DOI: 10.5061/dryad.n8pk0p32t
title: Stabilizing selection on a plumage-based foraging adaptation: hooded warblers with average-sized white tail spots live longer

description: Avian flush-pursuit insectivores typically use contrasting white plumage
 patches in their tails or wings to startle potential prey. Although
 experimental evidence indicates that the extent of white has been
 fine-tuned by natural selection to optimize foraging performance, the
 hypothesis that within-population plumage variation directly influences
 survival or lifetime reproduction and is subject to stabilizing selection
 has not been tested. Here I provide such a test using data collected as
 part of a 14-year study of a colour-ringed breeding population of the
 hooded warbler (Setophaga citrina), a migratory flush-pursuit insectivore
 that shows inter-individual variation in the extent of white in the tail
 that is highly repeatable across molts and likely heritable. As expected
 under stabilizing selection, warblers with average-sized white tail
 patches achieved significantly higher long-term apparent survival than
 individuals with either a lesser or greater extent of white in the tail.
 Evidence of stabilizing selection was especially strong in males, an
 observation that is likely related to pronounced sexual habitat
 segregation on the wintering range. My results provide infrequently
 observed evidence of stabilizing selection operating in a natural
 population and also illustrate how stabilizing selection can act on avian
 plumage traits outside the context of sexual and social signaling.
description: Field data were collected as part of a
 long-term study of hooded warblers at Hemlock Hill Field Station (41.8°N,
 79.9°W), 2010-2023. Intensive efforts were made May-August each year to
 completely census the breeding population of 45–70 pairs, capture and
 individually colour-ring all breeding adults, and locate and monitor the
 reproductive success of all nests. Complete details on general field
 methods and hooded warbler breeding biology are available elsewhere [Mumme
 2018, Mumme et al. 2023, Lignac and Mumme 2023].  When birds were captured for ringing, I took a
 standard series of morphometric measures, including wing chord, tail
 length, body mass, and the maximum linear extent of white in each of the
 outer 3 tail feathers, rectrices 4–6 (figure 1a). I then calculated a tail
 white index (TWIndex) as the sum of the 3 measures divided by 3 times the
 total tail length, an index that approximates the proportion of the outer
 rectrices that is white. TWIndex is strongly correlated with the extent of
 white in each rectrix (electronic supplementary material, figure S3) and
 constitutes an effective summary index of the extent of white in the tail
 that simultaneously controls for variation in tail length. About 28% of
 males and 14% of females have an additional small white spot on rectrix 3.
 However, because this tail spot is absent from most individuals, is
 considerably smaller in both length and width, and is only weakly
 correlated with the extent of white in the other rectrices (electronic
 supplementary material, figure S3), rectrix 3 was not included in the
 calculation of TWIndex. For individuals captured and measured in multiple
 years (electronic supplementary material, figure S1), I used their mean
 measurements in statistical analyses.  For all colour-marked breeding individuals I
 calculated their apparent long-term survival, defined as the difference
 between the last year an individual was observed or captured in the study
 area and the year it was initially captured and marked. Thus, apparent
 long-term survival would be 0 years for a bird present only 1 year.
 Survival data were available only for breeding birds rung 2010–2022; mean
 apparent long-term survival was 1.05 (range 0–8, n = 317) for males and
 0.73 (range 0–7, n = 308) for females. A small number of marked
 individuals (8 males, 14 females) were absent from the study area (or
 present but undetected) 1–2 years before returning in a subsequent year,
 but their apparent long-term survival was calculated in the same way as
 birds present continuously. <strong>(b) Statistical analysis: apparent
 long-term survival</strong> Apparent long-term survival was modeled as a
 Poisson variable (log link function) in a Generalized Linear Mixed Model
 (GLMM). I chose this approach over alternatives, such as
 Cormack-Jolly-Seber mark-recapture models [Schaub and Royle 2013] or Cox
 proportional hazards models [Moore 2016], because the primary analytical
 focus for stabilizing selection is the effects of variation in tail white
 on the observed lifespan of individual birds, not on population-level
 parameter estimates of either annual survival or hazard. First- and
 second-order (quadratic) polynomials of TWIndex, and their interactions
 with sex, were modeled as fixed factors; including the quadratic terms
 allows statistical testing of the most likely forms of selection acting on
 variation in tail white, including positive and negative directional
 selection, disruptive selection, and stabilizing selection [Kingsolver et
 al. 2012]. Initial year on the study site was included in the model as a
 random effect.  Apparent long-term
 survival clearly underestimates true long-term survival, for two reasons.
