DOI: 10.5061/dryad.jm63xsjh5
title: Climate change outpaces adaptive potential via hybridization in nesting female Saltmarsh and Nelson’s Sparrows

description: Hybridization and introgression can promote adaptive potential and
 evolutionary resilience in response to increased pressures of climate
 change; they can also disrupt local adaptation and lead to outbreeding
 depression. We investigated female fitness consequences of hybridization
 in two sister species that are endemic to a threatened tidal marsh
 ecosystem: Saltmarsh (Ammospiza caudacutus) and Nelson’s Sparrows (A.
 nelsoni). We found increasing nest flooding rates due to rising sea levels
 are outpacing potential adaptive benefits of hybridization due to very low
 overall nesting success in both the Nelson’s and Saltmarsh Sparrows. In
 the center of the hybrid zone across two years, we determined the success
 of 201 nests of 104 pure and admixed Saltmarsh and Nelson’s Sparrow
 females, genotyped using a panel of Single Nucleotide Polymorphisms (SNPs)
 from double digest restriction-site associated DNA (ddRAD) Sequencing. We
 evaluated five metrics of female fitness and modeled nesting success in
 relation to genotypic, environmental, and nesting characteristics. We
 found differential fitness among Saltmarsh, Nelson’s, and hybrid females,
 such that birds with predominantly Saltmarsh Sparrow alleles had higher
 reproductive success than birds with predominantly Nelson’s Sparrow
 alleles, and hybrids were intermediate. Fledging success increased with
 two known tidal marsh nesting adaptations: nest height and nesting
 synchrony with tidal cycles. We found a positive relationship between
 hybrid index and fitness in daily nest survival in 2016, but not in 2017,
 likely due to differing levels of precipitation and nest flooding between
 years. The strongest and most consistent predictors of daily nest survival
 were nesting synchrony with lunar tidal flooding cycles and daily maximum
 tide height. Fitness patterns suggest there may be an adaptive benefit of
 interspecific geneflow for the Nelson’s Sparrow at the detriment of the
 Saltmarsh Sparrow; however, flooding rates are so high in many years they
 mask any fitness differences between the species, and all females had poor
 nesting success, regardless of genetic makeup.
description: We captured, banded, and genotyped a
 total of 104 female sparrows across the two sites and years. We monitored
 202 nests (including 301 nestlings) of pure and admixed Saltmarsh and
 Nelson’s Sparrows across the two sites across the 2016 and 2017 breeding
 seasons. We conducted nest monitoring at both sites during May-August,
 encompassing three nesting cycles each in 2016 and 2017, as described in
 Maxwell et al. 2021 and following protocols from Saltmarsh Habitat and
 Avian Research Program (SHARP, www.tidalmarshbirds.org). Nests were
 visited every 3–4 days until completed and assigned an overall fate
 (fledged, failed due to flooding, failed due to predation, or failed due
 to unknown causes), following established standardized protocols (Ruskin
 et al., 2017). We calculated date of nest initiation based on known
 duration of egg-laying (3–5 days), incubation (11–12 days), and chick
 development (8–11 days) to determine first egg date following methods
 developed by Shriver et al. (2007). We collected vegetation and nest
 characteristic data to test predictions about nesting characteristics as
 drivers of reproductive success. Vegetation data were collected within 1
 m<sup>2 </sup>surrounding each nest upon its completion
 (fledge/fail/unknown failure). Measurements included thatch depth (height
 of vegetative thatch layer from marsh surface in cm) at nest and across
 four cardinal points in 1 square meter surrounding the nest, average
 vegetation height directly above and surrounding the nest (in cm across
 four cardinal points in 1 square meter and at the center), as well as
 species composition of the vegetation to determine the percent high marsh
 vegetation surrounding the nest (percent <em>Spartina
 patens)</em>. We recorded physical characteristics of the nests
 including, height above the ground (cm from cup lip and cup bottom to
 surface of the marsh), presence/absence of nest canopy (woven/domed
 structure that effectively covers the nest cup and prevents eggs from
 washing out of the nest during high tides (Humphreys et al. 2007)),
 percent of nest visible from above, and the species of vegetation of which
 the nest was made. To determine timing of nest initiation in relation to
 the nearest flood tide, we calculated the number of days after the new
 moon (the highest tidal amplitudes and flooding were on new moon dates due
 to lunar tidal cycles) that the nest was initiated, with initiation
 determined as described above, following Shriver et al. (2007).
 HOBO water level loggers (ONSET,
 Bourne, MA) were placed at the bottom of a central channel at each study
 site to monitor the water levels on each day of the breeding season. These
 loggers measure the total pressure above their location at 15-minute
 intervals. With barometric pressure collected from the National Oceanic
 and Atmospheric Administration Stations nearest the study site locations,
 a compensation was made using HOBOware Pro software to determine water
 level seen at each marsh in 15-minute intervals throughout the entire
 three-month breeding season (in 2016 and 2017). Nestlings were banded with a USGS aluminum
 leg band and a single site-specific color band when they were ~6 days old.
 Additionally, standard morphological measurements were taken on each
 nestling during banding, including weight, tarsus length, bill length,
 head length, and wing cord. Proxys
 of fitness used in this study were short-term reproductive metrics taken
 across the two-year study period and included: daily nest survival
 estimates, fledging success (number of offspring fledged from each nest,
 including 0), hatching success (number of eggs hatched from each nest,
 including 0), clutch size (maximum number of eggs/nestlings in a nest),
 average nestling size, and maximum nestling size in a nest (measured in
 grams at ~ 6 days of age at banding). These measures of reproductive
 success represent a snapshot in time (one breeding cycle), and do not
 reflect lifetime fitness of an individual. Data are
 provided in their raw form in columns in Excel
 spreadsheets. 
description: Data files are provided as CSV spreadsheet files and do not
 require any special software to open or use. From here, data can be used
 for modeling or for creation of capture histories for survival
 analysis.