DOI: 10.5061/dryad.4j0zpc8fd
title: Plasticity in female timing may explain earlier breeding in a North American songbird

description: Many species have shifted their breeding phenology in response to climate
 change. Identifying the magnitude of phenological shifts and whether
 climate-mediated selection drives these shifts is key for determining
 species’ resilience to climate change. Birds are a strong model for
 studying phenological shifts due to numerous long-term research studies;
 however, generalities pertaining to drivers of phenological shifts will
 emerge only as we add study species that differ in life history and
 geography. We investigated 32 years of reproductive timing in a
 non-migratory population of dark-eyed juncos (Junco hyemalis). We
 predicted that plasticity in reproductive timing would allow females to
 breed earlier in warmer springs. We also predicted that selection would
 favour earlier breeding, and we asked whether temperatures throughout the
 breeding season would predict the strength of selection. To test these
 predictions, we examined temporal changes in the annual median date for
 reproductive onset (i.e., first egg date), and we used a sliding window
 analysis to identify monthly spring temperatures driving these patterns.
 Next, we explored plasticity in reproductive timing and asked whether
 selection favoured earlier breeding. Lastly, we used a sliding window
 analysis to identify the time during the breeding season that temperature
 was most associated with selection favouring earlier breeding. First egg
 dates occurred earlier over time and strongly covaried with April
 temperatures. Further, for individual females that bred in more than one
 year, they typically bred earlier in warmer Aprils, exhibiting plastic
 responses to April temperature. We also found significant overall
 selection favouring earlier breeding (i.e., higher relative fitness with
 earlier first egg dates) and variation in selection for earlier breeding
 over time. However, temperature across diverse climatic windows did not
 predict the strength of selection. Our findings provide further evidence
 for the role of phenotypic plasticity in shifting phenology in response to
 earlier springs. We provide evidence for the role of selection favouring
 earlier breeding, regardless of temperature, thus setting the stage for
 adaptive changes in female breeding phenology. We suggest for
 multi-brooded birds that advancing first egg dates likely increases the
 length of the breeding season, and therefore, reproductive success.
description: <strong>Study system and
 breeding data</strong> Since 1983, a breeding population of Dark-eyed
 Juncos has been monitored at Mountain Lake Biological Station (MLBS) and
 the surrounding Jefferson National Forest (37°22’N, 80°32’W)
 (Chandler et al.
 1994). All monitoring
 protocols were approved by Indiana University’s Institutional Animal Care
 and Use Protocol (#12-050). At the beginning of each breeding season
 (April-May), birds on the study site were caught using mist nets or Potter
 traps and banded with a unique USFWS metal band and distinctive
 combinations of colour bands. Researchers searched for nests every year,
 identifying parents and tracking the progress of the nest. First egg date,
 expressed as ordinal date, was observed directly, or for nests found after
 the start of egg-laying, was calculated based on the day nestlings hatched
 or left the nest (Nolan et
 al. 2002). Breeding data
 from 1983–2015 were used for this study except for 2013 due to limited
 research effort. Records where female ID or first egg date were unknown
 were removed. Female subjects that were implanted with exogenous
 testosterone during a separate five-year study in the population were also
 removed (Clotfelter et al.
 2004; Ketterson et al. 2005). To calculate true first egg dates, we excluded
 any known re-nests. Also, knowing that the first nest found for a female
 might not be her true first nest, we eliminated nests whose first egg
 dates came later than each year’s median first egg date from known
 re-nests. Our data filtering resulted in 1,244 first nests of 935 female
 juncos between 1983 and 2015; females had one to five years of data (x̃ =
 1). Annual differences in research effort (number of nests found) did not
 explain variation in first egg dates (see Supplementary Materials; Fig.
 S1). Because the distributions of first egg dates were not normal in some
 years, we calculated median annual first egg dates from first nests. Using
 both first nests and re-nests for each year, we calculated the annual
 total number of eggs and total number of fledglings produced by each
 female. Females were grouped into two age classes based on plumage
 (Pyle
 1997) or records from
 previous breeding seasons: second years (SY; first breeding season) and
 after second years (ASY; second or later breeding season).
   <strong>Temperature
 data</strong> Between November 16, 1971 and January 31, 1998,
 temperature data (daily minimum; T<sub>min</sub> and maximum
 temperature; T<sub>max</sub>) were collected from MLBS via a
 National Oceanic and Atmospheric Administration (NOAA) weather station
 (Network ID GHCND: USC00445828, hereafter, “Logger A”). On June 24, 1994,
 a second data logger (Campbell CR10) was established at MLBS that records
 temperature every half hour. To permit comparing data across devices, we
 calculated daily T<sub>max</sub> and
 T<sub>min</sub> from this MLBS data logger (hereafter, “Logger
 B”). From T<sub>min</sub> and T<sub>max</sub>, we
 calculated a daily midpoint (median) temperature (T<sub>mid</sub>) for
 both loggers. Since the two weather stations overlapped from 1994-1997, we
 confirmed that Datasets A and B were strongly correlated and then combined
 the datasets (see Supplementary Materials).
 Monthly average
 T<sub>min</sub>, T<sub>max</sub>, and
 T<sub>mid</sub> were calculated for
 March–August for each year. Data were available for all
 years (1983-2015), except for missing March and April data for 1991 and
 2002 and missing March data for 2004.
description: Files have been analysed with R.