2013 NORTHEASTERN NATURALIST 20(1):171–184 The Value of Native and Invasive Fruit-Bearing Shrubs for Migrating Songbirds Susan B. Smith1,*, Samantha A. DeSando1, and Todd Pagano2 Abstract - The success of annual migrations for songbirds is greatly affected by habitat quality at stopover sites, particularly in relation to food needed for rapid refueling. The abundance and nutritional quality of important food resources may be linked to the presence of deciduous shrub species that provide seasonal fruits in the fall and support insects in the spring. The objective of this study was to determine whether migrating songbirds benefit from resources provided by native or invasive fruit-bearing shrubs found at 2 birdbanding stations in Rochester, NY. We conducted nutritional analyses (energy density, fat content, total soluble solids) on the fruits of common shrub species at the study sites, monitored removal of the fall fruits of focal native and invasive shrub species in the field, and measured the abundance of midges—a common insect resource for migrating songbirds— supported by the focal shrub species in the spring. The highest fat content and energy densities were found in fruits of native shrubs, ranging from 6.57 to 48.72% fat and 18.83 to 28.68 kJ/g of energy. All invasive fruits had ≤0.99% fat and ≤17.17 kJ/g of energy. We also found a significant positive correlation between fat and energy content of the fruits. Native dogwood fruits were consumed by migrating songbirds at higher rates than invasive fruits over the fall migration period. However, there was no clear pattern of midge abundance between native and invasive shrub species during the spring migration period. Our results suggest that fruits of native shrubs are of greater nutritional value to migrating songbirds than the fruits of invasive shrubs during fall migration, which is supported by the higher removal rates by songbirds of native dogwood fruits than fruits of the 4 other invasive fruit species. This finding suggests that removal of invasive fruit-bearing shrubs or plants will not negatively impact migrating birds when high-quality native fruit-bearing shrubs are available. However, additional study on the relative value of these shrubs in the spring and over multiple seasons is needed to provide insight into their overall value for birds during annual migrations. Introduction Annual migrations between breeding and wintering grounds are vulnerable periods in the life cycle of songbirds (Sillett and Holmes 2002). Migration is energetically challenging, and songbirds must consume large quantities of food at stopover sites used along the migration route in order to quickly refuel and continue migration. The loss of suitable stopover habitat is a major threat to migrating birds and has contributed to recent declines in bird populations (Askins et al. 1990, Butchart et al. 2010). Thus, stopover sites with high-quality food resources and habitats are essential for successful completion of annual migrations (Schaub and Jenni 2000). 1Thomas H. Gosnell School of Life Sciences, Rochester Institute of Technology, Rochester, NY 14623. 2Laboratory Science Technology Program, National Technical Institute for the Deaf, Rochester Institute of Technology, Rochester, NY 14623. *Corresponding author - sbssbi@rit.edu. 172 Northeastern Naturalist Vol. 20, No. 1 Invasive plant species are drastically changing important habitats/ecosystems, and the environmental and economic costs associated with the spread of invasive plant species are substantial (Gordon 1998, Pimentel et al. 2005). The invasion of exotic plants may impact the habitat quality at stopover sites given their relatively high growth rates and competitive ability compared to native plants (Daehler 2003), particularly if they compete with native plants that provide important food resources for migrating birds. Research focused on identifying how the habitat and food requirements of migrating songbirds are impacted by the presence of invasive plant species may benefit the conservation and management of migratory bird populations. Fruits are a major food resource for many songbirds during fall migrations in the eastern US (Parrish 1997, 2000), and omnivorous songbirds may consume the fruits of both native and invasive fruit-bearing plants during critical refueling periods in the fall. In addition to the ecological implications of avian seed dispersal as a vector for the spread of invasive species (Bartuszevige and Gorchov 2006, Buckley et al. 2006, Gosper et al. 2005), the negative