DOI: 10.5061/dryad.2jm63xsxj
title: The interplay of resource availability and parent foraging strategies on juvenile sparrow individual specialization

description: Temporal variation in resource availability, amplified by global change,
 may have strong impacts on species breeding at temperate and high
 latitudes that cue their reproduction to exploit seasonal resource pulses.
 This study examines how resource availability and parental care influence
 niche partitioning between and within age classes in the rufous-collared
 sparrow, which provides extensive parental care. We hypothesized juveniles
 would exhibit narrower niches focused on high-quality resources compared
 to adults, regardless of resource availability. We used stable isotope
 analysis to quantify individual and population niches in juveniles and
 adults across the breeding season in two cohorts experiencing contrasting
 resource landscapes. Contrary to our initial hypothesis, juveniles
 exhibited greater among individual diet variation and smaller total niche
 widths (i.e., higher levels of individual specialization) during periods
 of high food availability in comparison to periods of food scarcity.
 Interestingly, total niche width and individual specialization of adults
 remained stable across seasons despite a shift in trophic level,
 highlighting their potential role in providing a consistent diet for their
 young. These findings reveal a dynamic interplay between resource
 availability, parental care, and individual specialization, with important
 implications for understanding population resilience under variable
 resource scenarios. The study also suggests that adult sparrows modify
 their provisioning strategies based on resources, potentially buffering
 offspring from environmental fluctuations. Understanding age-specific
 responses to resource variation is crucial for predicting species
 responses to ecological conditions, particularly in regions like central
 Chile where seasonal resource limitation is expected to become more
 variable in response to climate change.
description: <em>Fieldwork and Tissue Collection. </em>The study
 site, Quebrada de la Plata (33°31’S 70°50’W) in central Chile, is
 characterized by a Mediterranean climate with cold wet winters and
 relatively hot dry summers (Di Castri and Hajek, 1976). The average
 accumulated winter precipitation is 230 mm, and during the breeding season
 in our study, it was 180 mm (CR2, 2024). In addition, at this site the
 availability of insects and plants varies seasonally being highest in
 spring and lowest in winter; summer values also are lower than in spring
 (Lopez-Calleja, 1995). We captured rufous-collared sparrows with mist nets
 in spring (November 2020) and summer (January 2021). We categorized
 captured individuals into adults (N<sub>spring</sub>= 16,
 N<sub>summer</sub>= 20) or juveniles (fledglings;
 N<sub>spring</sub>= 12, N<sub>summer</sub>= 8)
 according to plumage characteristics described in Miller and Miller
 (1968). We ringed the captured birds to ensure the independence of our
 samples, avoiding the resampling of the same individuals across
 seasons. Parental care in this species extends from
 hatching until complete independence at 40 days old, with both parents
 actively feeding the chicks. Fledglings leave the nest after 18-20 days;
 at which time they start learning to eat on their own and can feed
 independently from four weeks old. Thus, they continue to solicit and
 receive food from their parents until about 40 days old, when they become
 completely independent (Miller and Miller, 1968; Tubaro, 1990).
 Accordingly, individuals captured during November (spring) would
 correspond to late September broods, while those captured in mid-January
 (summer) would correspond to early December broods, ensuring that both
 broods were raised under markedly different ecological
 conditions. Immediately after capture, individuals were
 placed in a bird-holding bag (Ecotone, Poland), weighed to the nearest 0.1
 g on a digital scale, and claw tips were sampled and placed in an
 Eppendorf tube. We then collected ~50μl blood from the brachial vein using
 heparinized microhematocrit capillary tubes. Blood was centrifuged at
 10,000 rpm for 10 min to separate plasma and red blood cells. All samples
 were stored frozen in liquid nitrogen prior to preparation for stable
 isotope analysis. All tissue samples were dried at 80°C, homogenized, and
 ~0.5–0.6 mg of each sample was loaded into tin capsules for stable isotope
 analysis. <em>Resource Availability</em>.
 The rufous-collared sparrow is an omnivorous species that routinely
 consumes arthropods, seeds, and small herbaceous plants (Lopez-Calleja,
 1990, 1995).<em> </em>In order to support the information
 given by previous research by Lopez-Calleja (1995) at the study site, we
 examined seasonal variation in resource availability measured as the
 abundance and diversity of herbaceous plants and terrestrial arthropods.
