DOI: 10.5061/dryad.2rbnzs7rt
title: Data From: Long-term winter food supplementation shows no significant impact on reproductive performance in Mountain Chickadees in the Sierra Nevada Mountains

description: Supplemental feeding of wild animal populations is popular across many
 areas of the world and has long been considered beneficial, especially to
 avian taxa. Over four billion dollars are spent by hobby bird feeders in
 the United States each year alone. However, there is mixed evidence
 whether wildlife feeding is beneficial, including when it is implemented
 as a conservation management tool, a targeted experimental design, or an
 avocation. Much of the current evidence suggests that providing
 supplemental food is advantageous to the reproductive output and general
 survival of focal taxa. However, many of these studies are limited in
 scope and duration, leaving possible negative impacts unaddressed. This is
 particularly true regarding passive backyard feeding, which describes the
 majority of supplemental feeding, including the immense effort of millions
 of public enthusiasts. Here we show that winter supplemental feeding prior
 to reproduction had no significant impact on a range of reproductive
 parameters in a resident, montane passerine species, the Mountain
 Chickadee (Poecile gambeli). This population resides in an intact natural
 environment with no exposure to supplemental food beyond our experimental
 treatments, and individual birds were tracked across six years using radio
 frequency identification technology. Our results add to the growing
 evidence that supplemental feeding alone, isolated from the effects of
 urban environments, may have little to no impact on the population
 dynamics of some avian taxa.
description: Subjects
 and study site All data for this study
 were collected from December 2015–August 2021 at Sagehen Experimental
 Forest (Sagehen Creek Field Station, University of California, Berkeley)
 in the Sierra Nevada, USA. This population has been monitored year-round
 at two elevational sites (high: ca. 2400 m; low: ca. 1900 m separated by
 3.49 km) since 2013 (Kozlovsky et al., 2018, Croston et al., 2016). Nest
 boxes and feeders were placed along service roads within the forest (5.87
 km of non-linear road at low elevation and 4.83 km at high elevation), but
 nest boxes cover a larger area compared to the area with available
 feeders. Adult and juvenile chickadees were captured with mist nets during
 the fall and early spring at feeders and by hand at nest boxes during the
 summer (e.g. Croston et al., 2016, 2017, Tello-Ramos et al., 2018). Birds
 were banded with unique combinations of colored PIT-tags and plastic or
 aluminum leg bands. Individuals that were not first captured as nestlings
 were assigned an approximate age (adult or juvenile) at time of initial
 capture based on multiple plumage characteristics and sexed via
 physiological evidence if possible (e.g. brood patch or cloacal
 protuberance; Pyle, 1997). Individuals in their first year of life were
 classified as juveniles and those older were classified as adults.
 Nestlings were captured in nest boxes and banded with U.S. Geological
 Survey aluminum bands; if recaptured as adults, they were banded with an
 additional plastic color band and a PIT-tag. Birds are highly resident and
 do not disperse between sites. A total of 4,264 nestlings have been banded
 in nest boxes since 2013 and not a single individual was detected breeding
 at a different elevation than it was initially banded during the duration
 of the study. Food
 supplementation We provided chickadees
 with a constant supply of black oil sunflower seeds during the winter
 months (December through April) of each year. While multiple feeders were
 used during early autumn to capture and band chickadees, only two feeder
 locations per elevation were continually stocked with supplementary food
 throughout the winter. All other feeders were closed before winter. These
 permanent locations were the only feeding sites across approximately 5 km
 (at each elevation) and some birds regularly utilized the feeders while
 other more distant individuals did not visit at all. These geographic
 details allowed us to compare reproductive performance of birds with and
 without access to supplementary food during the winter prior to
 reproduction. Winter feeding locations were associated
 with feeder arrays used for testing cognitive abilities and included eight
 feeders arranged equidistantly in a circular array format on a 122 x 122
 cm square frame. Visitation of feeders was detected via RFID readers that
 record when birds marked with a PIT-tag land on a feeder’s perch (Croston
 et al., 2016, 2017; Tello-Ramos et al. 2018, Bridge et al., 2019). Tagged
 birds detected at these feeders were categorized as individuals with
 access to supplemental food and tagged birds that never visited the
 feeders were categorized as those without access to supplemental food.
 Birds trapped at feeders in the fall (September-November) who did not
 continue visiting the main feeding locations were classified as
 individuals without access to supplemented food (non-supplemented: 109 and
 supplemented: 128). Many of the adult individuals banded at nest boxes
 were far away from the main feeding locations and were never detected at
 these locations (non-supplemented: 83 and supplemented: 45). Thus, our
 sample of birds represents some individuals that were never near our
 feeders, some that may have been near our feeders but did not visit them,
 and others that visited the feeders. There may have been birds that
 remained undetected during the winter months due to pilfering seeds
 underneath the feeders or from the caches of those visiting feeders but we
 expect this number to be quite small. The coniferous forest habitats at
 each elevation are extremely homogenous so it is unlikely that habitat
 quality influenced the choice to visit feeders. We did
 not incorporate individual visitation rates into the study as our data are
 inherently incomplete due to the timing of when an individual was captured
 in each season (early or late fall or winter) and due to limited time in
 which the feeders recorded visits by the tagged birds (feeders were active
 all winter, but we only collected visitation data during the specific
 interval of cognitive testing), making this comparison problematic.
