alien plant parts in nest construction; (3) if Veeries used alien plant parts, determine their use relative to native plant parts in the construction of different portions of the nest; and (4) determine if nest success was related to the use of alien plant parts. Further, Veeries are single-brooded (Heckscher, 2007), thus, the progression of the nesting season increases motivation to complete the nest cycle in adults that are not successful in the first half of the nesting season (Heckscher, 2007). Therefore, we surmised if Veeries use alien plants to construct their nests, females may have minimized search time by incorporating more alien plant parts into nests later in the nesting season, assuming that alien plant parts are more readily available than native parts in our study area. Therefore, our fifth objective was to test for an association of alien plant mass with the progression of the nesting season.
The study took place at White Clay Creek State Park, New Castle County, Delaware (39.44[degrees]N, 75.45[degrees]W). Veery nests were located by following individually marked (i.e., color banded) adults canting nesting material or food to nest sites. Nests were monitored at 2-3 d intervals to determine whether they fledged young or failed. Veeries do not re- use nest structures; therefore, we collected nests from the field [greater than or equal to] 3 d after the termination of the nesting cycle. Once collected, nests were immediately frozen to halt decay and to eliminate any detritivores that might potentially affect the nest's structural integrity.
Nests were dried (mean: 377, SD [+ or -] 174, range: 97-648 min) in a dessication oven and subsequently weighed prior to dissection. Each nest was dismantled by two researchers both using forceps to carefully remove each individual piece of litter. The position and type of each item that comprised the nest structure was noted, identified, and categorized by species when possible. Several nests took multiple days for two researchers to fully process. To store partially dissected nests overnight, we returned them to the sealed dessication ovens and stored at room temperature.
Eight nests were dissected in 2009 and 11 in 2010. Mean dry weight did not differ between the 2 y (Student's t-test, P = 0.76) so we pooled all data. We assumed that the function of the plant part in the architectural configuration (how materials were arranged to construct the nest) of the nest was the most important aspect of use to consider rather than the number of parts used (e.g., three small leaves may be equivalent to one large leaf). Therefore, we quantified the dry weight of plant parts and considered that value as a relative index to the importance of each species. We used linear regression to test for an association of time with alien plant mass. For all regressions, we used ordinal date and data were log transformed after examining their distributions. In an attempt to test for an association of alien plant mass with nest failure, we used an exact binomial test of goodness-of-fit to compare the number of nests that failed when the dry weight of alien material was below the sample median, in comparison to the number of failed nests that contained alien material that exceeded the median dry weight of aliens. We considered a nest successful if it fledged at least one young.
We analyzed 19 Veery nests constructed by 19 different females (unlike males, females are not known to attend multiple nests; Halley and Heckscher, 2012). Twelve nests failed and nine fledged at least one young. To satisfy the assumption of independence for statistical purposes, second nests constructed by females were not used in the study. Of the 19 nests used in this study, female Veeries initiated construction from 9 May to 29 May in 2009 (n = 7) and 11 May and 12 Jun. in 2010 (n = 12). All nests were placed in the alien shrub R. multiflora and supported by that species and often fallen dead branches. The mean dry weight of all nests was 54.6 g SD + 23 (range: 21-113 g). Total nest weight was not associated with the progression of the breeding season ([r.sup.2] = 0.004, P = 0.8).
All nests consisted of three distinct structural layers: outer, inner, and nest lining. Veeries used parts from 27 plant species to construct these layers (Tables 1, 2). Veeries initiated nest construction by placing a platform-like layer of robust leaves across existing supporting branches. This platform was reinforced by various other plant parts (e.g., petioles, small stems) some of which acted as interior support beams for the platform itself. This platform formed the outer layer and acted as the primary foundation that held the inner layer in place. Many leaves used for this portion of the nest were from litter dropped the preceding autumn in minimal stages of decay and included robust leaves from various species (Tables 1, 2). The inner layer was a firm cup largely consisting of inter-woven material. Thin strips of bark were used by all 19 birds to integrate rootlets, leaf petioles, and leaves. Within the inner layer of all nests, drying moisture and limited mud on leaves acted as an adhesive, improving the nest's structural integrity. Finally, the nest lining consisted of flexible material to hold the eggs, young, and brooding female. Typically, rootlets, barkstrips, Armillaria mellea, and seeds from various species were used for the nest lining (Table 1).