impacts of the outcompeting of native species by invasive species may also extend to the physiological condition of avian frugivores, particularly if birds are attracted to, and consume, their fruits as a food resource rather than their native counterparts (Aslan and Rejmanek 2012, LaFleur et al. 2007, Sallabanks 1993). In effect, if songbirds are consuming an invasive fruit that has little nutritional value to the bird then this could serve as an ecological trap (Robertson and Hutto 2006, Rodewald 2012) whereby birds could experience a decrease in body condition because they cannot feasibly consume enough fruits to meet their daily energy requirements (S.B. Smith et al. 2007). Conversely, removal of invasive species could be counterproductive to songbird conservation and negatively impact migrating songbirds by eliminating a food resource needed to replenish fat reserves quickly if their fruits represent a high-quality resource that is selectively consumed by migrants (Gleditsch and Carlo 2011). Therefore, assessing the consumption of fruits of invasive species in combination with the analysis of nutritional composition of the fruits is necessary to assess their value for migrating birds. Previous research focused on the relative importance of native and invasive plants for songbird consumers has linked physical or chemical fruit traits to indices of bird use, such as seed dispersal and fruit consumption (Aslan and Rejmánek 2012, Gosper and Vivian-Smith 2010, Greenberg and Walter 2010, LaFleur et al. 2007, White and Stiles 1992). However, studies that have attempted to relate the nutritional quality of fruits with consumption rates by migrating birds have yielded mixed results. For example, frugivorous birds did not discriminate native versus invasive fruits based on energy density in central Maine (Drummond 2005), yet birds selected energy-dense, high-fat native fruits in New England (S.B. Smith et al. 2007)—although direct comparisons among native and invasive fruits was not the focus of that work. Other field and captive studies have demonstrated that fruits with high levels of pulp lipids are often selectively consumed by avian frugivores (Stiles 1993). To our knowledge, few if any studies have focused on assessing the combined aspects of fruit nutritional content 2013 S.B. Smith, S.A. DeSando, and T. Pagano 173 and selective consumption of native and invasive fruits by migrating birds in the fall along with the value of the native and invasive plant species for supporting insects that are used as a primary food resource by migrating birds during spring. The potential for certain shrub species to attract and harbor more insects than others may result in spatial differences in the availability of insect resources, which can ultimately influence the quality of habitat and foraging behavior of birds at spring stopover sites (R.J. Smith et al. 1998, 2007). The availability of insects is particularly important during early spring in the Great Lakes region when midges represent an important food resource for migrating songbirds (Ewert et al. 2011, R.J. Smith et al. 2007). The goal of our study was to assess the value of native and invasive fruit-bearing shrub species for migrating songbirds during stopover periods through the analysis of fruit nutritional quality in relation to consumption in the fall and their potential to support flying insects as food resources in the spring. We designed our study to determine if native shrubs provide high-quality food resources for songbirds in both spring and fall migrations and, as such, are of higher year-round value when present at stopover sites used by migrating songbirds compared to invasive species. We predicted that native fruits would have higher nutritional value than invasive fruits and would also be consumed at higher rates than invasive fruits by migrating songbirds in the fall. We also hypothesized that there would be a difference in the abundance of insects supported by native and invasive shrubs during the spring migration period. Methods Study sites We collected data from 2 study sites that are continuously monitored as migration stopover sites in Rochester, NY. The first site was the Rochester Institute of Technology Bird Observatory (hereafter “RITBO”; 43°54'N, 77°40'W), which is a mid-successional forested wetland area adjacent to suburban development. Dominant trees include Fraxinus pennsylvanica Benth. (Green Ash) and Populus deltoides W. Bartram ex Marshall (Eastern Cottonwood), and dominant shrubs include Cornus