DOI: 10.5061/dryad.bg79cnpbj
title: Geographic variation in dispersal of western burrowing owl (Athene cunicularia hypugaea) populations

description: Dispersal is one of the key elements of species’
 metapopulation dynamics and, hence, influences global
 conservation status. Furthermore, determining the geographic variation in
 magnitude and direction of dispersal throughout a species’ distribution
 may expand our understanding of the causes of population declines in
 species of conservation concern. For instance, western burrowing owl
 (Athene cunicularia hypugaea) populations have declined at the northern
 and eastern edge of their breeding distribution during the
 20th century. In the same period, large areas of thornscrub that
 did not support breeding owls were converted to irrigated agriculture in
 the southern edge of the subspecies’ breeding distribution in northwestern
 Mexico. These farmlands now support some of the highest breeding densities
 of owls. We tested the hypothesis that owls that colonized this recently
 created habitat originated from declining migratory populations from the
 northern portion of the subspecies’ range. We used stable
 isotopes 2H, 13C, and 15N in owl feathers to
 infer breeding dispersal patterns throughout the subspecies’ breeding
 range. Populations near the northern edge of the subspecies’ breeding
 range had immigrants that dispersed over larger distances than immigrants
 at low and mid latitude populations. However, agricultural populations in
 northwestern Mexico disrupted this latitudinal pattern, attracting owls
 from more distant locations. We also found immigrants originated from
 further distances in declining populations than increasing populations.
 Stable isotopes provided no evidence of contemporaneous breeding dispersal
 from Canadian populations to northwestern Mexico but suggest that
 agricultural areas in the southern edge of the subspecies’ distribution
 have altered the continental dispersal pattern.
description: We collected feathers from nestling and adult burrowing owls during the breeding seasons of 2004-2009 at 36 study locations throughout the species’ breeding range in Canada, Mexico, and the United States. We pulled breast, back, and head feathers from nestlings that were 10-40 days-old, and we pulled the third right rectrix from adult burrowing owls. We processed all our samples in the Environmental Isotope Laboratory at the University of Arizona. We estimated the non-exchangeable δ<sup>2</sup>H in burrowing owl feathers using a modification of the “comparative equilibrium” technique of Wassenaar and Hobson (2003, Isot. Environ. Healt. S. 39:211–217) by Fan and Dettman (2014, Isot. Environ Health Stud. 51(2):214-30) as described by Macías-Duarte and Conway (2015, Auk 132:25-36). We measured δ<sup>13</sup>C and δ<sup>15</sup>N on a continuous-flow gas-ratio mass spectrometer (Finnigan Delta PlusXL). We combusted samples with added oxygen in an elemental analyzer (Costech) coupled to the mass spectrometer. Standardization was based on acetanilide for elemental concentration, NBS-22 and USGS-24 for δ<sup>13</sup>C, and IAEA-N-1 and IAEA-N-2 for δ<sup>15</sup>N. Precision based on repeated internal standards was better than 0.08‰ for δ<sup>13</sup>C and better than 0.2‰ for δ<sup>15</sup>N. Values of δ<sup>2</sup>H, δ<sup>13</sup>C, and δ<sup>15</sup>N are computed for the Vienna Standard Mean Ocean Water standard, PeeDee Belemite standard, and atmospheric N<sub>2</sub>, respectively.
description: File isotope_data.csv contains raw isotope ratio
 measurements (permil) for individual burrowing owls. File
 locations_coordinates.csv contains longitud and latitude of study
 locations. File sample_info.csv contains study location and year of
 collection, age and sex for each individual burrowing owl.