Stabilizing selection on a plumage-based foraging adaptation: hooded warblers with average-sized white tail spots live longer

Abstract

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 moults and probably heritable. As expected under stabilizing selection, warblers with average-sized white tail patches achieved significantly higher apparent long-term 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 probably 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 signalling.

1. Introduction

Stabilizing natural selection plays a critical role in evolutionary theory and the process of adaptation. Once a trait reaches an adaptive peak via directional or other forms of selection, stabilizing selection is hypothesized to maintain this evolutionary optimum and prevent further phenotypic changes that might arise via mutation and genetic drift [1,2]. Despite its theoretical importance, however, there is surprisingly little evidence of widespread stabilizing selection operating in natural populations [3,4], and detailed studies of biological traits suspected to be direct targets of stabilizing natural selection are clearly needed [5].

One group of organisms with traits likely to be under strong stabilizing selection are avian flush–pursuit insectivores. Species in this ecological guild typically have contrasting white plumage patches in their tails or wings that are revealed during stereotypic foraging displays and function to startle and flush winged insects that are then pursued and captured in flight [6–8]. Displays of contrasting plumage patches exploit evolutionarily conserved insect escape responses [9,10], and evidence from the genus Myioborus (family Parulidae) suggests that plumage patterns have been fine-tuned by natural selection; field and laboratory experiments with both birds and their insect prey have demonstrated that plumage patterns which depart from the population mean are less effective at startling potential prey and lead to reduced overall foraging performance [11–13]. However, there is currently no evidence that natural within-population variation in plumage pattern affects survival or lifetime reproduction and is subject to stabilizing selection.

The hooded warbler (Setophaga citrina) is a flush–pursuit forager that is a good candidate for stabilizing selection on plumage pattern. In all individuals the inner vanes of the three outer tail feathers (rectrices 4–6) are broadly tipped with white (figure 1a). By continuously flicking open their tails while foraging, hooded warblers startle winged insects that constitute the major portion of their diet [8,14,15]. Field experimental evidence indicates that the white tail spots are likely to be a target of selection, as birds with temporarily darkened tail spots have significantly reduced prey capture rates relative to sham-darkened controls (figure 1b; [8]). Hooded warblers employ tail-flicking behaviour while foraging during all phases of their complex migratory life cycle; they breed in forested habitats in eastern North America and winter primarily in the Yucatan Peninsula and neighbouring regions of Mexico, northern Central America and the Caribbean [16]. On the wintering grounds, males and females have strikingly different habitat preferences; males winter primarily in closed-canopy tropical and subtropical forest, while females prefer more open shrubby habitats [16–18].

Figure 1.

(a) Typical tail pattern of a hooded warbler, showing the broad white spots on the inner vanes of the three outer tail feathers, rectrices R6–R4. Box plots in (b) portray previously published findings showing that birds with temporarily darkened tail spots have significantly lower prey attack rates than sham-darkened controls [8]. A video showing how the white tail spots are revealed by tail-flicking during foraging is available at https://macaulaylibrary.org/asset/403553.

The extent of white in an individual hooded warbler's tail is highly repeatable across years and feather replacement during the annual late-summer moult [16]. An index of the amount of white in the outer three rectrices has an intraclass correlation coefficient of 0.745, suggesting that nearly 75% of the variation in the extent of white is attributable to consistent differences among individuals, with only 25% attributable to annual or measurement variation within individuals (electronic supplementary material, figure S1). Estimating trait heritability in hooded warblers is challenging because (i) low rates of natal philopatry mean that few individuals of known parentage are encountered and measured as adults [16], and (ii) high rates of extra-pair fertilization make paternity of offspring ambiguous [16,19,20]. Nonetheless, limited data suggest that inter-individual variation in tail white has a significant heritable component, particularly through the mother (electronic supplementary material, figure S2). Collectively, these findings suggest that inter-individual variation in the extent of white in the tail of hooded warblers has a substantive underlying genetic basis and is only modestly influenced by annual changes in environmental conditions during moult.

