Effects of Exotic Morrow's Honeysuckle (Lonicera morrowii) on Migratory Songbirds.

摘要

In much of the eastern United States, bush honeysuckles (Lonicera spp.) threaten the integrity of old-fields used as songbird habitat. From 2007--2009, we studied old-field systems in two areas of southwestern Pennsylvania: Fort Necessity National Battlefield and Ohiopyle State Park. Old fields at Fort Necessity consisted of a dense cover of Morrow's honeysuckle (Lonicera morrowii), whereas those at Ohiopyle were relatively free of this shrub species. We captured birds in mist nets during the breeding and post-breeding seasons to assess species composition at both locations and found that the species composition of breeding birds was correlated with the presence of Morrow's honeysuckle but that the species composition of post-breeding birds and juveniles was correlated with other microhabitat variables.;To determine if Morrow's honeysuckle had an impact on reproductive success, we examined the nesting success of Field Sparrows (Spizella pusilla) based on the use of Morrow's honeysuckle as a substrate and on the density of Morrow's honeysuckle surrounding each nest site. We developed a population model to determine if reproductive success was sufficient to maintain a stable population. Nest success was not different at Fort Necessity and Ohiopyle and was combined for both sites giving a nest success rate of only 16.8% +/- 4.8 yielding 1.21 female offspring/adult female. This number was likely not sufficient to maintain a stable population, resulting in a population sink at both locations. We used an information-theoretic approach to model nesting success as a response to nest substrate, % cover of Morrow's honeysuckle, midpoint date of each nest, and clutch size. An averaged composite model indicated that Morrow's honeysuckle had a negative impact on field sparrow nesting success both as a substrate and when found in dense patches surrounding the nest, thereby potentially causing a further decline in field sparrow populations.;We examined the effect of Morrow's honeysuckle on arthropod availability to determine if Morrow's honeysuckle was acting as an ecological trap for songbirds by decreasing the availability of food sources while increasing nest predation. We used territory mapping to delineate Field Sparrow breeding territories, yet found no difference in territory density. We used the availability of dominant arthropod taxa in communities both with and without Morrow's honeysuckle as an indicator of habitat quality and resource availability. The availability of arthropod taxa frequently used by songbirds was reduced by as much as 75% in areas dominated by Morrow's honeysuckle. We used body condition as an indicator of social dominance to determine if songbirds preferentially chose to establish territories in areas dominated by Morrow's honeysuckle. We then compared the availability of major arthropod taxa to changes in body condition of five songbird species over the breeding season to assess the potential impact of resource changes on songbird health and fitness. Of the five species studied, only Field Sparrows and American Goldfinches (Carduelis tristis) had significantly greater body condition at the beginning of the breeding season in areas with a high density of Morrow's honeysuckle; moreover, only Field Sparrows showed a significant change in body condition in response to habitat differences. Although Field Sparrows that chose territories in areas with a high density of Morrow's honeysuckle were significantly larger than those selecting territories comprised of native vegetation, they experienced a greater decline in body condition over the breeding season. Our results indicate that Field Sparrows show a preference for areas with a high density of Morrow's honeysuckle, yet these areas may provide fewer resources over the breeding season causing a decline in overall fitness. Consequently, our research suggests that in addition to contributing to the formation of a population sink, Morrow's honeysuckle may also result in the formation of an ecological trap for some early successional songbird species.