DOI: 10.5061/dryad.612jm640g
title: Burrowing Owl (Athene cunicularia) nest phenology influenced by drought on nonbreeding grounds

description: Migratory birds are demonstrating changes in phenology linked to climate
 change. Understanding these changes requires connecting events that occur
 over the multiple regions occupied during their annual cycle. 
 The Burrowing Owl (Athene cunicularia) is a species of concern in North
 America, with pronounced declines in regions of the Great
 Plains.  Using a dataset that spanned ten breeding sites from
 South Dakota to northern Mexico in various years during 1989-2017, we
 observed both advances and delays in nesting along with increasing
 variation in nest initiation dates.  We examined the effects of a
 large-scale climate system (El Niño Southern Oscillation), drought, and
 local weather patterns throughout the annual cycle as potential predictors
 of early and late nesting.  Moisture conditions during the winter
 and spring migratory period had the greatest influence on nest
 phenology.  Years with more intense drought on winter and
 migratory grounds increased the probability of nests initiating late
 relative to early.  Correspondingly, wet conditions were
 associated with an increased probability of early nest
 initiation.  Drought likely has cascading ecological effects that
 negatively influence food abundance for Burrowing Owls, resulting in
 delays in the ability of individuals to meet energetic demands required
 for migration.  How climate change will impact Burrowing Owl
 phenology is important considering a projected increase in magnitude and
 frequency of drought and declining owl population trends. 
description: Our nest
 initiation dataset consists of ten breeding sites distributed
 latitudinally across the Great Plains from northern South Dakota to Janos,
 Chihuahua Mexico. Six sites occur on US Forest Service National
 Grasslands: Grand River in northwestern South Dakota and Buffalo Gap in
 southwestern South Dakota are in mixed and short-grass prairie; while
 Pawnee in northeastern Colorado, Comanche in southeastern Colorado, Kiowa
 in northeastern New Mexico, and Rita Blanca in northwestern Texas are all
 in short-grass steppe.  Our site in Nebraska is in the western panhandle
 portion of the state on privately-owned grasslands consisting of mixed and
 short-grass prairie.  Kirtland Air Force Base in Albuquerque, New Mexico
 has pinyon-juniper and predominantly mixed grassland vegetation
 communities. The Armendaris Ranch is within Chihuahuan desert grassland in
 southwestern New Mexico.  The Las Cruces site is a mixture of urban and
 agricultural areas in southwestern New Mexico.Janos Biosphere Reserve is
 in northern Chihuahua, Mexico, within Chihuahuan desert grassland. For
 each breeding site, nest searching began according to latitudinal location
 (approximate breeding period start dates in Appendix Table 6).  Nest
 searching was conducted by methodically walking or driving ATVs along
 transects through prairie dog colonies, targeted based on current prairie
 dog activity and historic owl nest locations.  For the site in Las Cruces,
 New Mexico nest searching was conducted by vehicle along roads and canals,
 walking in appropriate habitat, communication with the public and through
 historical nest locations.  Additionally, individual nests were recorded
 opportunistically across sites when owls were encountered on accessible
 land.  Burrowing Owl broadcast calls via playback speakers were sometimes
 used to detect breeding owls.  Signs of owl activity at a burrow entrance
 include presence of white wash, owl pellets, prey items, and nest material
 (shredded cow dung and plant fragments).  When an active burrow was
 identified, it was recorded via GPS coordinates and/or marked with a metal
 tag flush to the ground.  Because Burrowing Owls tend to nest in
 aggregations, the surrounding area of known nests were thoroughly searched
 to ensure all nests were found.  Nest initiation date was defined as the
 day the first egg was laid, and was estimated by backdating from fledging
 data (earlier datasets), hatch date or by directly viewing the interior
 nest burrow with a camera peeper probe (Sandpiper technologies, Manteca,
 CA). A camera probe was used at all sites except Nebraska 1989-1997,
 Armendaris Ranch 2001, Las Cruces 2001-2005, and Buffalo Gap, Pawnee,
 Comanche, and Kiowa-Rita Blanca National Grasslands in 2006.  Camera
 probes allow observers to confirm nest presence, count clutch size, and
 estimate hatch date.  Each nest was revisited approximately a week after
 initial examination and repeat visits were continued until the clutch was
 complete (about one week with the same number of eggs).  In cases when the
 clutch was complete at first visit, nests were checked until chicks
 hatched.  Chicks were aged by plumage and behavioral traits, and nest
 initiation was estimated using hatch date backdating methods described
 above.  Because owls are known to lay an egg at a mean of 1.5-day
 intervals, initiation date can accurately be estimated.  We were unable to
 determine nest initiation date for certain nests due to depredation or
 abandonment between nest visits or nest failure before juveniles emerged. 
