Daniel P. Shustack

Anthropogenic resource subsidies decouple predator—prey relationships

The extent to which resource subsidies affect food web dynamics is poorly understood in anthropogenic landscapes. To better understand how species interactions are influenced by subsidies, we studied breeding birds and nest predators along a rural-to-urban landscape gradient that varied in subsidies provided to generalist predators. We hypothesized that resource subsidies in urban landscapes would decouple predator-prey relationships, as predators switch from natural to anthropogenic foods. From 2004 to 2009, we surveyed nest predators and monitored 2942 nests of five songbird species breeding in 19 mature forest stands in Ohio, USA. Eighteen species were video-recorded depredating nests. Numbers of avian and mammalian nest predators were positively associated with the amount of urban development surrounding forests, with the exception of Brown-headed Cowbirds (Molothrus ater). Although nest survival strongly declined with detections of nest predators in rural landscapes, nest survival and predator numbers were unrelated in urban landscapes. Thus, the strength of interaction between breeding birds and nest predators diminished as landscapes surrounding forested parks became more urbanized. Our work suggests that decoupling of predator-prey relationships can arise when synanthropic predators are heavily subsidized by anthropogenic resources. In this way, human drivers can alter, and completely disarticulate, relationships among species that are well established in more natural systems.

Exotic shrubs as ephemeral ecological traps for nesting birds

Certain exotic plants may increase risk of nest predation, and, in this way, may act as ecological traps. We hypothesized that the greater vulnerability to predation was a consequence of either (1) reduced nest height due to architectural differences among plant species or (2) seasonal changes in the distribution of nests among forest strata. To test this, we examined temporal variation in nest survival of 888 nests of Northern Cardinal (Cardinalis cardinalis) in native substrates and two exotic shrubs (Lonicera maackii and Rosa multiflora) in Ohio, USA, 2001–2006. We evaluated evidence for an ecological trap by monitoring the annual reproductive productivity of 245 breeding pairs of cardinals. Only nests in Rosa experienced relatively constant survival rates across the season, whereas probability of survival increased over the season for nests in other substrates. Interestingly, the relative vulnerability of nests in different substrates varied across the season. Most strikingly, nests in Lonicera in early spring showed the lowest survival rates but exceeded survival rates of nests in native substrates late in the season. Nest height failed to explain seasonal changes in nest survival, as only nests in native plants significantly increased in height as the season progressed. Rather, predation risk seemed to be a function of the proportion of nests within each substrate, as illustrated by the decreased predation in Lonicera as the relative proportion of nests in native substrates increased. The patterns of temporal variation in predation risk that we detected show that impacts of Lonicera are not a function of plant architecture alone and may be related to leaf phenology, changes in nest density, nest site location, and/or nest synchrony. Examination of the reproductive productivity of cardinals showed that pairs that made their first nest attempt in Lonicera fledged 20% fewer cardinal young than birds that began the season using other substrates. Thus, we suggest that exotic plants may represent an ephemeral ecological trap for certain nesting birds, where negative effects persist only during certain periods.

CONSUMER RESOURCE MATCHING IN URBANIZING LANDSCAPES: ARE SYNANTHROPIC SPECIES OVER-MATCHING

Population responses of synanthropic species to urbanization may be explained by the resource-matching rule, which postulates that individuals should distribute themselves according to resource availability. According to the resource-matching rule, urban habitats will contain greater densities if they provide better resources than rural habitats. However, because resource availability is density dependent, individuals in urban areas would ultimately achieve fitness levels comparable to, but no better than, individuals in less urban areas. Some ecologists suggest that synanthropic birds may not conform to the resource-matching rule and may instead overmatch (i.e., overexploit) in urban habitats, ultimately leading to lower fitness despite greater resource levels. Using the Northern Cardinal (Cardinalis cardinalis) as a focal species, we evaluated if Cardinal populations in urban and rural habitats were consistent with predictions of consumer resource matching. During 2003-2006 we documented population density, adult body condition, apparent survival, and annual reproductive productivity of Cardinals in riparian forest stands within urban (n = 8 stands) and rural (n = 6 stands) landscapes in Ohio, USA. Density of Cardinals in urban forests was four times that found in more rural forests. Mark-resight data from 147 males and 125 females over four years indicated that apparent survival rates were similar between urban and rural landscapes (phi = 0.64, SE = 0.039 for males and phi = 0.57, SE = 0.04 for females). Similarly, body condition indices of 168 males, 142 females, and 118 nestlings did not differ significantly between landscapes. Annual reproductive productivity (mean number of fledglings per pair over breeding season) of 294 pairs was comparable for urban (2.4 +/- 0.18 [mean +/- SE] and rural (2.1 +/- 0.18) young birds. Thus, contrary to recent suggestions, we find that high densities of certain synanthropic species in urban landscapes are consistent with expectations of consumer resource matching.

