Not Singing in the Rain: Linking Migratory Songbird Declines With Increasing Precipitation and Brood Parasitism Vulnerability

Abstract

Few empirical studies have quantified relationships between changing weather and migratory songbirds, but such studies are vital in a time of rapid climate change. Climate change has critical consequences for avian breeding ecology, geographic ranges, and migration phenology. Changing precipitation and temperature patterns affect habitat, food resources, and other aspects of birds’ life history strategies. Such changes may disproportionately affect species confined to rare or declining ecosystems, such as temperate grasslands, which are among the most altered and endangered ecosystems globally. We examined the influence of changing weather on the dickcissel (Spiza americana), a migratory songbird of conservation concern that is an obligate grassland specialist. Our study area in the North American Great Plains features high historic weather variability, where climate change is now driving higher precipitation and temperatures as well as higher frequencies of extreme weather events including flooding and droughts. Dickcissels share their breeding grounds with brown-headed cowbirds (Molothrus ater), brood parasites that lay their eggs in the nests of other songbirds, reducing dickcissel productivity. We used 9 years of capture-recapture data collected over an 18-year period to test the hypothesis that increasing precipitation on dickcissels’ riparian breeding grounds is associated with abundance declines and increasing vulnerability to cowbird parasitism. Dickcissels declined with increasing June precipitation, whereas cowbirds, by contrast, increased. Dickcissel productivity appeared to be extremely low, with a 3:1 ratio of breeding male to female dickcissels likely undermining reproductive success. Our findings suggest that increasing precipitation predicted by climate change models in this region may drive future declines of dickcissels and other songbirds. Drivers of these declines may include habitat and food resource loss related to flooding and higher frequency precipitation events as well as increased parasitism pressure by cowbirds. Positive correlations of June-July precipitation, temperature, and time since grazing with dickcissel productivity did not mitigate dickcissels’ declining trend in this ecosystem. These findings highlight the importance of empirical research on the effects of increasing precipitation and brood parasitism vulnerability on migratory songbird conservation to inform adaptive management under climate change.

Bobolink (Dolichonyx oryzivorus) Declines Follow Bison (Bison bison) Reintroduction on Private Conservation Grasslands

“…During the time period of our study (2002–2019), precipitation significantly increased in our study area, consistent with climate change predictions [ 64 ], and there was a non-significant trend towards warmer temperatures. Bobolink population dynamics exhibited significant responses to local climatic changes in our study area, with both abundance and productivity increasing with increasing January minimum temperatures and precipitation ( Figure 4 and Figure 5 ).…”

“…By contrast, where warmer winter temperatures and higher levels of precipitation allow plants to start growing, flowering, and producing seeds earlier in the year, Bobolinks arriving from spring migration to breed may benefit from improved conditions for nesting and raising chicks that in turn results in greater Bobolink productivity. In addition, our findings that warmer summers are followed by increased Bobolink productivity in subsequent years, and that Bobolink productivity in heavily grazed grasslands increased in response to wetter summer conditions, suggest that the regional trend toward warmer and wetter winters and summers in this region [ 63 , 64 ] has the potential to benefit both Bobolink abundance and productivity.…”

“…Bobolink productivity demonstrated positive responses to warmer and wetter summer conditions. Specifically, Bobolink productivity increased with warmer temperatures during the previous breeding season ( Figure 5 ), highlighting the carry-over effects of weather on Bobolinks in addition to the impacts of weather on grasslands and birds in real time [ 63 , 64 , 89 ]. Further, Bobolink productivity showed a positive correlation with the interaction of August PDSI, which represents cumulative drought conditions during the summer breeding season (May–July), and both time since active grazing and grazing intensity.…”

“…Next, we compared Bobolink abundance and productivity in bison-grazed grasslands with that in grasslands grazed by cattle and grasslands that were hayed after the bird breeding season. Finally, we investigated the effects of specific land management (grazing, burning, haying) and climate (temperature, precipitation, drought) parameters using Bobolink adult abundance (hereafter, abundance) and juvenile abundance (hereafter, productivity) as response variables [ 37 , 62 , 63 , 64 ].…”

“…We used Bobolink CMR data across all 17 sites, removing recaptures of individuals that occurred at the same site during the same year to avoid double-counting. We corrected for annual capture effort by dividing the number of captures by the cumulative mist net-meter-hours for the given year [ 63 , 64 ]. We divided the banding season into two halves for data analysis, in which the first half (21 May–29 June) comprised adults arriving and establishing breeding territories and nests, and the second half (30 June–8 August) comprised both adults and juvenile birds that had fledged from successful nests, e.g., [ 37 , 63 , 64 ].…”