spp. (dogwood). The second study site was near the Braddock Bay Bird Observatory (hereafter “BBBO”; 43°19'N, 77°43'W), approximately 15 miles north-northwest of the RITBO study site, located on the south shore of Lake Ontario. This study site is a mixture of secondary growth habitat types characterized by dominant tree species of Fraxinus spp. (ash) and Alnus spp. (alder) and dominant shrub species of Cornus spp. and Viburnum spp. (viburnum) (Bonter et al. 2007). Fruit nutritional analyses We collected ripe fruits during peak migration in early October of 2010. Fruit was collected from the following native shrub species: Cornus racemosa Lam. (Gray Dogwood), Cornus amomum Mill. (Silky Dogwood), Cornus sericea L. (Red Osier Dogwood), Viburnum dentatum L. (Arrowwood Viburnum), and Lindera benzoin L. Blume (Spicebush). We also collected fruits from the following 174 Northeastern Naturalist Vol. 20, No. 1 invasive shrub species: Rhamnus cathartica (Common Buckthorn), Rosa multiflora Thunb. (Multiflora Rose), Viburnum opulus L. (European Cranberrybush), and Lonicera spp. (bush honeysuckles). All fruits were collected at BBBO, except Lindera benzoin, which was collected at RITBO. Harvested fruit was frozen and stored at -20 °C. Thawed fruit was dissected to remove seeds and stems, and for all species except Viburnum dentatum and Lindera benzoin, the pulp was dried in a convection oven at 50 °C for 5 to 7 days until the mass was constant for at least 24 hr. The high-fat content of Viburnum dentatum and Lindera benzoin precluded accurate dry-mass determination via oven drying. Thus, dissected fruits of Viburnum dentatum and Lindera benzoin were freeze-dried in a Labconco Freezone TRIAD freeze dryer system for 48 hours. Dried samples were then homogenized with a mortar and pestle. We determined the average percent water in each fruit species by measuring the mass lost over the drying period in subsamples of 4 to 30 fruits. We then averaged mass loss over all dissected subsamples for a single species (2 to 11 subsamples per species). We measured the lipid content of each fruit species using Soxhlet lipid extraction with 1 g of dried sample in a ceramic thimble. Petroleum ether was refluxed through the Soxhlet apparatus for 6 hours. Following the Soxhlet lipid extraction, the extracted samples were placed in a Thermolyne muffle furnace (Thermo Scientific) at 550 °C for 3 hours to obtain ash content. We measured the energy density of fruit samples using 1 g of dried sample in a Parr 1341 oxygen bomb calorimeter. The total soluble solids (hereafter “TSS”) were determined by thawing frozen fruits and measuring the refractive index of the fruit juice of 2 to 8 randomly selected ripe fruits using a Bausch and Lomb Abbe refractometer. We repeated refractive index measurements 3 times for each species. The refractive indices were compared to a published percent sucrose table (AOAC Official Methods of Analysis 1990-Refractive Index [n] of sucrose solutions at 20 °C) which reflects TSS as °Brix. Fruit consumption by songbirds We monitored the fruit consumption by songbirds over the fall migration season at both the RITBO and BBBO study sites. The focal shrub species at RITBO were Cornus racemosa, Cornus amomum, Rhamnus cathartica, and Rosa multiflora. The focal shrub species at BBBO were Cornus racemosa, Cornus amomum, Viburnum opulus, and Lonicera spp. Three shrubs of each species were selected at each study site and monitored similarly to S.B. Smith et al. (2007). Two branches with similar amounts of ripe fruit were chosen on each shrub; one branch was enclosed with 25-mm-mesh netting to prevent birds from accessing the fruit but allow measurements of natural abscission rate, and the other branch was marked and left exposed to frugivory. The initial number of fruits on each branch was recorded, including the number of ripe and unripe fruits, which represents both current and potential future fruit resources. Decayed fruit was excluded from all of the fruit counts. Measuring fruit loss based on initial counts accounted for the differences in number of fruits between branches. The fruit was counted once weekly for a total of 5 weeks. We concluded that the fruit loss (after accounting for natural abscission) was due to songbird consumption because the experimental branches were carefully chosen so they would not support the weight of small mammals. Further, 2013 S.B. Smith, S.A. DeSando, and T. Pagano 175 there was no evidence of deer browsing, and songbirds are the most common frugivores in the area during the fall