DOI: 10.5061/dryad.qrfj6q5j7
title: Black-capped and mountain chickadee range-wide condition

description: Both abiotic and biotic drivers influence species distributions. Abiotic
 drivers, such as climate, have received considerable attention, even
 though biotic drivers, such as hybridization, often interact with abiotic
 drivers. We sought to explore the (1) costs of co-occurrence for
 ecologically similar species that hybridize and (2) associations between
 ecological factors and condition to understand how abiotic and biotic
 factors influence species distributions. For two closely related and
 ecologically similar songbirds, black-capped and mountain chickadees, we
 characterized body condition, as a proxy for fitness, using a 1,358
 individual range-wide dataset. We compared body condition in sympatry and
 allopatry with several abiotic and biotic factors using species-specific
 generalized linear mixed models. We generated genomic data for a subset of
 217 individuals to determine the extent of hybridization-driven admixture
 in our dataset. Within this data subset, we found that ~11% of the
 chickadees had hybrid ancestry, and all hybrid individuals had typical
 black-capped chickadee plumage. In the full data set, we found that birds
 of both species, independent of demographic and abiotic factors, had
 significantly lower body condition when occurring in sympatry than birds
 in allopatry.  This could be driven by either the inclusion of
 cryptic, likely poor condition, hybrids in our full dataset, competitive
 interactions in sympatry, or be due to range edge effects. We are
 currently unable to discriminate between these mechanisms. Our findings
 have implications for mountain chickadees in particular, which will
 encounter more black-capped chickadees as black-capped chickadee ranges
 shift upslope and could lead to local declines in mountain chickadee
 populations.
description: As part of other ongoing projects in three research labs, we
 opportunistically sampled both black-capped and mountain chickadees from
 across most of their contemporary North American distributions over a
 12-year period (2007-2016; 2018-2019) from May-August (i.e., during the
 breeding season) at 238 sites (n = 118 sympatric sites; n = 120 allopatric
 sites). Chickadees were identified to species using plumage
 characteristics in the field by trained individuals. The main sampling
 goal was to describe patterns of gene flow within both species’ ranges
 (Adams & Burg, 2015; Bonderud et al. 2018; Grava et al. 2012).
 Thus, our sampling did not focus exclusively on regions of overlap between
 the species and was relatively evenly distributed between species and
 sites through time, minimizing potential bias from only sampling in
 sympatry (Supplementary Figures 1, 2). Chickadees of both species were
 captured using audio lures at mist-nets or baited Potter traps. Small
 blood samples (&lt; 20 µl) were collected from the brachial
 vein from captured chickadees and stored either as whole blood in 2% lysis
 buffer, ethanol, or blood on filter paper stored in ethanol. Tissue
 samples, pectoral muscle, were stored in ethanol. Individual sex was
 determined using sex-specific characteristics during the breeding season
 (e.g., brood patches for females and cloacal protuberances for males)
 (Desrochers 1990). Individuals lacking either a brood patch or obvious
 cloacal protuberance were classified as unknown sex. Age was determined
 using plumage (Meigs et al. 1983) and breeding characteristics.
 Individuals were classified as: hatch year (HY), after hatch year (AHY),
 and after second year (ASY) in the field. These age classes were then
 combined as 1) HY and 2) AHY/ASY for statistical analyses based on
 previous work showing that dominance status in chickadees correlates
 strongly with age, where HY individuals are the most subordinate
 individuals (versus AHY/ASY birds; Smith, 1976).  We measured the length
 of the right tarsus to the nearest 0.01 mm and mass to the nearest 0.5 g.
 Evidence from a single study in black-capped chickadees suggests that
 weight can increase by as much as 1 g (~8% total body mass) over the
 course of the day as individuals forage, with individuals gaining ~0.5 g
 (~4% total body mass) from sunrise to midday (Graedel and Loveland 1995).
 While we did not record exact time of capture for each bird, all birds
 were captured between 05:00-13:00 and our methods for weighing birds was
 only accurate to 0.5 g. Thus, while we have not controlled for specific
 time of capture in our downstream analyses, we feel that we have
 reasonably minimized variation by standardizing the range of capture
 times.  Further, our method for weighing birds cannot characterize the
 total variation needed to see effects of time of day (finer than 0.5 g
 intervals). Over the 12 years, multiple banders (n =10) collected these
 size measurements using standard techniques and bander was included as a
 crossed random effect in downstream statistical analyses to control for
 tarsus variation from bander alone.             Birds
 were recorded as occurring either in sympatry or allopatry using current
 distribution maps, eBird observations, and whether or not individuals of
 both species were sighted and / or captured at a single site (Sullivan et
 al. 2009). If individuals from both species were captured in a single
 location, we scored them as sympatric, regardless of distribution maps or
 eBird data. This allowed for allopatry to occur within the range of
 overlap (i.e., at high elevation sites where only mountain chickadees were
 sampled, or low elevation where only black-capped chickadees were
 sampled). We sampled 582 black-capped chickadees from sympatry and 431
 from allopatry and 294 mountain chickadees from sympatry and 51 from
 allopatry. All protocols were approved by University of Colorado, Boulder
 IACUC panel (protocol 2683), University of Northern British Columbia ACUC
 (protocols 2004-07; A2008.0109.002; 2011.05; 2014.06 & 2017.01),
 and  University of Lethbridge(protocols 1028 and 1504) animal care
 committees and all methods in this study were performed in accordance with
 relevant guidelines, permits, and regulations.  
 <em>Calculating body condition</em>
             We used body size measurements to calculate the
 scaled mass index (SMI) of black-capped and mountain chickadees as
 outlined in Peig et al. (2009). Body condition measures should control for
 the correlation between length and mass. Peig et al. (2009) developed SMI
 for calculating body condition that accounts for the covariation between
 length and mass measurements when measuring body condition by
 standardizing body mass to a fixed value of a length measurement based
 upon a scaling relationship between mass and length. We calculated SMI for
 chickadees using the equation:             Where
 <em>M<sub>i</sub></em> and
 <em>L<sub>i</sub> </em>represent individual
 specific mass and tarsus length measurements respectively.  is the scaling
 component estimated from the standardized major axis (SMA) regression of
 the ln  on ln  and  is the predicted body mass for individual  with a
 length measurement (here, tarsus) standardized to , a species-specific
 mean tarsus length. Importantly, comparisons of body condition between
 groups can only be made when SMI is calculated using the same scaling
 component. To account for slight size differences between the species (and
 potential differences in fat storage), we calculated SMI separately for
 each species. To allow us to compare condition within species (i.e.,
 sex-specific differences), we calculated a separate scaling component ( )
 for each species (black-capped chickadees = 1.09, mountain chickadees =
 1.12) and used species-specific tarsus length means (black-capped
 chickadees = 17.83 mm, mountain chickadees = 18.62 mm) for  to calculate
 the SMI. We used individual mass (g) as  and individual tarsus length (mm)
 as <em>L<sub>i</sub></em>.