Dynamic selective environments and evolutionary traps in human-dominated landscapes

Human activities can alter selective environments in ways that can reduce the usefulness of certain ornamental traits as honest signals of individual quality and, in some cases, may create evolutionary traps, where rapid changes in selective environments result in maladaptive behavioral decisions. Using the sexually dichromatic, socially monogamous Northern Cardinal (Cardinalis cardinalis) as a model, we hypothesized that urbanization would erode the relationship between plumage coloration and reproductive success. Because the exotic Amur honeysuckle (Lonicera maackii) provides carotenoids, is a preferred habitat attribute, and increases vulnerability to nest predation, we predicted the presence of an evolutionary trap, whereby the brightest males would achieve the lowest reproductive success. Working at 14 forests in Ohio, USA, 2006-2008, we measured plumage color, monitored reproduction, and quantified habitat within territories. In rural landscapes, the brightest males bred earliest in the season and secured more preferred territories; however, annual reproduction declined with plumage brightness. Coloration of urban males was not associated with territory attributes or reproduction. Female redness across all landscapes was negatively related to reproduction. Poor reproductive performance of otherwise higher-quality males probably resulted from preferences for honeysuckle, which reduces annual reproduction when used as a nesting substrate early in the season. In this way, exotic shrubs prompted an evolutionary trap that was avoided in urban forests where anthropogenic resources disassociated male color and reproductive phenology and success. Our study illustrates how modified selective environments in human-dominated landscapes might shape microevolutionary processes in wild bird populations.

Springtime in the city: exotic shrubs promote earlier greenup in urban forests

Despite the widespread recognition that urban areas are frequently dominated by exotic and invasive plants, the consequences of these changes in community structure have not been explicitly considered as an explanation for the pattern of advanced leaf phenology, or early greenup, reported in many urban areas. As such, we evaluated two hypotheses that could account for advanced greenup in forests along an urban to rural gradient: advanced phenology within individual species or differences in woody plant community. We monitored the spring leafing phenology of Aesculus glabra (Ohio buckeye), Lonicera maackii (Amur honeysuckle), and Acer negundo (box elder) in 11 forests spanning an urban to rural gradient in central Ohio, USA. From February to April 2006, we monitored these species, recorded woody plant composition, and documented daily minimum and maximum temperatures at each site. We found a weak but general trend of advanced phenology within species in more urban landscapes. Monthly average minimum temperatures were higher with increasing urbanization while monthly average maximum temperatures were similar across the urban to rural gradient. We also found evidence for shifts in woody plant communities along the urbanization gradient, mainly driven by the abundance of L. maackii, an invasive exotic species, in the more urban forests. Because L. maackii leafs out weeks earlier than native woody species and is very abundant in urban forests, we suggest that the invasion of forests by this species can generate earlier greenup of urban forests.

Variation in plumage coloration of Northern Cardinals in urbanizing landscapes.

Abstract Biologists know relatively little of how one of the most important avian phenotypic signals, feather coloration, may be affected by anthropogenic changes resulting from urbanization. We examined the relationship between urbanization and carotenoid-based plumage color of Northern Cardinals (Cardinalis cardinalis) in 13 riparian forests distributed across a rural-to-urban landscape gradient in central Ohio, USA. Feathers and morphometric measurements were collected from breeding territorial males (n  =  129) and females (n  =  145) during March–August 2006–2008. Plumage brightness of males, but not females, increased with body condition (i.e., size-adjusted mass) and declined with amount of urbanization surrounding forests in which cardinals bred. The extent plumage coloration reflected condition was partially mediated by landscape composition. Specifically, the relationship between brightness and body condition was most pronounced in the most rural landscapes. The interdependency of male coloration and body condition may be more relaxed in urban than rural landscapes if carotenoid-rich foods from anthropogenic and/or invasive sources are more available, or are accessible to birds across a wide range of condition.