 Visitation numbers also reflect both consumption and caching in this
 species which cannot be easily disentangled. Our categorization tests the
 idea of whether the presence and access (birds that visited the feeders
 even a few times were considered to have full access to the feeders while
 birds that never visited the feeders were considered to have no access to
 supplementary food) of human-provided food during winter has any effect on
 reproduction in the following season. We also conducted a more
 conservative analysis that only included individuals never detected at
 feeders and individuals that completed at least 20 trials during cognitive
 testing (a trial results in a bird getting food during a visit to the
 feeder array, Croston et al. 2016, 2107; Tello-Ramos et al. 2018). All
 birds that completed at least 20 trials were visiting the feeders
 regularly for several weeks leading up to cognitive testing and most
 exceeded 20 trials during their participation (Trials: range 20–4,444,
 mean: 603.35, median 337), so this analysis amplified potential
 differences between pairs that regularly visit the feeders and those that
 and did not access supplemental food. Breeding
 We monitored chickadees for breeding
 behavior starting in mid-April each year. Chickadees began nesting
 approximately one to two months after food supplementation was completely
 removed. Mountain chickadees are secondary cavity-nesting birds that
 readily use human-made nest boxes (McCallum et al., 2020). Nest boxes were
 monitored for the onset of nest building, first egg date, onset of
 incubation and hatch date on a weekly to biweekly basis (Kozlovsky et al.,
 2018, Pitera et al., 2021, Sonnenberg et al., 2020). After the onset of
 incubation was detected, nests were targeted for monitoring based on
 expected incubation duration to limit disturbance and allow for precise
 detection of hatch dates. Thus, first egg dates are approximations with an
 error rate of ±1 day but all hatch dates are exact. After nestlings
 hatched, any unbanded parents were captured and banded between eight and
 twelve days post-hatch. Nestlings were counted, banded, and weighed using
 an electronic scale at 16 days post-hatch (Kozlovsky et al., 2018, Pitera
 et al., 2021, Sonnenberg et al., 2020). We assigned parental breeding
 pairs to four categories depending on their visitation to feeders in the
 winter prior to each breeding attempt: pairs in which both individuals
 visited feeders, pairs in which only one individual visited feeders (male
 or female), and pairs in which neither individual visited feeders. Any
 nest with a parent that was not banded in the previous winter was
 excluded. Statistical methods
 We tested whether access to supplemental
 food in winter influenced breeding performance of mountain chickadees
 using five breeding variables: first egg date, clutch size, brood size
 (number of living nestlings on day 16 post-hatch), mean nestling mass, and
 the coefficient of variation of nestling mass. We only included data from
 initial nests, leaving out renests within a season, as these attempts are
 very infrequent and often unsuccessful in this system. We modeled each
 breeding variable separately for our high- and low-elevation sites due to
 previously reported major differences in breeding performance between
 elevations in this population which were not the main focus of this
 particular study (Kozlovsky et al., 2018, Pitera et al., 2021). Analyzing
 elevations separately allowed us to test whether supplementary food had an
 effect on reproduction within each elevation. This resulted in 10 models,
 five for each elevation. Each model contained the breeding variable as the
 response variable, parent access to supplemental food as a fixed effect
 (four levels: both parents accessed feeders, male only, female only, and
 neither parent accessed feeders), breeding year as a fixed effect (six
 levels: 2016 – 2021) and parent age as a fixed effect (four levels: both
 parents adults, adult male and juvenile female, juvenile male, and adult
 female and both parents juveniles). Including year and parent age as fixed
 effects allowed us to control for environmentally driven differences in
 breeding across years (Kozlovsky et al., 2018), and effects of parent
 breeding experience (Pitera et al., 2021) previously reported for this
 system. We included male ID and female ID as separate random effects in
 each model to control for individual variation and repeated nesting
 attempts across years. We did not include an interaction between year and
 parent access to food due to small sample sizes when the data were divided
 this way. We used linear mixed models to model the
 effects of winter feeding on first egg date, mean nestling mass, and the
 coefficient of variation of nestling mass. To model the effects of winter
 feeding on clutch size and brood size, we used generalized linear mixed
 models with a Generalized Poisson distribution and log link in the R
 package ‘glmmTMB’ due to our data being underdispersed (Brooks et al.,
 2017, Joe & Zhu, 2005). When modeling clutch size, we included
 first egg date of the nest as a fixed effect, as previous research in this
 system has indicated that earlier nests have larger clutches (Kozlovsky et
 al., 2018). When modeling mean nestling mass and coefficient of variation
 of nestling mass, we included brood size as a fixed effect in the model,
 as brood size is known to influence nestling mass in some species
 (Dijkstra et al., 1990). Coefficient of variation of nestling mass was
 log-transformed prior to modeling to improve residual fit. We tested the
 importance of each fixed effect in all models using type II Wald
 chi-square tests. When the ‘parent access to food’ fixed effect was
 significant (p &lt; 0.05), we calculated the estimated marginal mean
 for each level using the ‘emmeans’ R package (Lenth, 2021) and calculated
 the significance of differences among the categories using a Tukey HSD
 test and Tukey correction for multiple comparisons through the ‘pairs’
 function in R. We tested the residual fit of all models using the R
 package ‘DHARMa’ (Hartig, 2018). We calculated marginal R2 values for each
 model following Nakagawa et al. (2017) implemented in the R package
 ‘performance’ (Lüdecke et al 2021). All statistics and graphics production
 were performed in R version 4.1.1 (R Core Team, 2021, Wickham et al.,
 2019).
description: The program R is the only required software.