DOI: 10.5061/dryad.9cnp5hqh5
title: Forest birds select food-rich habitat during migratory stopover

description: 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.
description: 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 m<sup>2</sup> plot at each
 randomly selected location and within each plot, I estimated percent cover
 of each tree or shrub species in the canopy (&gt; 5 m), mid-canopy (2
 – 5 m) and shrub layer (&lt; 2 m).