 First, birds rung later in the study had fewer years of potential survival
 available to them than birds from earlier cohorts, an issue that can be
 controlled statistically by including initial year on the study site as a
 random effect in the GLMM. Second, apparent long-term survival confounds
 actual mortality with undetected emigration off the study area [Schaub and
 Royle 2013], a problem that is especially prevalent in females, which show
 less breeding-site fidelity than males [Howlett and Stutchbury 2003, Mumme
 et al. 2023]. However, because it is unlikely that the extent of white in
 the tail has any effect on emigration or breeding site fidelity, apparent
 long-term survival is likely an unbiased underestimate of true long-term
 survival in relation to tail white. All statistical analyses were
 conducted using the Fit Model platform of JMP Pro 17.0 (SAS Institute
 Inc., Cary, North Carolina, USA). <strong>(c) Statistical analysis:
 reproductive performance and body condition</strong> Reproductive performance for each individual hooded
 warbler was calculated as the total number of young fledged from nests
 throughout its reproductive life on the study area. Mean ± SD lifetime
 reproductive output was 4.6 ± 4.8 (range 0–25, n = 317) for males and 4.2
 ± 3.9 (range 0–27, n = 308) for females. Because of high rates of
 extra-pair fertilizations [Stutchbury et al. 1994, Chiver et al. 2008,
 Mumme et al. 2023], conclusions about male reproductive performance should
 be interpreted cautiously. I investigated the effects of tail white on
 breeding-season body condition by analyzing data on body mass after
 controlling for the effects of wing chord—a strong indicator of overall
 body size—and sex. Data on body mass were available for only a subset of
 individuals in the complete data set; mean ± SD body mass was 11.3 ± 0.5 g
 (n = 266) for males and 11.9 ± 0.8 g (n = 186) for females.
 Reproductive performance and body mass were
 both modeled as normal (Gaussian) variables in General Linear Mixed
 Models. Initial year on the study site was included as a random effect,
 and the first- and second-order polynomials of TWIndex, and their
 interactions with sex, were modeled as fixed factors. The model for
 lifetime reproductive performance included number of years present on the
 study site as a fixed factor, while the body mass model included wing
 chord as a fixed factor. Examination of normal quantile plots indicated
 that residuals of both models approximated a normal
 distribution. <strong>References</strong> Chiver I, Stutchbury BJM, Morton ES. 2008 Do male plumage and song characteristics influence female off-territory forays and paternity in the hooded warbler? <em>Behav. Ecol. Sociobiol. </em><strong>62</strong>, 1981–1990. (doi:10.1007/s00265-008-0629-x) Howlett JS, Stutchbury BJM. 2003 Determinants of between-season site, territory, and mate fidelity in hooded warblers (<em>Wilsonia citrina</em>). <em>Auk</em> <strong>120</strong>, 457–465. (doi:10.1093/auk/120.2.457) Kingsolver JG, Diamond SE, Siepielski AM, Carlson SM. 2012 Synthetic analyses of phenotypic selection in natural populations: lessons, limitations and future directions. Evol. Ecol. <strong>26,</strong> 1101–1118. (doi:10.1007/s10682-012-9563-5) Lignac C, Mumme RL. 2023 Brood parasitism of hooded warblers by brown-headed cowbirds: severe impact on individual nests but modest consequences for seasonal fecundity and conservation. Ornith. Appl. 125, duac041. (doi:10.1093/ornithapp/duac041) Moore DF. 2016 Applied survival analysis using R. Springer International Publishing, Cham, Switzerland. Mumme RL. 2018 The trade-off between molt and parental care in hooded warblers: simultaneous rectrix molt and uniparental desertion of late-season young. Auk: Ornithol. Adv. 135, 427–438. (doi:10.1642/AUK-17-240.1) Mumme RL, Chiver I, Evans Ogden LJ, Stutchbury BJ. 2023 Hooded warbler (<em>Setophaga citrina</em>), version 2.0. In <em>Birds of the World</em> (Rodewald PG, Ed.). Cornell Lab of Ornithology, Ithaca, NY, USA. (doi:10.2173/bow.hoowar.02) Schaub M, Royle A. 2014 Estimating true instead of apparent survival using spatial Cormack–Jolly–Seber models. <em>Meth. Ecol. Evol.</em> <strong>5</strong>, 1316–1326. (doi: 10.1111/2041-210X.12134) Stutchbury BJ, Rhymer JM, Morton ES. 1994 Extra-pair paternity in hooded warblers. <em>Behav. Ecol.</em> <strong>5</strong>, 384–392. (doi:10.1093/beheco/5.4.384)