migration period. Insect abundance We measured the number of flying insects (Order Diptera) supported by 4 focal shrub species using hanging insect traps throughout the spring migration season of 2011 (late-April through mid-May). We focused on Diptera because flying insects, namely midges (Chironomidae), are an important food resource for songbirds in early spring prior to leaf-out, especially in areas near wetlands or lake shores as is typical at our study sites (Ewert et al. 2011, R.J. Smith et al. 2007). Flying insect density was measured at the RITBO study site for 2 native species (Cornus racemosa and Cornus amomum) and 2 invasive species (Rhamnus cathartica and Rosa multiflora). Two 10-cm by 20-cm plexiglass boards covered in Tanglefoot® insect trapping compound were placed at 1-m and 2-m height intervals on 2 shrubs of each species. The traps were left hanging on the shrubs for a 24-hr period after which the number of flying insects on each board was recorded. We repeated this process once a week over a 3-week period. Statistical analyses We compared the energy density, percent fat, and TSS between native and invasive fruit species using Wilcoxon-Mann-Whitney Tests with computation of exact P-values. We calculated Spearman correlation coefficients to correlate the energy density, percent fat, and TSS of fruit species. A repeated measures ANOVA was used to compare the proportion of fruits remaining on enclosed and unenclosed branches by species over time for each site separately. Results of Shapiro-Wilk normality tests indicated that the data deviated from normality; therefore, we arcsine transformed data after correcting for initial fruit numbers on each branch, which normalized the data. We conducted follow-up Tukey tests to compare fruit removal between the fruit species measured at a given site. We calculated a fruit consumption index similarly to S.B. Smith et al. (2007) using the equation: Consumption index = 1 - (unenclosed branch % remaining / enclosed branch % remaining) for each species and for each sampling date of the consumption study. We calculated the average number of insects per shrub by averaging the number of insects counted on all boards at all heights for a given collection date. We then used repeated measures ANOVA to compare the number of insects found on each fruiting shrub species over time. We ran a Tukey follow-up analysis to examine specific differences between species and time. All statistical analyses were conducted using SAS 9.2 program (SAS Institute 2008). Results Fruit nutritional analyses The average water content of fresh fruit ranged from 55.91% (Rosa multiflora) to 88.77% (Lonicera spp.) (Table 1). The average ash content of the dried fruits ranged from 3.22% (Viburnum opulus) to 8.38% (Lonicera spp.) (Table 1). The fruits of native shrubs had significantly higher energy density (S = 66.0, P < 0.001) and percent fat (S = 78.0, P < 0.001) than the fruits of invasive shrubs 176 Northeastern Naturalist Vol. 20, No. 1 Table 1. Average nutritional values ± standard error for native and invasive fruit species collected during October 2010. Sample sizes for fat, energy density, total soluble solids (TSS), and ash are provided in parentheses. Sample sizes for water content estimates are discussed in the Methods. Species Fat (% dry mass) Energy density (kJ/g dry mass) TSS (°Brix) Ash (% dry mass) Water (% fresh mass) Rhamnus cathartica 0.51 ± 0.16 (3) 16.77 ± 0.62 (3) 40.32 ± 1.14 (3) 3.87 ± 0.09 (3) 68.48 ± 0.91 Rosa multiflora 0.99 ± 0.18 (3) 17.17 ± 0.01 (2) – 6.98 ± 0.11 (3) 55.91 ± 1.20 Lonicera spp. 0.70 ± 0.25 (3) 16.29 ± 0.27 (3) 10.82 ± 0.77 (3) 8.38 ± 1.24 (3) 88.77 ± 0.21 Viburnum opulus 0.86 ± 0.11 (3) 16.73 ± 0.20 (3) 13.26 ± 0.29 (3) 3.22 ± 0.26 (3) 86.31 ± 0.87 Cornus racemosa 34.90 ± 1.71 (3) 27.16 ± 0.03 (3) 19.00 ± 0.14 (3) 3.54 ± 0.14 (3) 67.17 ± 0.30 Cornus amomum 6.57 ± 0.78 (3) 18.83 ± 0.18 (3) 15.79 ± 0.15 (3) 5.59 ± 0.18 (3) 79.83 ± 0.12 Cornus sericea 23.37 ± 0.96 (3) 20.74 ± 0.49 (2) 13.02 ± 0.53 (3) 4.99 ± 0.17 (3) 84.52 ± 0.65 Viburnum dentatum 48.72 ± 0.45 (3) 28.06 ± 0.11 (2) – 4.70 ± 0.01 (3) 58.88 ± 0.99 Lindera benzoin 48.01 ± 0.73 (3) 28.68 ± 0.28 (3) 15.25 ± 0.37 (3) 6.24 ± 0.12 (3) 66.05 ± 1.05 2013 S.B. Smith, S.A. DeSando, and T. Pagano 177 (Table 1). However, TSS was not significantly different between native and invasive fruits (S = 87.0, P = 0.41, Table 1). Energy density was positively correlated to with percent fat of fruits (rs[24] = 0.85, P < 0.001; Fig. 1). TSS was not significantly correlated with energy