Aerial Insectivorous Bird Ecology and Conservation

Goal: To evaluate factors that influence riparian aerial insectivorous birds at the individual, population, and community levels.

Background and Rationale: North American birds that feed on aerial insects are experiencing widespread population declines (reviewed in Nebel et al. 2010). The patterns of decline are more prevalent in northeastern North America, and are greater in long-distance migrants. For example, data from the second Ontario Breeding Bird Atlas suggest major decreases in the occurrence of swallows, swifts, and nightjars between 1981-85 and 2001-05, with particularly rapid decreases in populations of Bank Swallows, Cliff Swallows, Barn Swallows, Chimney Swifts, and Eastern Kingbirds (ABBO 2013). Because aerial insectivores rely on flying insects, they are particularly sensitive to changes in insect populations both terrestrial and aquatic, as many aerial insectivores depend on aquatic insects that emerge from the water as adults.  In fact, high omega-3 long-chain polyunsaturated fatty acid (LCPUFA) content found in aquatic insects has been shown to be more important to Tree Swallow nestling growth than food quantity (Twining et al. 2016). Thus, whereas the cause for declines in aerial insectivores is unknown, population declines across species within the foraging guild potentially implicate decreasing or fluctuating insect food quality and quantity. In the North, mismatches between spring arrival dates and the timing of food availability caused by climate change can lead to starvation, loss of body condition, and mortality in some insectivorous bird species (Both et al. 2006, Fraser et al. 2013). Therefore, changes in climate may also be an important factor in declines of aerial insectivorous birds, including Tree Swallows and Eastern Phoebes, which seemingly have experienced more severe declines farther north. Environmental contaminants and habitat loss have also been considered as potentially contributing to declines in aerial insectivorous bird populations. In this project, we focus on riparian ecosystems, as they tend to harbor high levels of bird diversity, and aquatic emergent insects may be a key linkage between aerial insectivorous birds and changes in water quality.

Some key findings of our work include:

1. Urban-nesting Tree Swallows initiated laying 7.9 days earlier and fledged 35% more young per nest than those at protected sites (see animated plots below for results by year; click on plot for full-sized image and caption).  Local climate conditions and measures of water quality were identified as drivers behind these patterns.  Local climate, in particular, was associated with earlier breeding and and fledgling success; the higher overall temperatures characteristic of urban sites may therefore offer a longer breeding season and more favorable conditions for egg and nestling survival.

2.  Hg concentrations were elevated in urban-nesting Tree Swallows.  Concentrations were 482% greater in adult swallow blood at urban sites than at protected sites.  We developed an Urban Stream Index which reflects a suite of continuous variables of urbanization (e.g., tree canopy cover, nutrient concentrations), and this index strongly predicted Hg concentrations in adult swallows along a gradient of urbanization (see figure below).

Concentrations of Hg in adult Tree Swallow blood samples (2014–2016), by land use type: pink = protected sites, blue = urban sites. Error bars = +/- 1 SE.

3.  Emergent aquatic insects constituted an important component of Tree Swallow diets.  Tree Swallows at urban and protected sites relied on emergent insects for 37.4 and 30.8% of their nutritional subsidies, respectively.  The dietary/nutritional link between emergent insects and swallows in our study system was evidenced by the positive relationship we found between between δ13C values of aquatic emergent insects and Tree Swallows (see figure below).

Relationships between δ13C of Tree Swallow blood and δ13C of emergent aquatic insects from the two numerically dominant families, 2014-2017 at urban (gold) and protected (gray) study sites. Solid lines = relationship with emergent aquatic insects; dashed lines = terrestrial insects.

Read more about our findings in these stories from OSUNews and The Wildlife Society.

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Hayes, J.T. and S.M.P. Sullivan. Beyond the water’s edge: interactive effects of land use and water quality on community dynamics of aerial insectivorous birds. Poster. Ohio Biodiversity Conservation Partnership.  Nov. 2019. Columbus, OH.

Corra, J.W. and S.M.P. Sullivan.  Linking water quality and Tree Swallow populations across an urban-forested gradient. Oral.  Society for Freshwater Science Annual Meeting.  May 2018. Detroit, MI.

Corra, J.W. and S.M.P. Sullivan.  Aerial insectivorous birds: conservation across the aquatic-terrestrial boundary. Oral.  Ohio Biodiversity Conservation Partnership.  Nov 2017.  Columbus, OH.

Corra, J.W. and S.M.P. Sullivan.  Aerial insectivorous birds: conservation across the land-water interface. Poster. Society for Freshwater Science Annual Meeting.  May 2017. Raleigh, NC.

Tyl, R. and S.M.P. Sullivan. Variability in Tree Swallow reproductive success and body condition in urban and protected riparian areas of central Ohio: preliminary evidence. Poster. Ohio Biodiversity Conservation Partnership.  Nov. 2015.  Columbus, OH.


Sullivan, S.M.P., Corra, J.W., and J.T. Hayes. Urbanization mediates the effects of water quality and climate on a model aerial insectivorous bird. Forthcoming.  Ecological Monographs.

Sullivan, S.M.P., D.W.P. Manning, and R.P. Davis. 2018. Do the ecological impacts of dam removal extend across the aquatic-terrestrial boundary? Ecosphere 9:e02180.

Alberts, J.M., Sullivan, S.M.P. and A. Kautza. 2013. Riparian swallows as integrators of landscape change in a multiuse river system: implications for aquatic-to-terrestrial transfers of contaminants. Science of the Total Environment. 463-464: 42-50