Are urban forests ecological traps for understory birds? An examination with northern cardinals (Cardinalis cardinalis)
Lionel Leston, MS
Advisor: Amanda Rodewald
Although urbanization is one of the most ubiquitous and profound changes underway in landscapes throughout the world, its ecological consequences are not entirely understood. Most previous studies have examined only general patterns of diversity and community structure within urban habitats and have generally documented declines in many native species (e.g. Neotropical migrants) and increases in exotic species and a few native species. A variety of explanations for these patterns have been proposed, such as reductions in habitat area that usually accompany urbanization, land uses within the matrix between habitats (e.g. agriculture or urban development), disturbance by human activity, and changes in nesting substrates, food resources, predator communities, or competitors. Although many studies have focused on the negative ecological consequences of urbanization upon species, few studies have explicitly examined processes (e.g. habitat selection, demographics) responsible for high densities of urban-associated native birds such as Northern Cardinals (Cardinalis cardinalis). Whether or not urbanizing landscapes are really good for some native species that seem to increase with urbanization remains poorly answered. The Northern Cardinal is an excellent model species for addressing this question. Although previous research shows that Northern Cardinals increase in urbanizing landscapes, perhaps because they benefit from urban-associated increases in fruiting exotic shrubs, bird feeders, or winter temperature, some research suggests that cardinals nesting in urban forests experience greater rates of nest depredation than those in more rural forests. This could be a serious cost to urban birds if repeated nesting attempts do not compensate for predation. Thus, urban forests may represent an ecological trap for Northern Cardinals, whereby birds actively select habitats that ultimately reduce their fitness. For example, fruiting exotic shrubs that increase with urbanization, such as Lonicera are frequently used as nesting substrate and food by cardinals, and may provide a “false” cue for quality habitat. The existence of an ecological trap would have serious implications for how and where we manage and maintain habitat for wildlife.
The central purpose of my research was first to determine if and why Northern Cardinals are more abundant in urban forests, and second, to evaluate if urban riparian forests might constitute an ecological trap for this species and other understory birds with similar nesting habits. I identified cues used by cardinals to select habitats and evaluate if these cues were related to urbanizing landscapes and to the reproductive success of cardinals. Cardinals were surveyed within either rural (n = 6) or urban (n = 6) landscapes at riparian forest sites in central Ohio in the breeding (April - August) and non-breeding (December - February) seasons 2003 - 2005. In both seasons, habitat features, including vegetation structure, floristics, winter temperatures, and direct and indirect estimates of food resources, were measured within occupied cardinal territories, nest patches and randomly and systematically-located plots. Fate and productivity of 288 cardinal nests were monitored over both years and hourly provisioning rates were recorded at 24 nests in 2003 and 96 nests in 2004. Discriminant function analysis was used to identify differences between random and nest plots as well as between plots in urban and rural landscapes. An information theoretic approach was used to select the most parsimonious models for predicting nest fate, reproductive productivity, and abundances of cardinals in the breeding and non-breeding seasons.
Cardinals occupied forests in urban landscapes in 67% greater densities in the breeding season and over 500% greater abundances in the winter than cardinals in rural forests. Results suggest that these differences in abundance stem from urban-associated changes in habitat features used by cardinals to select habitats. In particular, cardinals selected nest sites containing 1.42 times as many tree stems and nearly 2.96 times as many exotic shrub stems, especially Lonicera spp., which were respectively 1.16 and 1.38 times denser in urban forests. At the site scale (2 ha), cardinals were most abundant in forests with greater minimum January temperatures, lower canopy heights, and more nearby bird feeders, which were positively associated with urban forests. Thus, urban forests appear to have more habitat features typically used by cardinals to select habitats compared to rural forests, and such habitat features seem responsible for their high densities in urban forests. Interestingly, there appeared to be some mismatches between the habitat features that cardinals used to select habitat and how those features affected reproductive success. In particular, the strong selection for exotic shrubs seems maladaptive, because nests in exotic shrubs were over twice as likely to fail as nests in native trees and shrubs. The tendency of cardinals to occupy areas with relatively lower canopy height also may be maladaptive in urban forests, as nest sites with lower canopies had higher densities of exotic shrubs and thus an increased risk of nest predation. In addition, birds nesting lower in substrates (which presumably would happen more often in low stature forests) had lower productivity. Although these mismatches were evident at the level of individual birds, there were no significant differences in productivity and return rates of adults between urban and rural forests as a whole. Thus, these findings provide little support for the idea that urban forests represent ecological traps for synanthropic understory birds. Ultimately, data on lifetime reproductive success of individual birds in urban and rural sites are necessary to evaluate if urban forests constitute ecological traps.