density (rs[20] = 0.27, P = 0.25) or percent fat of fruits (rs[21] = 0.15, P = 0.51). Fruit consumption by songbirds At RITBO, the proportion of fruit remaining on enclosed branches changed over time (time: F3,32 = 5.9, P = 0.003) and differed among species (species: F3,32 = 126.9, P < 0.001) (Fig. 2a). There was also a significant interaction between species and time (time x species: F9,32 = 3.4, P = 0.005; Fig. 2a). Cornus amomum lost its fruits faster than all other species (Fig. 2a). Also, the proportion of fruit remaining on unenclosed branches differed among species (species: F3,32 = 33.2, P < 0.001; Fig. 2a). Rhamnus cathartica and Rosa multiflora lost less fruit over time than Cornus racemosa and Cornus amomum (Fig. 2a). In addition, the 2 native species, Cornus racemosa and Cornus amomum, differed from each other in the rate of fruit loss, with Cornus amomum losing more fruit over time than Cornus racemosa (Fig. 2a). The proportion of fruit remaining on the enclosed branches at the BBBO site changed over time and among species (time: F3,32 = 5.6, P = 0.003; species: Figure 1. Relationship between average energy density and average fat content of native (solid symbols) and invasive (open symbols) fruits collected during fall 2010. There was a significant correlation between percent fat and energy density of fruits (rs[24] = 0.85, P < 0.001). 178 Northeastern Naturalist Vol. 20, No. 1 F3,32 = 9.9, P < 0.001; Fig. 2b), with Cornus racemosa, Cornus amomum, and Lonicera spp. losing fruit faster than Viburnum opulus (Fig. 2b). The proportion of fruit remaining on the unenclosed branches also changed over time and among species (time: F3,32 = 13.0, P < 0.001; species: F3,32 = 9.5, P = 0.001; Fig. 2b), with Cornus racemosa and Cornus amomum losing fruits faster than Lonicera spp. and V. opulus (Fig. 2b). Figure 2. Fruit consumption index at the RITBO site (a) and the BBBO site (b) during fall 2010. The consumption index was determined by the formula: 1 - (unenclosed % remaining / enclosed % remaining). The error bars denote the standard error for each group of fruit. 2013 S.B. Smith, S.A. DeSando, and T. Pagano 179 The fruit consumption indices indicated that the 2 native species, Cornus amomum and Cornus racemosa, were consumed at a faster rate over the study period than the invasive species at RITBO (Fig. 2a). At BBBO, invasive Lonicera spp. was consumed at the lowest rate, and Cornus racemosa was consumed at the highest rate over the study period (Fig. 2b). Insect abundance Ninety-five percent of the insects captured on the sticky traps were midges, and all were adults of similar size. At the RITBO site, the average number of insects per shrub species was significantly different for each week (time: F6,8 = 58.9, P < 0.001; Fig. 3). There was also a significant difference in overall insect abundance among the 4 species (species: F3,4 = 6.7, P = 0.049; Fig. 3), and there was a significant interaction between species and time (species x time: F6,8 = 8.8, P = 0.004). Cornus amomum and Rosa multiflora consistently had higher average numbers of insects over the 3 weeks than Cornus racemosa and Rhamnus cathartica (Fig. 3). Rhamnus cathartica had significantly lower numbers of insects captured than Cornus amomum and Rosa multiflora (Fig. 3). In addition, Rosa multiflora had significantly higher numbers of insects captured than Cornus racemosa (Fig. 3). Overall, the third week of the study period had the lowest number of flying insects (Fig. 3). Discussion Fruit nutritional analyses The native fruits in this study generally had higher percent fat and energy density than the invasive fruit species. We found a positive correlation between Figure 3. Average midge (Chironomidae) density measured over a 24-hr period at weekly intervals for 4 different fruit-bearing shrub species during spring 2011. The error bars denote the standard error for each species. 