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Forest birds select food-rich habitat during migratory stopover

Citation

Thompson, Bill (2021), Forest birds select food-rich habitat during migratory stopover , Dryad, Dataset, https://doi.org/10.5061/dryad.9cnp5hqh5

Abstract

Many species of migrant songbirds require stopover habitat during migration, as places to rest, and gain additional weight prior to proceeding on in migration. For some birds, the amount of time spent at stopover habitat exceeds the amount of time spent in flight during migration.  While the amount of time spent at stopover decreases as habitat quality increases, it is unclear what cues migrant birds use to identify high quality habitat.  A set of thirty sites north of Lake Ontario were surveyed weekly during April and early May for the presence of both migrant songbirds, and the emergence of arthropods, as a potential food source. Neither distance from Lake Ontario nor size of the forest had a significant influence on number of migrants or arthropod biomass observed at each stopover site.  Total arthropod biomass did however have a significant positive influence on the total number of migrants observed over the course of the migration season.  The relationship between arthropod biomass and migrant abundance remained from week to week, although the sites of greatest abundance changed.  These results suggest that current land use planning rules, which focus primarily on forest size and connectivity may be insufficient for protecting migratory stopover habitat.

Methods

I conducted bird surveys in the study sites during the spring of 2013, 2014, 2018 and 2019. I began bird surveys in these sites during the first week of April and continued them until the third week of May, on days of suitable weather (i.e. no precipitation and little or no wind). I conducted the surveys using line transects, to maximize the number of individuals and species observed during every visit.  I visited sites weekly and randomized the order of visitation within each week with the use of a random number generator.  I conducted the surveys between 06:00 and 10:00 hours, along transects 200 m long, perpendicular to the forest edge.  Each survey consisted of slowly walking the transect, with pauses approximately every 25m, and recording all birds seen or heard within 25 m of either side of the transect centerline, excluding all flyovers (sampling a 200m by 50m rectangular plot).  I recorded ambient air temperature at the beginning of each survey.

Prior to beginning each transect, I generated five random numbers in a random number generator and used each number as a distance along the transect at which to collect arthropods.  Distances were approximated by pacing, and a twig was collected from the nearest tree or shrub at each stop, by placing a bag over a branch between 1 m and 3 m in height, clipping a twig into the bag, and fumigating the contents.  I identified all arthropods collected to taxonomic order and determined their length to the nearest 0.1 mm.

During the last visit of each season, I generated four additional random numbers and used those at distances along the transect at which to quantify habitat structure.  I located a 100 m2 plot at each randomly selected location and within each plot, I estimated percent cover of each tree or shrub species in the canopy (> 5 m), mid-canopy (2 – 5 m) and shrub layer (< 2 m).