The effects of flying insects and urbanization on insectivorous bird communities in northern Wisconsin and Madison

Author / Creator

Schilke, Paul, author

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Summary

North American temperate breeding bird species have experienced large scale population declines. While causes for these declines, such as loss of habitat, have been well studied, for some groups of...

North American temperate breeding bird species have experienced large scale population declines. While causes for these declines, such as loss of habitat, have been well studied, for some groups of birds the causes are not well understood. One group of birds that have experienced significant declines are aerial insectivores, which feed on flying insects, many of which are emergent aquatics that depend on lakes and streams to complete their life cycle. And while conversion of native habitat to urban centers is clearly accompanied by a reduction in bird species richness, it's not clear how declining species may respond to urbanization when major natural elements, such as mature trees, are retained. My first two goals were to determine how emergent aquatic insects are distributed around lakes in north-temperate forests and whether aerial insectivorous birds and the larger insectivorous bird community responds to flying insect abundance. My third goal was to determine how urban neighborhoods with different degrees of tree cover shape bird communities in Madison, WI. To determine how insects were distributed around lakes I used transects of insect sticky traps placed around five lakes in northern Wisconsin. The insect traps were deployed during the avian breeding season of late May through August in 2013-2015. To determine the composition of the bird community I conducted point counts along the same transects on which insect trapping occurred. Birds were surveyed at points close to lakeshores and points farther inland. I determined the distances where insect abundance peaked using generalized additive models. I determined whether bird abundance was correlated with insect abundance over time using multilevel models. To determine how birds were affected by urbanization I surveyed bird communities in neighborhoods of different ages in Madison in 2016-2017. I used multilevel models to determine whether tree cover, distance to lakes or streams, and neighborhood age were correlated with the abundance of different foraging guilds of birds. Flying insect abundance around northern lakes was generally highest closer to lakes than farther from lakes, but exhibited high variability over time, over distance from the lake, and depending on the taxon. Diptera comprised the greatest abundance and biomass of insects and peaked in abundance between 10 and 20 m from lakeshores. The bird community tracked the abundance of insects in time and space, with aerial insectivorous birds significantly more abundant when and where insect abundance was high. Ground foraging birds were less abundant near roads and lakeshores. In urban areas aerial insectivorous bird were more abundant in older neighborhoods and gleaning birds were associated with higher urban tree cover. However the bird community of Madison was primarily composed of urban adapted species. The patterns of insect abundance I observed around north temperate lakes are consistent with patterns observed around streams and in theoretical models, with flying insect abundance highest close to shore and rapidly decreasing at distances greater than 20 m inland from shore. The correlation between aerial insectivore abundance and aerial insect abundance revealed by my analysis suggests that these birds track flying insects during the breeding season and thus that the insects are important to maintaining populations of aerial insectivores. I found that urbanization had a strong effect on the composition of bird communities in Madison. Areas with higher tree cover had greater bird abundance and diversity, but bird communities in urban Madison differed substantially from communities in native natural savanna, suggesting that not only tree cover, but other natural elements are required to make cities, even those with remnant large oaks, more hospitable to breeding insectivorous birds.

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