180 Northeastern Naturalist Vol. 20, No. 1 the mean percent fat and mean energy density of the fruits tested in our study, similar to findings by S.B. Smith et al. (2007). Our nutritional values, specifically fat content and energy density, were comparable to previously published data of similar fruiting plant species in the northeastern US (Borowicz and Stephenson 1985, Drummond 2005, Johnson et al. 1985, LaFleur et al. 2007, S.B. Smith et al. 2007, Witmer 1996). However, measured fat content of Cornus racemosa collected in Illinois (Johnson et al. 1985) was slightly higher than fat content of Cornus racemosa fruits in our study (39.86% vs. 34.90%). Variation in nutritional values of fruits among different studies may be attributed to the seasonal progression of fruit development. We collected fruit at the peak of fall migration; however, fruits analyzed at different points during the fall season may vary in nutritional content, particularly in regard to certain nutrients like fat or energy density (Borowicz and Stephenson 1985). Nevertheless, our results suggest that fruits of native species will have a relatively high nutritional value compared to invasive species present in the area during the peak of fall migration for songbirds. Fruit consumption by songbirds During the fall migration season, the native fruit species were consumed at a faster rate than the invasive species at both study sites, and the high consumption of native Cornus racemosa and Cornus amomum was consistent between both sites throughout the season. Further, native dogwood fruits that were consumed at the fastest rate also had the highest percent fat and energy density. This finding suggests fruit nutritional quality, particularly fat and energy content, influences selective consumption of different fruit species by birds during fall migrations. However, our sample sizes were relatively small and focused to cover the localized area of our study site. Continued monitoring of higher numbers of shrubs of each species and over multiple seasons will provide more robust evidence of selective consumption by migrating birds in relation to the nutritional content of fruits. Fruit selection by birds during the fall could also be influenced by other fruit characteristics such as ripeness, secondary compounds, size, shape, or color of the fruits (Gosper et al. 2005, Levey and Martínez del Rio 2001). At the RITBO site, the 2 invasive species, Rhamnus cathartica and Rosa multiflora, were consumed at very low rates, and at the BBBO site, Viburnum opulus and Lonicera spp. were consumed at similarly low rates. Plant secondary compounds can have adverse effects on the digestion and gut retention time of avian consumers (Levey and Martínez del Rio 2001). The lack of consumption of Rhamnus cathartica may be related to the presence of a toxic secondary compound that acts as a purgative, which could deter birds from consuming the fruits (Heimpel et al. 2010). Similarly, the low consumption rate of Viburnum opulus could be due to the tartness and unpalatability of the fruit (Jones and Wheelwright 1987). Rosa multiflora fruits tended to ripen later in the season than other fruits in our study area, which could also have affected consumption of the fruits by migrating birds. The low consumption of Lonicera spp. and Viburnum opulus could also be attributed to the high water content coupled with the low energy density of the fruits, which is consistent with observations that fruits of low-quality invasive honeysuckle tend to be avoided by birds in the fall and winter (Ingold and Craycraft 1983). 2013 S.B. Smith, S.A. DeSando, and T. Pagano 181 Insect abundance Few studies have investigated the relationship between insect density harbored by invasive and native shrubs during the spring migration period in combination with the value of shrubs as fruit-bearing food resources for migrating birds in the fall. Insects are an important source of protein for migrating songbirds (R.J. Smith et al. 2007), especially during the spring migration when fruits and seeds may be limited in their availability at stopover sites. Though we recognize that our study is limited to measures of flying insect (midge) abundance rather than direct observations of insect consumption by birds, our assessment may provide a useful approximation for comparing the spatial distribution of a common early spring food resource for birds in the area. Interesting patterns emerged from our data, and though there was no clear relationship between insect density and native or invasive shrub status in our study, there were differences in insect density among shrub species on the early spring sampling dates. Cornus amomum and Rosa multiflora had higher average insect density over the first 2 weeks of the insect study than Cornus racemosa and Rhamnus cathartica, a pattern that could be related to the phenology of leaf emergence for the shrubs. Rosa multiflora was the first of the 4 species to produce leaves during the spring in this study, followed by the 2 Cornus species, whereas Rhamnus cathartica produced its first leaves much later than the other 3 species. Our results may have been influenced by weather conditions