 Herbaceous plants were sampled in five permanent 1m<sup>2
 </sup>quadrats distributed across the study area which covered
 approximately 1000 m² within a ravine, ensuring each quadrat was
 positioned at a minimum distance of 35 meters apart. The greatest distance
 between two quadrats was 140 meters. On each quadrat, we identified the
 percent cover of living herbaceous species during both seasons:
 mid-September and mid-October for spring and mid-December and mid-January
 for summer. Also, we sampled terrestrial arthropods with Barber pitfall
 traps filled with water and biodegradable detergent (Southwick, 1978).
 They were placed every 1 m along five linear transects of 10 m distributed
 across the study area, at close proximity from the vegetation quadrats,
 and collected after 24 h (Jaksic and Lazo, 1999). We counted and
 identified the collected arthropods and classified them into morphotypes
 corresponding to the lowest taxonomic resolution possible. We then
 estimated the mean total abundance per sample unit for herbaceous plants
 and terrestrial arthropods for each season and its confidence interval by
 bootstrap procedure with 10,000 iterations. Resource
 diversity was estimated by quantifying the Hill numbers for the plant and
 arthropod assemblages (Chao et al., 2014). Seasonal species richness and
 Shannon diversity index were calculated by using sample-size-based
 rarefaction and extrapolation estimation of the Hill numbers of order q
 equal to 0 and 1, respectively. The estimates were based on species
 abundance, measured as the percent cover of living plant species and
 number of individuals within each arthropod morphotype (Chao et al., 2014;
 Hseih et al., 2019). All estimations were performed with the package iNEXT
 (Hseih et al., 2019). Moreover, we collected leaf and
 seed samples from 12 plant species during the spring and seed samples from
 6 species during the summer, as live plant cover was absent in our study
 quadrats. The plant and seed species were selected based on their relative
 abundance and their inclusion in the rufous-collared sparrow's diet
 as described by Lopez-Calleja (1990,1995). This collection allowed us to
 characterize the d<sup>15</sup>N composition of the trophic
 web baseline required to estimate relative trophic level (see
 below). <em>Stable Isotope Analysis</em>.
 We measured d<sup>15</sup>N values in blood plasma, red blood
 cells, and claw tips of juvenile and adult individuals to quantify niche
 characteristics in seasons characterized by high (spring) and low (summer)
 resource availability according to previous studies (Lopez-Calleja, 1995).
 Although many tissues can be compared to construct a time series of
 isotopic variation, blood plasma, red blood cells, and claws can be easily
 collected in the field without sacrificing animals (Podlesak et al., 2005;
 Carleton et al., 2008). d<sup>15</sup>N values of claw tips
 integrate dietary inputs over ~6 weeks prior to sample collection, while
 blood plasma and red blood cells integrate diet over ~1-2 and ~3-4 weeks
 prior to collection, respectively (Hobson and Clark, 1992; Carleton et
 al., 2008; Hahn et al., 2014). Young birds initiate the post-juvenile molt
 either at the time of independence or within about 10 days thereafter
 (Miller and Miller, 1968). In our study, the captured juveniles showed no
 evidence of molting, which suggest that they probably were not yet fully
 independent in their feeding. This implies that the variance in diet
 composition based on analysis of these three tissues, with the most recent
 one reflecting diet integrated over ~1-2 weeks prior to capture,
 represents mostly the diet of juvenile individuals during the parental
 care period when they had not yet achieved foraging
 independence. Nitrogen isotope
 (d<sup>15</sup>N) values of sparrow tissues were measured on a
 Costech 4010 elemental analyzer coupled to a Thermo Finnigan Delta Plus XP
 isotope ratio mass spectrometer at the University of New Mexico Center for
 Stable Isotopes (Albuquerque, NM). Stable isotope data are expressed in
 delta (d) notation as parts per thousand (‰) according to the equation,
 d<sup>15</sup>N = (R<sub>sample</sub> /
 R<sub>standard</sub> – 1) X 1000, where R represents the ratio
 heavy to light isotopes
 (<sup>15</sup>N/<sup>14</sup>N) of the sample
 relative to the international reference standard. Measured
 d<sup>15</sup>N values were calibrated relative to air
 with certified reference materials. Within-run estimates of analytical
 precision were obtained by measurements of an internal laboratory
 reference material and yielded a precision (SD) ≤ 0.2‰.