Alien plant species were used in all nests and in all three layers (Table 2). Among the 19 nests, six of the 27 plant species used by Veeries (22%) were alien to the Mid-Atlantic region (Table 2). The total number of alien plants used was greatest in the outer layer and least in the inner layer (Fig. 1). The progression of the breeding season was not related to the category of plant material (i.e., aliens: [r.sup.2] = 0.05, P = 0.4 or natives: [r.sup.2] = 0.01, P = 0.8). There was no significant difference in the mean weight of natives and aliens comprising the outer layer; however, in the inner layer and nest lining alien species comprised significantly greater weight than that of native species (Fig. 2). Of the six alien species used, all were represented in the outer layers (100%), three in the inner layers (50%), and four in the nest linings (67%; Fig. 1). Stems of Alliaria petiolata and R. multiflora were prevalent components of outer and inner nest layers (Table 2). There was no difference in the number of failed nests below the median alien weight (n = 5; 24.84 g) compared to the number of failed nests above the median weight (n = 7) of aliens (Exact binomial test; P = 0.77).
The construction of an outer layer, an inner layer, and nest lining by Veeries at our study site was similar to Veery nests described from other regions. For example, Annan (1961) reported that Veeries in Michigan used a platform of twigs and coarse bark shreds, an outer cup of bark, pine needles, and large "leafy pieces," and an inner lining consisting of fine bark, rootlets, needles, and "shreds of decaying leaves." Similar to nests reported by Annan (1961), the leaves used to construct the outer layer of our nests were interspersed with bark shreds and several other plant parts. Hansell (2000) proposed that the outer layer of open-cup leaf nests often aid in camouflaging the nest from predators; we believe this is the case for Veery nests as well. For example, a bulky outer leaf layer can conceal the inner layer from view and appear simply as an accumulation of dead leaves (see Meanley, 1971).
The inner layer of the nests we examined were largely comprised of bark strips woven together to form the bulk of the cup wall. The nest cup lining contained a large amount of rootlets and rhizomorphs of Armillaria mellea. Similarly, rhizomorphs are an important component of the inner lining of Bicknell's Thrush (Catharus bicknelli) nests (e.g., McFarland and Rimmer, 1996).
The use of mud in nest construction is believed to be an adaptive behavior among <5% of bird species including many species of Turdidae (Rowley, 1970). Although limited mud was present on some leaves in all Veery nests we examined, we believe the birds did not make any concerted effort to collect mud, as do sympatric Wood Thrush (Hylocichla mustilena); rather, based on this study and prior field observations we believe Veeries often searched for nest material when leaves and detritus were moist (e.g., shortly after rainfall, on wetland margins), such that the structural integrity of the nest improved through the adhesive action that took place during the natural dessication process. Rowley (1970) implied that Catharus use mud in a concerted fashion to "hold sticks and grass together." In contrast, we believe the presence of limited mud in our Veery nests was coincidental. Obtaining moist leaf litter to improve structural integrity and collecting moist mud, we believe, are evolutionarily convergent solutions to prevent nest collapse through structural reinforcement.
Alien plant parts were a prevalent component of each nest layer. In particular, the stems of A. petiolata and R. multifloraseemed to function as important internal support beams that contributed to the structural integrity of the outer and inner layers unlike any comparable native plant stem. Leaves of R. multiflora were common in the outer nest layers but these leaves did not appear to fulfill a critical structural role. Seeds of A. petiolata were found in the outer and inner layer of one nest. The use of seeds in nest material is thought to be a significant dispersal mechanism for some plants (Dean et al., 1990) and could also be so for alien species. However, A. petiolata is common on the floodplain where our nests were discovered and its use in Veery nests at our study site was probably unrelated to the plant's invasion event as Veeries usually collect material close to the nest site (Heckscher, 2007). We found no effect of time on the mass of alien plants suggesting that alien plant use was uniform through the course of the nesting season and was not a consequence of seasonal effects or otherwise differed significantly among nest attempts.