Here, I test the hypothesis that the natural within-population variation in the extent of white in the tail of hooded warblers is under stabilizing selection for long-term survival. I also examine how tail-white variation affects reproductive performance and body condition during the nesting season. The analysis is based on a 14-year study of a colour-marked breeding population in northwest Pennsylvania, USA.

2. Methods

(a) . Study site and data collection

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 [16,21,22].

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 three tail feathers, rectrices 4–6 (figure 1a). I then calculated a tail white index (TWIndex) as the sum of the three measures divided by three 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 on the study area and the year it was initially captured and marked. Thus, long-term apparent survival would be 0 years for a bird present only 1 year. Survival data were available only for breeding birds ringed 2010–2022; mean long-term apparent 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 (eight 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.

(b) . Statistical analysis: apparent long-term survival

Apparent long-term survival was modelled 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 [23] or Cox proportional hazards models [24], 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 modelled 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 [5]. 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 ringed 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 [23], a problem that is especially prevalent in females, which show less breeding site fidelity than males [16,25]. 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 probably an unbiased underestimate of true long-term survival in relation to tail white. All statistical analyses were conducted using JMP Pro 17.0 (SAS Institute Inc., Cary, North Carolina, USA).

(c) . Statistical analysis: reproductive performance and body condition

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 ± s.d. lifetime reproductive output was 4.6 ± 4.8 fledged young (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 [16,19,20], conclusions about male reproductive performance should be interpreted cautiously. I investigated the effects of tail white on breeding-season body condition by analysing 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 dataset; mean ± s.d. 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 modelled as normal (Gaussian) variables in GLMMs. 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 modelled 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.

3. Results

(a) . Apparent long-term survival

The extent of white in the tail (TWIndex) is normally distributed in both sexes, with males having slightly but significantly more white (ANOVA, F_1,623 = 27.6, p < 0.0001; figure 2a,b); an average male has about 2 mm more white in each of the three outer rectrices than does an average female (electronic supplementary material, figure S3).

Figure 2.

The extent of white in the outer three tail feathers of hooded warblers is normally distributed, and individuals with intermediate levels of white have higher apparent long-term survival. (a,b) The frequency distribution of the tail white index (TWIndex) for males and females, with fitted normal distribution curves; both distributions are consistent with normality (Shapiro–Wilk W = 0.996, p > 0.52 for both). (c) Predicted and observed values of apparent long-term survival for males and females in relation to the TWIndex. Predicted survival values are the red and blue curves plotted against the left y-axis and derived from the Poisson generalized linear mixed model shown in table 1; observed values are the jittered points plotted against the right y-axis.

Predictions from the Poisson GLMM indicate that individuals with intermediate amounts of tail white achieved the highest apparent long-term survival (figure 2c); the model identified significant effects of sex and the quadratic term TWIndex² but no significant interaction between them (table 1). Although the GLMM suggests a stronger effect of TWIndex on survival in males, the shape of the curve relating apparent survival to TWIndex was similar for the sexes and consistent with stabilizing selection; males and females with a TWIndex of about 0.50–0.53 had the highest apparent long-term survival, while individuals with either lesser or greater tail white experienced lower apparent survival (figure 2c).

Table 1.

Poisson generalized linear mixed model (GLMM) examining the effects of sex and the extent of white in the tail (TWIndex) on apparent long-term survival (number of years survived) in the hooded warbler. (Initial year present on the study site was included in the model as a random effect. The second-order polynomial of tail white (TWIndex²) and sex both had significant effects on apparent survival, but there was no significant TWIndex² * sex interaction. Model predictions of apparent long-term survival are plotted in figure 2c. Marginal and conditional R² for the model were 0.063 and 0.216, and the intraclass correlation coefficient for the random effect was 0.163. The model was calculated using the GLMM personality of the Fit Model platform of JMP Pro 17.0 (SAS Institute Inc., Cary, North Carolina, 2023).)