 <em>Total Niche Width and Individual Diet
 Specialization</em>. To quantify the total niche width and degree of
 individual diet specialization between age groups and seasons, we used a
 modified version of Roughgarden index for use with
 d<sup>15</sup>N data (Roughgarden, 1972; Maldonado et al.,
 2017, 2019). Roughgarden used a simple framework to quantify individual
 diet specialization: the total niche width (TNW) of the (adult or
 juvenile) age group is the sum of two components: (1) the
 within-individual component (WIC) which reflects the average of the
 variability of resources used by individuals, and (2) the between
 individual component (BIC) that represents the between-individual
 variation in the average use of resources. The degree of individual diet
 specialization is reflected in the WIC/TNW ratio where low ratios close to
 zero indicate a higher prevalence of specialization within an age class,
 indicating a generalist population composed of individual specialists that
 each use a small subset of the total niche width.
 <em>Trophic Level</em>. In food web studies involving
 animals, d<sup>15</sup>N measurements are commonly used to
 infer the relative trophic position of consumers due to the predictable
 and significant enrichment of <sup>15</sup>N at higher trophic
 levels, known as trophic enrichment factors (TEFs). This enrichment occurs
 because of protein catabolism. During catabolism, amino acids with amine
 groups containing the heavier <sup>15</sup>N isotope are
 preferentially retained over those with the lighter
 <sup>14</sup>N isotope. Consequently, excreted urinary
 nitrogen is depleted in <sup>15</sup>N compared to the
 animal's tissues. This leads to a progressive accumulation of
 <sup>15</sup>N along the food chain, with each successive
 trophic level exhibiting higher
 <sup>15</sup>N/<sup>14</sup>N ratios than the
 previous level (Post, 2002; see also Karasov & Martinez del Rio,
 2007 and references therein). Additionally, using stable isotopes to
 estimate trophic levels requires a priori estimates of discrimination
 factors, which represents the differences in isotopic composition between
 an animal's tissue and its diet. These factors are crucial for
 accurately interpreting d<sup>15</sup>N data and inferring
 trophic positions in bird studies, as they vary depending on the tissue
 being analyzed. We used previously determined discrimination factors for
 bird tissues in our trophic level calculations (Caut et al.2009). We
 calculated the relative trophic level (TL) of each individual as follows
 (Post, 2002): TL= (1 +
 (d<sup>15</sup>N<sub>bird</sub> −
 d<sup>15</sup>N<sub>producers</sub>) /
 Δ<sup>15</sup>N, where the
 d<sup>15</sup>N<sub>bird</sub> represents the mean
 isotopic signature of tissues,
 d<sup>15</sup>N<sub>producers</sub> correspond to
 the baseline isotopic signature of the primary producers (plants) and
 Δ<sup>15</sup>N is the trophic discrimination factor. We used
 a trophic discrimination of 2.37‰ for collagen (claws), 2.25‰ for blood,
 and 2.82‰ for plasma (Caut et al., 2009). To calculate the baseline, we
 estimated the d<sup>15</sup>N values of plants and seeds for
 both seasons, but a t-test showed no significant differences between them.
 Therefore, we used a pooled sample d<sup>15</sup>N value of
 3.47‰ for the baseline at the study site.
 <em>Statistical Analysis</em>. To assess for
 differences in levels of IS of juveniles and adults against a null model,
 we employed a non-parametric Monte Carlo procedure with 10,000 iterations
 to derive p-values. The null model characterizes the total niche width of
 either juveniles or adults as being comprised of generalist individuals
 who forage randomly across the entire niche of their respective age
 classes (Bolnick et al., 2002). This Monte Carlo procedure is implemented
 in the R package RInSp (Zaccarelli et al., 2013). To evaluate whether
 significant differences exist in TNW, BIC, WIC, and WIC/TNW between
 juveniles and adults, we also used a Monte Carlo permutation procedure. To
 calculate a p-value, the observed mean difference in WIC/TNW between
 juveniles and adults was compared against the mean differences generated
 from 10,000 random permutations of juvenile-adult groups. We then assessed
 potential differences in trophic levels between the two age classes across
 seasons by linear mixed models (lmm; package lme4, Bates et al., 2015)
 with age class (juveniles, adults), seasons (spring, summer) and the
 interaction among them as fixed factors and tissue (nails, blood plasma
 and red blood cells) as random intercept. Model assumptions (normality,
 homoscedasticity and absence of residual autocorrelation) were confirmed.