Alien plant invasions can have conflicting effects on songbird community ecology; for example, some native bird species may benefit from the presence of alien plants and some native species may be negatively affected. Complicating matters, it is possible that a single alien plant species can create conflicting intraspecific effects (Heckscher, 2004). However, despite the dearth of convincing empirical evidence, the invasion of alien plants in temperate forests is assumed to have a net negative effect on songbird ecology regardless of whether some interactions may be positive (e.g., Rodewald, 2012). In the current study, alien plant material was used by Veeries to construct nests and apparently did fulfill a critical structural role. However, we doubt that Veeries were dependent on alien plant parts to construct nests. Instead, we believe alien plant material was used in place of a comparable native material simply due to the abundance of alien plants in our study area. However, this interpretation does not eliminate the possibility that the unnatural abundance of invasive plants may decrease the energy otherwise necessary for females to locate suitable native material at a time when energy reserves are at a premium. If that scenario is correct, the presence of suitable nest material from alien plants might be considered beneficial. It is equally possible that alien plant parts are not the best material for Veery nests and Veeries are "making the best of a bad situation" because comparable native plant parts are not readily available. Although we could not associate the use of alien plant parts with nest failure our sample size may have been too small to detect such a relationship; however, if that relationship exists it would be puzzling considering Veeries have increased in this region during a period of alien plant establishment (Heckscher, 2004).
Although the nest construction process can reflect the evolutionary history of taxa, the ability of Veeries to incorporate alien plant parts into the nest construction process corroborates earlier studies that show some bird species can exhibit a high degree of plasticity in the use of materials (Mennerat et al., 2009; Walsh et al., 2011). This suggests that at least some songbirds are not dependent on specific plan t species but rather have an adaptive ability to acquire the structural elements that are needed for successful nest building regardless of a plant species' taxonomic origin. However, we caution that this does not necessarily imply that plant species composition is not important in the nest construction process. For example in the present study the mass of Fagus grandifolia in the outer layer is over six times that of Acer rubrum (the species with the next greatest mass). Fagus grandifolia leaves are relatively resistant to decay (Melillo et al., 1982), and we believe their use reflects a critical need to maintain a robust outer layer for structural support. Further, F. grandifolia are absent on the broad floodplain of our study area where most nests are found (Heckscher, 2004) yet it is common on surrounding upland slopes. Although Veeries usually collect material close to the nest site, the lack of proximity of F. grandifolia to nest sites suggests that some female Veeries flew far beyond their territorial boundaries to acquire that resource.
Alien species were a significant component of Veery nests at our study site. Although our study focused on one avian species, managers that attempt to restore native shrub diversity through the eradication of alien invaders might want to consider the potential negative effects on songbird nest construction; if a resource of nonnative origin is suddenly removed comparable native species may not recover in a timely manner to produce the resources necessary to fulfill the roles left vacant by alien species. In that vein, managers need to be aware in some cases aliens may provide a physiognomic resource unlikely to be met by native species alone (Heckscher, 2004). Thereibre, to mitigate for potential devastating effects (e.g., local extirpation of a desirable native avian species), managers might consider a patchwork restoration approach whereby sections of forest understory are restored over a prolonged period of time rather than single eradication events aimed at large scale removal of alien forest understory.
Acknowledgments.--This research was funded in part by the Center for Integrated Biological and Environmental Research, College of Agriculture and Related Sciences, Delaware State University. We thank Nicholas McFadden, Christopher Bennett, Robert Line, and the staff of White Clay Creek State Park, Delaware Division of Parks and Recreation, for their support. This manuscript benefitted from two anonymous reviewers.
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CHRISTOPHER M. HECKSCHER (1) AND SYRENA M. TAYLOR, Department of Agriculture and Natural Resources, Delaware State University, 1200 N. DuPont Highway, Dover 19901; AND CATHERINE C. SUN, New York Cooperative Fish and Wildlife Research Unit, Department of Natural Resources, Cornell University, Ithaca 14853. Submitted 31 August 2012; Accepted 16 September 2013.