at the study site in early spring. The spring of 2011 was cooler than 2010 and relatively wet compared to recent years, with the greatest total rainfall in the month of April since before 1940 and the highest rainfall for the month of May since 2002, according to NOAA weather records (NOAA 2011). Environmental conditions likely affected the timing of leaf out for the shrubs as well as the emergence of aquatic insects in the area. This portion of the study could be repeated in subsequent years to assess potential seasonal differences in insect density at the RITBO site. Conclusions and management implications The results of this study show that certain native fruiting shrub species are of higher quality to songbirds during fall migration than invasive species based on the nutritional quality of their fruits. Since we did not measure insect abundance harbored by all the focal shrub species used for fruit nutritional analysis in our study during the spring migration, we do not have a full understanding of the value of all of the shrub species during the spring. However, when considering the nutritional value of the fruits, migrating songbirds consumed more high-energy and high-fat native dogwood fruits; therefore, these native dogwood species are likely to have greater value for songbirds in the fall than the invasive species included in our study. Results of our study suggest that the growth of nutritionally valuable native shrubs in areas that are important stopover sites during fall migration should be encouraged to help support high-quality stopover habitats. It is important to note that the fruits of all native woody species may not be nutritionally beneficial to birds. High-quality shrub species that are likely to be consumed by birds may be identified by a strong, positive correlation of fat content and energy density of the fruits. Our study focused on fat content and 182 Northeastern Naturalist Vol. 20, No. 1 energy density of fruits; however, future studies should expand the nutritional analyses of these fruits to include protein, fiber content, and micronutrients such as amino acids and plant secondary compounds that may be beneficial to birds in terms of balancing oxidative stress (e.g., plant polyphenols) or act as deterrents to avian consumers, in order to obtain a fuller representation of the value of the fruits in terms of nutritional content. Future studies should also focus on direct observations to assess avian fruit selection in order to more strongly link fruit loss and consumption by songbirds. Invasive fruits did not appear to be of greater nutritional value for migrating songbirds during the fall compared to native species, and invasive shrubs did not clearly support more insects for birds in the spring. Therefore, it appears that the removal of invasive species like Rhamnus cathartica, Rosa multiflora, Viburnum opulus and Lonicera spp. would not negatively impact populations of migrating songbirds in the study area. The presence of invasive fruit-bearing shrubs may further affect habitat quality for bird populations during breading and nesting periods (Borgmann and Rodewald 2004, McCusker et al. 2010, Rodewald et al. 2010, Schmidt and Whelan 1999). Although the fruits of native species were removed at the highest rates in our study during fall migration, the fruits of invasive species may be consumed by wintering birds if they persist throughout the winter months (Drummond 2005, Greenberg and Walter 2010, Stiles 1982). Future studies could continue the consumption measurements into the late fall and winter to determine if persisting fruits are selectively consumed by local wintering birds. It may also be important to assess annual and regional variation in nutritional quality, in addition to variation in insect abundance, over multiple seasons to gain a broader and more widely applicable understanding of the value of fruits and fruit-bearing woody plant species for migrating birds. Acknowledgments The authors thank the Braddock Bay Bird Observatory for permission to conduct parts of this study on the observatory grounds and the staff and student volunteers at the RIT Bird Observatory for field support. We especially thank Allyson Miller, Stephanie Schroeder, Matthew Robbins, and Gloria Wink for technical assistance with laboratory analyses. We also thank the editor and two anonymous reviewers for their helpful comments on a previous version of this manuscript. Funding for this study was provided by the Thomas H. 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