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. 

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:

hatch to fledge period (42 days) + incubation period (31 days) = 73 days

fledge date – 73 days = nest initiation date

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.

The second tab of the excel file has the code description for each heading across the datafile