Managing Grass Height for Birds and Livestock: Insights from the Río de la Plata Grasslands

Description

Our hypothesis was that it is possible increase grass height and maintain grassland bird specialist in ranches. To do so we measured grass height and counted different bird species in 300 m transects in paddocks accross 11 ranches, before and after changing grazing regime to increase grass height.

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Grass height was assessed in each paddock by measuring it at regular intervals along a 300-meter transect, with measurements taken every 50 meters. This resulted in a total of six measurements per paddock. A metal ruler was used to determine the maximum height at which the majority of the grass biomass was concentrated. The standard deviation of grass height along each transect was utilized as an indicator of grassland heterogeneity. We conducted our study on grassland bird species that depend entirely on grasslands for their whole life cycle (Vickery et al., 1999). The continuous conversion of grasslands into alternative land uses in the Rio de la Plata Grasslands (Baeza and Paruelo, 2020) presents a significant risk to these species. Hence, conserving the species that rely exclusively on native grasslands becomes a matter of utmost importance (Azpiroz et al., 2012). In each paddock, we conducted bird identifications and counts before (December 2017) and after (December 2018) implementing cattle grazing management changes. To sample obligate grassland birds, we used linear transects of 300 m without a fixed bandwidth. A unique transect line was randomly placed within each paddock, and two observers walked along each transect, recording and identifying individual birds. We only recorded birds that were actively using the habitat within the transect, such as feeding, resting, or perching, excluding those birds that were merely flying over the surveyed area. Bird surveys were conducted during the first 4 hours of the morning or the last 4 hours of the afternoon. To account for detection heterogeneity (Miller et al., 2015), the timing of bird surveys was randomized across ranches and paddocks. To estimate the detection probability of bird species (MacKenzie et al., 2002), we repeatedly surveyed birds in the same transects established in different paddocks (between 5 and 7 transects in each ranch), within a short period of time (2 to 4 times over a 3-day period).

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