Habitat Use Patterns of Bobolinks and Savannah Sparrows in the Northeastern United States

In the northeastern United States, grassland birds regularly use agricultural fields as nesting habitat. However, birds that nest in these fields regularly experience nest failure as a result of agricultural practices, such as mowing and grazing. Therefore, information on both spatial and temporal patterns of habitat use is needed to effectively manage these species. We addressed these complex habitat use patterns by conducting point counts during three time intervals between May 21, 2002 and July 2, 2002 in agricultural fields across the Champlain Valley in Vermont and New York. Early in the breeding season, Bobolinks (Dolichonyx oryzivorus) used fields in which the landscape within 2500 m was dominated by open habitats. As mowing began, suitable habitat within 500 m became more important. Savannah Sparrows (Passerculus sandwichensis) initially used fields that contained a high proportion of suitable habitat within 500 m. After mowing, features of the field (i.e., size and amount of woody edge) became more important. Each species responded differently to mowing: Savannah Sparrows were equally abundant in mowed and uncut fields, whereas Bobolinks were more abundant in uncut fields. In agricultural areas in the Northeast, large areas (2000 ha) that are mostly nonforested and undeveloped should be targeted for conservation. Within large open areas, smaller patches (80 ha) should be maintained as high-quality, late-cut grassland habitat. RÉSUMÉ. Dans le Nord-est des États-Unis, les oiseaux de prairie nichent régulièrement dans les champs agricoles. Cependant, la nidification de ces oiseaux échoue souvent en raison des activités agricoles, comme le fauchage et le broutement. Il s’avère donc nécessaire de connaître les caractéristiques spatio-temporelles relatives à l’utilisation de l’habitat pour gérer efficacement ces espèces. Nous avons examiné ces patrons complexes de l’utilisation de l’habitat au moyen de dénombrements par points d’écoute effectués à trois périodes entre le 21 mai et le 2 juillet 2002, dans des champs agricoles de la vallée de Champlain dans les États du Vermont et de New York. Tôt dans la saison de nidification, les Goglus des prés (Dolichonyx oryzivorus) ont utilisé des champs situés dans des paysages où les milieux ouverts dominaient dans un rayon de 2 500 m. Lorsque le fauchage a commencé, l’habitat propice dans un rayon de 500 m est devenu plus important. Les Bruants des prés (Passerculus sandwichensis) ont utilisé des champs qui offraient un habitat propice dans un rayon de 500 m dès le début de la saison. Une fois le fauchage terminé, les caractéristiques des champs (c.-à-d. la dimension et la quantité de lisières boisées) sont devenues davantage déterminantes. Les deux espèces ont agi différemment face au fauchage : les bruants ont utilisé les champs fauchés et les champs non fauchés de façon égale, tandis que les goglus étaient plus nombreux dans les champs n’ayant pas subi de fauchage. Dans les zones agricoles du Nord-est, les vastes régions (2 000 ha) University of Vermont, The Rubenstein School of Environment and Natural Resources, U.S. Geological Survey, Vermont Cooperative Fish and Wildlife Research Unit Avian Conservation and Ecology 5(2): 11 http://www.ace-eco.org/vol5/iss2/art11/ qui ne sont ni boisées ni développées devraient être considérées à des fins de conservation. Dans les grands milieux ouverts, des parcelles plus petites (80 ha) – pour lesquelles le fauchage serait retardé – devraient être conservées en tant que milieux de prairie de qualité supérieure.