 Finally, we estimated the isotopic niche overlap between juveniles and
 adults in both seasons by calculating the overlapped areas (and their
 bootstrap standard errors with 1000 replicates, SE) relative to each pair
 of d<sup>15</sup>N kernel density estimations using the
 overlapping R package (Pastore, 2018; Pastore and Calcagnì, 2019). To
 assess whether the estimated area of overlap was significantly different
 from what would be expected by chance -due solely to sample size- we
 performed a permutation test with 10,000 random permutations of the
 d<sup>15</sup>N values. We then compared the observed overlap
 area to the 95% confidence interval (CI<sub>0.95</sub>)
 generated from the permuted data.
description: # **The Interplay of Ecological Opportunity and Adult Foraging Strategies
 in Juvenile Sparrow Individual Specialization Dataset**
 [https://doi.org/10.5061/dryad.2jm63xsxj](https://doi.org/10.5061/dryad.2jm63xsxj) This datasets contains data obtained for a study performed in Mediterranean central Chile. It contains three data files with information about Arthropod diversity, plant diversity and stable isotope measurements of three different tissues (nails, plasma and red blood cells) of rufous-collared sparrows (*Zonotrichia capensis*) juvenile and adult individuals obtained in two seasons: spring and summer. This data was used to describe the results of this study and support its main conclusions. ## Description of the data and file structure Data is separated in three different files: **Arthropod_Data_2020_2021.csv:** Contains data of abundance of arthropods classified into different morphospecies during sampling campaigns in spring and summer of 2020 and 2021, sampling was performed in five transects (grids) throughout the landscape, each sample contained 10 glasses separated by 1 meter. ###### Variables description *Year:* Year when the sample was taken *Season:* The season corresponding to when the sample was taken, it can be "Spring" or "Summer" *Month:* Number of the month in which the sample was taken *Day:* Day of the month in which the sample was collected *Grid:* The number of the transect corresponding to that sample (From 1 to 5) *Sampling_days:* The number of days that the pitfall traps (glasses) were left in the grid to sample *Number_glass:* The number of the pitfall trap (from 1 to 10) *Morphospecies:* Name of the morphotype classification. Morphotypes correspond to the lowest taxonomic resolution possible during identification and classification. *N:* Number of individuals observed. **Plant_data.csv:** Contains data of richness and cover of living plants sampled several times in 5 grids throughout the landscape during spring of 2020. ###### Variables description *Year:* Year when the sample was taken *Season:* The season corresponding to when the sample was taken, it can be "Spring" or "Summer" *Grid:* The number of the transect corresponding to that sample (From C1 to C5) *Species:* Currently valid species scientific name *Cover:* Percent cover of living herbaceous species **Z.capensis_data.csv:** Contains isotopic values of three tissues (nails, plasma, red blood cells) for *Z. capensis* adult and juvenile individuals captured in the study site during spring and summer of 2020 and 2021. ###### Variables description *ID_Bird:* Identification number for each *Zonotrichia capensis* individual captured and sampled *Season:* The season corresponding to when the bird was captured and the samples were taken, it can be "Spring" or "Summer" *Stage:* Life stage of the captured bird. Juvenile individuals are labeled "J" and adult individuals are labeled "A" *Nails:* d15N values for the nails (claw tips) samples of the individuals (Methods for obtaining isotopic values are described in the "Methods" section) *Plasma:* d15N values for the plasma samples of the individuals. Plasma and red blood cells were separated inmediatly after sampling via centrifugation at 10,000 rpm for 10 min.  *Red blood cells:* d15N values for the red blood cells samples of the individuals. ***Important: Empty cells*** A few empty cells can be found in the Z.capensis_data.csv file. This happens when values for that tissue could not be obtained due to lack of enough tissue for isotopic measurement. We chose to leave them empty and not fill them with N/A so this wouldn't affect the scripts used to analyze the data. ## Code/Software The following code was run using R software: Resource Analysis.R: Using Arthropod_Data_2020_2021.csv and Plant_data.csv calculates diversity and abundance of arthropods and plants. RG_analysis.R: Using the Z.capensis_data.csv file, computes RG indices and assesses the significance of differences through permutation testing. Additionally, it calculates overlap and confidence intervals using bootstrapping. Annotations are provided throughout the scripts through 1) library loading, 2) dataset loading and cleaning, 3) analyses, 4) figure creation