description: # Stabilizing selection on a plumage-based foraging adaptation: hooded
 warblers with average-sized white tail spots live longer
 [https://doi.org/10.5061/dryad.n8pk0p32t](https://doi.org/10.5061/dryad.n8pk0p32t) This archive contains all the data presented in both the published paper and the electronic supplemental material. ## Description of the data and file structure Four separate data files in .csv format are included in this archive; missing values are indicated by "null": **1\. FinalMaleFemaleDataset\.csv** This file is the primary data file for analysis. I contains records for 625 individual hooded warblers stored in 17 columns, A–Q: 1. IDCode – a unique identifier, based on colour-band combinations, for each bird. 2. BandNum – the unique U.S. Fish and Wildlife Service band number. 3. Sex – F indicates female, M indicates male. 4. DateBand – Date the bird was initially banded or measured. 5. FirstYr – The first year the bird was present on the study area. 6. LastYr – The last year the bird was present on the study area. 7. YrPr – How many years the bird was present on the study area. 8. YrSurv – How many years the bird survived, calculated as LastYr–FirstYr 9. YoungFl – Total number of young fledged from the bird’s nests throughout its entire reproductive lifespan on the study area. 10. WingC – Wing chord (length) in mm. 11. TailL – Tail length (in mm) 12. R6White – Maximum linear extent of white (in mm) on rectrix 6 13. R5White – Maximum linear extent of white (in mm) on rectrix 5 14. R4White – Maximum linear extent of white (in mm) on rectrix 4 15. R3White – Maximum linear extent of white (in mm) on rectrix 3 16. Mass – Body mass (in g) 17. TWIndex – Tail white index, calculated as the (R6White + R5White + R4White)/ (3 \* TailL). **2\. AnnualSurvivalData\.csv** This file was used to examine annual survival (see electronic supplementary material, figure S7). It contains records for 1161 individual bird-years, stored in 19 columns, A–S. Most of the column names are identical to those in the previous file (FinalMaleFemaleDataset.csv), but 4 are new: 9. Year – Year of potential survival for this particular bird. 10. Present – Y simply indicates that the bird was present on the study area this particular year. 11. Sur – 1 indicates this bird was known to have survived to the following year, while 0 indicates that it was not known to have survived to the following year. 12. Age – Best estimate of the bird’s age this particular year. **3\. RepeatabilityData\.csv** This file was used to estimate the repeatability (intraclass correlation coefficient) of the tail white index, TWIndex (see electronic supplementary material, figure S1). It contains 111 capture records for 52 individuals whose tails were measured in two or more years separated by the annual molt. Most of the columns (A–M) are identical or similar to those used in the previous two files, but two are new: 1. Age – Age at the time of capture; SY indicates a bird in its second year of life, ASY indicates an older bird (after second year of life). 2. N – Number of separate capture records available for this bird. **4\. HeritabilityData\.csv** This file was used to estimate the heritability of the tail white index, TWIndex (see electronic supplementary material, figure S2). It contains 7 records of tail measurements of birds for whom tail measurements are also available for their putative parents. The columns are: 1. OffspringID – The unique ID code of the offspring. 2. Sex – Offspring sex. 3. FatherTWI – Tail white index of the putative father. 4. MotherTWI – Tail white index of the putative mother. 5. MeanParTWI – Mean tail white index of both putative parents. 6. OffspringTWI – The tail white index of the offspring.