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Black-capped and mountain chickadee range-wide condition


Grabenstein, Kathryn; Taylor, Scott; Otter, Ken; Burg, Theresa (2022), Black-capped and mountain chickadee range-wide condition, Dryad, Dataset,


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.


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 (< 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.


Calculating body condition

            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 Mi and Li 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 Li.