Using systematically collected nest content and location data on warbler and blackbird nests gathered from May through July across three field seasons (2019–2021) in central Illinois, USA, we analyzed whether blackbirds that placed nests closer to actively breeding warbler or blackbird pairs experienced lower probabilities of brood parasitism. Detailed study site descriptions, nest searching/monitoring methodology, and how we determined warbler breeding status are described in detail in Lawson et al. (2020 (link)); Lawson, Enos, Mendes, et al. (2021 ). Briefly, we searched six sites in Champaign and Vermilion County two to three times a week for blackbird and warbler nests. For each nest, we noted parasitism status (parasitized vs. nonparasitized, binary status) and took location points for nest sites using GPS units to 3 m accuracy (Garmin model eTrex 10).
For breeding warbler pairs whose nests we could not find, we took a location point for one of the male's singing posts within the territory and used these points as proxies for active warbler nest locations. We determined breeding status by spot‐mapping pairs several times a week and noting behavioral cues strongly associated with an active nest present on territory (see Lawson, Enos, Mendes, et al., 2021 for a detailed description of warbler breeding status methods). For blackbirds, we only used known nests as location points, as we needed to include the parasitism status for each nest in our analyses.
We used ArcGIS (ver. 10.8.1; Redlands, 2022 ) to calculate the distance (m) between each blackbird nest and the closest warbler and blackbird nest or breeding pair location, within each year of the study. For analyses, we only used data from sites with both warbler and blackbird nests within a given year (2019, n = 6 sites; 2020, n = 4; 2021, n = 5). Blackbirds are facultatively polygynous and can have multiple active breeding nests in their territory at a time (Searcy & Yasukawa, 2014 ), which introduces biological pseudoreplication as an issue. Thus, we focused on whether the male blackbird's territory experienced any parasitism in any of his nests, by assigning male territories as “non‐parasitized” if no nests were parasitized or “parasitized” if at least one nest was parasitized. Distance to nearest warbler nest/pair or blackbird nest was averaged for all nests on a male blackbird's territory to produce a single distance metric to warbler and to blackbird nest per blackbird territory. Distances between blackbird nests and nearest warbler nest/pair ranged from 1.69 m to 2581.0 m, with most data points (>80%) falling linearly under the 300 m range. Therefore, for statistical purposes, we only included warbler nests and blackbird territories within 300 m of each other to minimize skewness of data and to increase model fit.
We ran a binary logistic regression model to test if distance to nearest warbler nest/pair significantly predicted the probability of brood parasitism on blackbird territories, with distance to the nearest nest as a fixed effect. We did not include year and/or site as fixed or random effects due to small sample sizes. No other variables were included in the model such as the blackbird or warbler nests' breeding stage (sensu Massoni & Reboreda, 2001 ). We also ran a second binary logistic regression model, with averaged distance to the nearest blackbird nest as a fixed effect instead of warbler nests. We ran all the analyses using R version 4.2.2 and the lme4 package (Bates et al., 2014 ) and set α = 0.05.
For breeding warbler pairs whose nests we could not find, we took a location point for one of the male's singing posts within the territory and used these points as proxies for active warbler nest locations. We determined breeding status by spot‐mapping pairs several times a week and noting behavioral cues strongly associated with an active nest present on territory (see Lawson, Enos, Mendes, et al., 2021 for a detailed description of warbler breeding status methods). For blackbirds, we only used known nests as location points, as we needed to include the parasitism status for each nest in our analyses.
We used ArcGIS (ver. 10.8.1; Redlands, 2022 ) to calculate the distance (m) between each blackbird nest and the closest warbler and blackbird nest or breeding pair location, within each year of the study. For analyses, we only used data from sites with both warbler and blackbird nests within a given year (2019, n = 6 sites; 2020, n = 4; 2021, n = 5). Blackbirds are facultatively polygynous and can have multiple active breeding nests in their territory at a time (Searcy & Yasukawa, 2014 ), which introduces biological pseudoreplication as an issue. Thus, we focused on whether the male blackbird's territory experienced any parasitism in any of his nests, by assigning male territories as “non‐parasitized” if no nests were parasitized or “parasitized” if at least one nest was parasitized. Distance to nearest warbler nest/pair or blackbird nest was averaged for all nests on a male blackbird's territory to produce a single distance metric to warbler and to blackbird nest per blackbird territory. Distances between blackbird nests and nearest warbler nest/pair ranged from 1.69 m to 2581.0 m, with most data points (>80%) falling linearly under the 300 m range. Therefore, for statistical purposes, we only included warbler nests and blackbird territories within 300 m of each other to minimize skewness of data and to increase model fit.
We ran a binary logistic regression model to test if distance to nearest warbler nest/pair significantly predicted the probability of brood parasitism on blackbird territories, with distance to the nearest nest as a fixed effect. We did not include year and/or site as fixed or random effects due to small sample sizes. No other variables were included in the model such as the blackbird or warbler nests' breeding stage (sensu Massoni & Reboreda, 2001 ). We also ran a second binary logistic regression model, with averaged distance to the nearest blackbird nest as a fixed effect instead of warbler nests. We ran all the analyses using R version 4.2.2 and the lme4 package (Bates et al., 2014 ) and set α = 0.05.
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