model term	estimate	s.e.	denominator d.f.	t	p-value
intercept	0.30	0.53	619	0.57	0.57
sex	–0.23	0.05	619	–4.20	<0.0001
TWIndex	–0.75	0.99	619	–0.76	0.44
TWIndex2	–44.19	15.20	619	–2.91	0.0038
TWIndex × sex	0.53	0.98	619	0.54	0.59
TWIndex2 × sex	20.11	15.32	619	1.31	0.19

Four alternative analytical approaches also show that birds with intermediate amounts of white in the tail have higher apparent survival, particularly for males (electronic supplementary material, figures S4–S7). The main target of stabilizing selection appears to be the amount of white in rectrix 5, with weaker evidence of stabilizing selection acting on rectrices 6 and 4 (electronic supplementary material, figure S8). Because TWIndex is uncorrelated with other morphometric characters (electronic supplementary material, figure S9), evidence that tail white is under stabilizing selection for long-term survival is unlikely to be confounded by selection acting on a correlated character.

(b) . Reproductive performance and body condition

Variation in the extent of tail white had no significant linear or curvilinear effects on either lifetime fledgling production or breeding-season body mass (table 2). Number of years on the study site was the only significant predictor of lifetime fledgling production (table 2a), and it alone explained 60% of the variation in reproductive performance (electronic supplementary material, figure S10). Body size (wing chord) and sex were the only significant predictors of breeding-season body mass (table 2b; electronic supplementary material, figure S11).

Table 2.

General linear mixed models (Gaussian distribution) showing that variation in the extent of white in the tail (TWIndex) has no significant linear or curvilinear effect on either lifetime reproductive performance (a) or breeding-season body mass (b) in hooded warblers. (Initial year present on the study site was included in both models as a random effect. Marginal and conditional R² and the intraclass correlation coefficient (ICC) for the random effect are shown for each model. Models were calculated using the Mixed Model personality of the Fit Model platform of JMP Pro 17.0 (SAS Institute Inc., Cary, North Carolina, 2023).)

model term	estimate	s.e.	denominator d.f.	t	p-value
(a) total young fledged: marginal R2 = 0.603, conditional R2 = 0.618, ICC = 0.038
intercept	–1.57	1.25	608.6	–1.26	0.21
sex	0.19	0.14	616.9	1.43	0.15
n years present	2.68	0.09	612.9	30.14	<0.0001
TWIndex	1.68	2.32	614.6	0.72	0.47
TWIndex2	37.06	32.48	615.4	1.14	0.25
TWIndex × sex	–0.94	2.30	608.7	–0.41	0.68
TWIndex2 × sex	41.59	32.49	616.1	1.33	0.20
(b) body mass: marginal R2 = 0.206, conditional R2 = 0.229, ICC = 0.029
intercept	7.33	1.26	444.9	5.81	<0.0001
sex	0.44	0.05	445.0	8.73	<0.0001
wing chord	0.067	0.018	444.4	3.60	0.0004
TWIndex	–0.21	0.67	440.8	–0.31	0.47
TWIndex2	12.67	9.37	443.1	1.35	0.18
TWIndex × sex	0.48	0.67	440.4	0.72	0.48
TWIndex2 × sex	–2.02	9.32	443.1	–0.22	0.82

4. Discussion

This study provides strong evidence that natural within-population variation in the tail pattern of hooded warblers is under stabilizing natural selection; birds with an intermediate extent of white in the tail have higher apparent long-term survival than individuals with more extreme amounts of white (figure 2c). The most likely mechanism to explain this finding is that the precise tail pattern of hooded warblers, as in other flush–pursuit insectivores [11–13], can have significant consequences for flush–pursuit foraging performance [8], which in turn affects an individual's survival prospects. In other words, the extent of white in the tail of hooded warblers is fine-tuned by stabilizing selection to optimize foraging performance and maximize long-term survival.