Attenuated Nesting Season of the Acadian Flycatcher (Empidonax virescens) in Urban Forests

ABSTRACT. Resident or short-distance migrant birds that occupy urban habitats are known to breed earlier than nearby rural populations, but similar data on long-distance tropical migrants are lacking. To understand the relationship between urbanization and reproductive phenology of a tropical migratory bird species, we (1) described variation in patterns and consequences of breeding phenology across an urban-to-rural gradient and (2) assessed underlying reasons why phenology may be related to urbanization. We studied Acadian Flycatchers (Empidonax virescens) between 2001 and 2007 in 35 forests spanning an urban-to-rural gradient in central Ohio. The general pattern of breeding phenology was opposite of that described for most resident and short-distance migrant birds. At higher levels of urbanization, site arrival dates and clutch initiations were later and cessation of breeding was earlier than in forests of more rural landscapes. The phenological shifts reduced the length of the nesting season in urban landscapes, with the result that birds in more urban landscapes initiated fewer nests and fledged fewer young than their rural-breeding counterparts. Underlying causes of phenological differences are less clear. One possibility supported by our data is that urban forests are less desirable and selected later by smaller females, which initiated first clutches later in the nesting season than larger females. This study provides the first evidence that urbanization is associated with attenuated nesting seasons for tropical migratory birds and that this shift in breeding phenology may have reproductive consequences.

A method for detecting undervalued resources with application to breeding birds

Anthropogenic changes to ecosystems can decouple habitat selection and quality, a phenomenon well illustrated by ecological traps in which individuals mistakenly prefer low-quality habitats. Less recognized is the possibility that individuals might fail to select high-quality habitat because of the absence of some appropriate cue. This incorrect assessment of resource quality can lead to relatively high-quality resources being undervalued, whereby they support fewer individuals than optimal. We developed a habitat selection model to predict the expected patterns in patch-level density, fitness, and individual quality derived from either accurate assessment of habitat quality or from undervaluing of habitat patches (i.e., quality is not correctly assessed). Unlike previous habitat selection models, we explicitly and simultaneously incorporated variation in both individual and habitat quality into our estimates of realized fitness. Although multiple mechanisms can reduce patch-average density, fitness, and individual quality in less preferred patches, only undervaluation results in the occupation of higher-quality territories by similar-quality individuals in less preferred vs. preferred patches. We then looked for evidence of undervaluation in our seven-year data set of Acadian Flycatchers (Empidonax virescens) occupying forests in urbanizing landscapes in Ohio, USA. We suspected that forests within more urban landscapes may be undervalued in our study system because (1) urban forests typically support lower densities of Neotropical migratory birds than rural forests and (2) anthropogenic disturbance and habitat alterations are likely to result in mismatches between cues typically used in habitat selection and actual habitat quality. In contrast to our predictions, field data suggest that urban forests are not undervalued. Our work not only expands upon previous habitat selection models by considering undervaluation, but also demonstrates how predictions derived from our model can be tested using a long-term empirical data set.

Diet explains red flight feathers in Yellow-shafted Flickers in eastern North America

ABSTRACT Yellow-shafted Flickers (Colaptes auratus auratus subspecies group of the Northern Flicker) occasionally have orange to red flight feathers in eastern North America far from the hybrid zone with the Red-shafted Flicker (C. a. cafer subspecies group). Blocks of feathers of anomalous color tend to show bilateral symmetry and may differ from one year to the next in the same bird. It has been suggested that hybridization with cafer must be the source of the red color and that selection for that color could explain the high incidence of red or orange shafts in eastern flickers in some localities. Complex, though largely unproven, physiological mechanisms have been hypothesized to explain the variegated look. We evaluated a simpler, dietary explanation for the variation: that the pigment rhodoxanthin acquired exogenously at the time of feather molt comes to rest alongside the carotenoids normally present in these feathers. An exogenous source of rhodoxanthin exists in the berries of nonnative bush honeysuckles (Lonicera spp.) now naturalized in eastern North America and the American Midwest. We confirm the presence of rhodoxanthin and a probable metabolite, rather than the 4-keto-carotenoids found in the Red-shafted Flicker, in the red flight feathers of 2 Yellow-shafted Flickers from eastern North America. In these individuals, dietary rhodoxanthin appeared to interfere with the production of lutein, the main carotenoid in auratus. A fairly abrupt appearance of red color in earlier-molted primaries (usually p4 or p5) followed by its gradual fading in subsequent primaries in a large series of aberrantly colored flickers from eastern North America and the American Midwest supports a dietary explanation. We use data on the timing of replacement of primaries in the Northern Flicker at Manomet in Plymouth, eastern Massachusetts, to infer that these aberrantly colored Yellow-shafted Flickers on average acquired the unusual red pigment in early August.