(1) Corresponding author: Telepone: (302) 857-6412; FAX: (302) 857-6455; e-mail: email@example.com
TABLE 1.-Native plant and fungus species and parts used in 19 Delaware Veery nests constructed in 2009 and 2010. Data are expressed as the number of nests and the total weight (g) of each species. Percent total mass is the contribution of each species or plant part from the total mass combined (both alien and native species) Number of nests (weight in grams) Outer Inner Species Common name layer layer Leaves Fagus grandifolia American Beech 18 (41.2) 17 (12.1) Acer rubrum Red maple 12 (6.7) 12 (5.1) Lindera benzoin Spicebush 7 (5.3) 10 (8.1) Liriodendron tulipifera Tulip poplar 11 (5.6) 12 (6.8) Platanus occidentalis Sycamore 5 (7.8) 3 (1.7) Quercus velutina Black oak 0 5 (5.2) Quercus rubra N. Red oak 7 (4.1) 4 (1.8) Carya glabra Pignut hickory 6 (2.2) 3 (0.2) Carpinus caroliniana American hornbeam 5 (1.2) 3 (0.2) Nyssa sylvatica Black gum 1 (0.1) 1 (2.0) Maclura pomifera Osage-orange 2 (1.8) 0 Populous deltoides Eastern cottonwood 2 (0.7) 0 Prunus serotina Black cherry 4 (0.3) 2 (0.8) Quercus alba White Oak 4 (0.9) 2 (0.1) Quercus palustris Pin oak 1 (0.9) 0 Viburnum acerifolium Maple leaf vibimum 1 (0.4) 0 Crataegus spp. Hawthorn 1 (0.1) 1 (0.1) Toxicodendron radicans Poison ivy 1 (0.1) 0 Fraxinus spp. Ash 1 (0.1) 0 Quercus prinus Chestnut oak 1 (0.1) 0 Seeds and stems Liriodendron tulipifera Tulip poplar seeds 17 (3.6) 18 (4.4) Platanus occidentalis Sycamore seeds 6 (0.1) 8 (0.4) Acerrubrum Red maple seeds 3 (0.01) 3 (0.1) Vitis spp. Grape petioles/stems 11 (6.0) 8 (1.8) Stems (unidentified) 11 (8.3) 10 (1.3) Other Grass (unidentified) 2 (1.2) 0 Bryophytes (unid.) 2 (0.4) 0 Rootlets (unidentified) 3 (0.1) 0 Barkstrips (unid.) 17 (54.6) 19 (211.3) Armillaria mellea Shoestring rot 9 (0.5) 13 (2.0) Unknown plant parts 19 (86.4) 19 (141) Number of nests (weight in grams) Nest Total Species Common name lining weight (g) Leaves Fagus grandifolia American Beech 7 (0.24) 53.5 Acer rubrum Red maple 13 (6.0) 17.8 Lindera benzoin Spicebush 6 (2.0) 15.4 Liriodendron tulipifera Tulip poplar 6 (1.3) 13.7 Platanus occidentalis Sycamore 0 9.5 Quercus velutina Black oak 3 (1.4) 6.6 Quercus rubra N. Red oak 1 (0.1) 6.0 Carya glabra Pignut hickory 5 (1.0) 3.4 Carpinus caroliniana American hornbeam 7 (1.1) 2.5 Nyssa sylvatica Black gum 1 (0.1) 2.2 Maclura pomifera Osage-orange 0 1.8 Populous deltoides Eastern cottonwood 1 (0.9) 1.6 Prunus serotina Black cherry 2 (0.3) 1.4 Quercus alba White Oak 1 (0.1) 1.1 Quercus palustris Pin oak 0 0.9 Viburnum acerifolium Maple leaf vibimum 2 (0.1) 0.5 Crataegus spp. Hawthorn 0 0.2 Toxicodendron radicans Poison ivy 0 0.1 Fraxinus spp. Ash 0 0.1 Quercus prinus Chestnut oak 0 0.1 Seeds and stems Liriodendron tulipifera Tulip poplar seeds 16 (1.2) 9.2 Platanus occidentalis Sycamore seeds 7 (0.1) 0.7 Acerrubrum Red maple seeds 4 (0.2) 0.31 Vitis spp. Grape petioles/stems 8 (1.0) 8.8 Stems (unidentified) 4 (0.2) 9.8 Other Grass (unidentified) 0 1.2 Bryophytes (unid.) 10 (0.1) 0.5 Rootlets (unidentified) 18 (15.8) 15.9 Barkstrips (unid.) 18 (59.2) 325.1 Armillaria mellea Shoestring rot 14 (5.6) 8.1 Unknown plant parts 18 (45.0) 272.4 Percent Species Common