Impacts of soil calcium availability and non-native plant invasions on an urban forest bird community
University of Delaware
Recent studies suggest that the effects of natural and anthropogenic caused decreases in soil calcium availability result in the decline of migrant bird species and reduced avian reproductive success; however few studies have demonstrated trends in multiple avian species’ habitat selection and nest success in relation to natural soil calcium characteristics. Herein I present results on how the availability of soil calcium affected snail abundance, understory non-native plant invasions, and breeding forest bird territory density and nest success in forest fragments. In 2010 and 2011, I sampled 21 forest fragment sites in Newark, Delaware, USA and estimated soil calcium, vegetation composition, snail abundance, and avian territory density. I used linear regressions to determine relationships between exchangeable calcium, snails, and vegetation with species’ territory densities. I utilized the unmarked package in R to fit hierarchical models to bird count data to estimate abundance, while accounting for observational and site covariates. I found that snail abundance and non-native shrub density were positively related to exchangeable calcium levels. Northern Cardinals (Cardinalis cardinalis) was the only species which had greater territory densities in areas with more non-native shrubs. Soil calcium did not serve as the top model to determine any species abundance, nor did the percentage of non-native stems. I hypothesized that soil calcium may have a weaker influence in the abundance of urban forest bird populations, compared to continuous habitats. Additionally, soil calcium may be an artifact of stand age and vegetation structure, which ultimately influences where avian species establish territories. Within the same sites in 2009-2011, I searched for and documented the outcomes of the nests of four songbird species: American Robin (Turdus migratorius), Gray Catbird (Dumetella carolinensis), Northern Cardinal, and Wood Thrush (Hylocichla mustelina). I modeled nest failure against landscape variables, vegetation, and nest characteristics in MCestimate, a Markov chain algorithm program used for estimating daily nest failure probabilities. Soil calcium availability was negatively related to American Robin nest success, and positively related to Northern Cardinal nest success. Northern Cardinals may benefit from an increase of soil calcium within their nest location for supplemental calcium, while American Robins may obtain calcium from other sources. Daily nest failure was positively related to date in the Gray Catbird, while Wood Thrush nest failure rates responded positively to nest height and negatively to vertical vegetation density. Stand structure and nesting habitats seem to drive daily nest failure across all four species, so forests should be maintained and edge area reduced to promote the nest success of a variety of urban forest birds.