 Only nests where we could absolutely estimate nest initiation date were
 used in the dataset. When nest initiation date was estimated based on
 fledge date, 42 days was used to backdate from fledge date to hatch date. 
 The Burrowing Owl’s incubation period is estimated to be approximately 31
 days, including the egg laying stage.  Therefore, initiation date for each
 nest was calculated by backdating 73 days from the recorded fledge date,
 as follows: <i>hatch to fledge period
 (42 days) + incubation period (31 days) = 73 days</i>
 <i>fledge date – 73
 days = nest initiation date</i> We collected data
 on ENSO, drought, and individual weather variables during the winter,
 migratory, and breeding periods as potential predictors of timing of
 nesting.  The standard metric used by the National Oceanic and Atmospheric
 Association (NOAA) to monitor the ENSO is the Oceanic Niño Index (ONI),
 where positive values of the ONI are associated with El Niño phases and
 negative values are associated with La Niña phases.  We used the mean ONI
 for December-January as a winter period variable, which includes the usual
 peak phase of the annual ENSO cycle, and the ONI in March as a migration
 period variable.  To examine the influence of drought, we used the Palmer
 Modified Drought Index (PMDI), which combines precipitation, temperature,
 and soil water capacity to measure drought intensity.  Positive PMDI
 values indicate abnormally wet conditions for a given location (extreme
 moisture ≥ +4.00), while negative values indicate abnormally dry
 conditions (extreme drought ≤ -4.00).  In addition to PMDI, we used total
 precipitation for each period because of precipitation’s potential
 influence on prey availability, as well as cumulative precipitation from
 the previous year for lag effects.  We also used mean daily precipitation
 as an indicator of heavy precipitation during migration, because frequent
 heavy precipitation events could interfere with movement during
 migration.  Lastly, we examined timing of nesting in relation to mean
 daily maximum temperature for each period.  We
 calculated values for drought and individual weather variables using
 weather station and gridded weather datasets associated with winter,
 migration, and breeding locations.  The winter grounds of owls in this
 study were estimated within northern and central Mexico based on satellite
 telemetry data and band recoveries.  Locations included for migration data
 for each breeding site were also based on satellite telemetry data and
 band recoveries, and were selected according to the region(s) located
 between the breeding site and the winter grounds. Stations that fell along
 the proposed migration pathway to a breeding site were used to calculate
 migration variables specifically for that site. A combination of weather station
 and gridded data sources were used for local data across the winter,
 migration, and breeding grounds.  Local weather data from winter and
 migration regions within Mexico were obtained from the Daymet dataset,
 through The Oak Ridge National Laboratory Distributed Active Archive
 Center (ORNL DAAC) (https://daymet.ornl.gov).  Local weather data for migration and breeding regions within the United States were obtained from the PRISM dataset (PRISM Climate Group, Oregon State University, http://www.prism.oregonstate.edu).   Records of the monthly Oceanic Nino Index and the station-based Palmer Modified Drought Index were obtained from NCEI (http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml and https://www.ncdc.noaa.gov/temp-and-precip/drought/nadm/indices.
description: The second tab of the excel file has the code description for
 each heading across the datafile