name total mass Leaves Fagus grandifolia American Beech 6.1 Acer rubrum Red maple 2.0 Lindera benzoin Spicebush 1.7 Liriodendron tulipifera Tulip poplar 1.5 Platanus occidentalis Sycamore 1.1 Quercus velutina Black oak 0.7 Quercus rubra N. Red oak 0.7 Carya glabra Pignut hickory 0.4 Carpinus caroliniana American hornbeam 0.3 Nyssa sylvatica Black gum 0.3 Maclura pomifera Osage-orange 0.2 Populous deltoides Eastern cottonwood 0.2 Prunus serotina Black cherry 0.2 Quercus alba White Oak 0.1 Quercus palustris Pin oak 0.1 Viburnum acerifolium Maple leaf vibimum <0.1 Crataegus spp. Hawthorn <0.1 Toxicodendron radicans Poison ivy <0.1 Fraxinus spp. Ash <0.1 Quercus prinus Chestnut oak <0.1 Seeds and stems Liriodendron tulipifera Tulip poplar seeds 1.0 Platanus occidentalis Sycamore seeds <0.1 Acerrubrum Red maple seeds <0.1 Vitis spp. Grape petioles/stems 1.0 Stems (unidentified) 1.1 Other Grass (unidentified) 0.1 Bryophytes (unid.) <0.1 Rootlets (unidentified) 1.8 Barkstrips (unid.) 36.8 Armillaria mellea Shoestring rot 0.9 Unknown plant parts 30.8 TABLE 2.-Alien plant species and plant parts used in 19 Delaware Veery nests constructed in 2009 and 2010. Data are expressed as the number of nests and the total weight (g) of each species. Percent total mass is the contribution of each species or plant part from the total mass combined (both alien and native species) Number of nests (weight in grams) Outer Inner Scientific name Common name layer layer Leaves Acer platanoides Norway maple 4 (5.5) 0 Euonymous alatus Burning bush 6 (0.1) 4 (2.4) Rosa multiflora Multiflora rose 13 (0.7) 0 Lonicera japonica Japanese honeysuckle 1 (0.3) 0 Elaeagnus umbellata Autumn Olive 2 (0.1) 0 Seed pods Alliaria petiolata Garlic mustard 1 (0.03) 1 (0.02) Stems Rosa multiflora Multiflora rose 16 (56.3) 10 (4.8) Alliaria petiolata Garlic mustard 17 (12.3) 17 (5.4) Euonymous alatus Burning bush 1 (1.5) 1 (0.60 Lonicera japonica Japanese honeysuckle 0 0 Number of nests (weight in grams) Nest Total Scientific name Common name lining weight (g) Leaves Acer platanoides Norway maple 3 (2.4) 7.9 Euonymous alatus Burning bush 2 (0.1) 2.6 Rosa multiflora Multiflora rose 7 (0.3) 1.0 Lonicera japonica Japanese honeysuckle 1 (0.02) 0.3 Elaeagnus umbellata Autumn Olive 0 0.1 Seed pods Alliaria petiolata Garlic mustard 0 0.05 Stems Rosa multiflora Multiflora rose 3 (0.1) 61.2 Alliaria petiolata Garlic mustard 0 17.7 Euonymous alatus Burning bush 0 2.1 Lonicera japonica Japanese honeysuckle 1 (0.1) 0.1 Percent Scientific name Common name total mass Leaves Acer platanoides Norway maple 0.9 Euonymous alatus Burning bush 0.3 Rosa multiflora Multiflora rose 0.1 Lonicera japonica Japanese honeysuckle <0.1 Elaeagnus umbellata Autumn Olive <0.1 Seed pods Alliaria petiolata Garlic mustard <0.1 Stems Rosa multiflora Multiflora rose 7.0 Alliaria petiolata Garlic mustard 2.0 Euonymous alatus Burning bush 0.2 Lonicera japonica Japanese honeysuckle <0.1
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|Title Annotation:||Notes and Discussion|
|Author:||Heckscher, Christopher M.; Taylor, Syrena M.|
|Publication:||The American Midland Naturalist|
|Date:||Jan 1, 2014|
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