Although foraging performance is the most likely way that variation in tail white affects apparent survival, birds with extensive tail white may be at a survival disadvantage for two other reasons: (i) white feather vanes abrade more readily than pigmented vanes [26], so excess white in the tail may reduce flight performance in birds in worn plumage, and (ii) larger tail spots may make birds marginally more visible and vulnerable to predators (e.g. [27]). Neither of these alternatives, however, can explain why individuals with below-average tail white have lower apparent survival (figure 2c); only the foraging performance hypothesis successfully predicts poor survival of individuals with both below-average and above-average extents of white in the tail.

Six different analytical approaches all suggest that stabilizing selection for survival acts more strongly on tail white in males than in females (figure 1; electronic supplementary material, figures S4–S8). This sex difference may partially reflect the lower breeding site fidelity of females [16,25]; many surviving females may have emigrated off the study area and were scored as having not survived, weakening any statistical relationship between tail white and survival. However, data simulations of off-site dispersal (electronic supplementary material, figure S12) suggest this cannot be the entire explanation for why male survival appears to be more strongly influenced by tail white. Instead, three lines of evidence suggest that stabilizing selection may be acting more strongly on males in the wintering range. First, male hooded warblers winter primarily in dark closed-canopy tropical forest while females prefer more open shrubby habitats [16,17,18], and it is possible that the strength of stabilizing selection on tail white may differ across these overwintering habitat types. Second, a growing body of evidence suggests that food availability on the wintering grounds may be a primary ecological driver of annual mortality and population size in migratory songbirds [28], increasing the opportunity for stabilizing selection to operate on a foraging adaptation during the wintering phase of the annual cycle. Third, birds with intermediate amounts of tail white do not enjoy either higher nesting success or higher breeding-season body condition (table 2), suggesting that stabilizing selection for optimum tail white, foraging performance and long-term survival is acting primarily outside of the breeding season. Additional work is needed, however, to clarify how stabilizing selection acts on tail white across the entire annual cycle and if it is indeed more important in males on the wintering grounds.

A related unresolved question is why males have more white in their tails than do females (figure 2a,b). Two hypotheses, not mutually exclusive, may bear on this question. First, selection for improved foraging performance on the wintering grounds may favour more extensive white in the tails of males; experimental evidence indicates that in dark habitats—like the closed-canopy tropical forests favoured by male hooded warblers in winter—plumage patterns with extensive white are more effective at flushing potential prey [12]. Second, intersexual selection may favour increased tail white in males. When courting females for extra-pair matings, males regularly spread their tails, revealing their contrasting white tail spots [16]. If males with larger tail spots are more successful in acquiring extra-pair copulations, intersexual selection could explain the sex difference in tail white. This intersexual selection hypothesis would also help explain why the extent of white in the tails of males (figure 2a) appears to be slightly greater than the extent that would maximize long-term survival (figure 2c; electronic supplementary material, figures S5–S7).

Variation in tail white had no significant effect on lifetime fledgling production once duration of reproductive lifespan was statistically controlled (table 2; electronic supplementary material, figure S11). This finding was not unexpected, as nesting success in hooded warblers is determined primarily by nest predation [16] and nest parasitism by brown-headed cowbirds (Molothrus ater) [22], not by the ability of adults to forage efficiently and provision nestlings [16]. Thus, variation in tail white probably influences lifetime reproductive success primarily via effects on survival and reproductive lifespan, not via effects on nesting success.

In conclusion, the results presented here provide a compelling example of how stabilizing natural selection acts on repeatable and probably heritable inter-individual variation in a natural population. They also illustrate (i) how stabilizing selection can promote evolutionary stasis of an adaptive trait that is closely associated with foraging performance and long-term survival, (ii) how selection on avian plumage patterns can occur outside the usual contexts of sexual and social signalling [29], and (iii) as Charles Darwin originally observed, how ‘natural selection is daily and hourly scrutinising, throughout the world, the slightest variations; rejecting those that are bad, preserving or adding up all that are good’ [30, p. 84].

Ethics

Data were collected with authorization from the Allegheny College Animal Research Committee (most recent protocol 2019-028) and with relevant Pennsylvania and United States research permits, including U.S. Federal Bird Banding Permit 21897, Pennsylvania Bird Banding Permit 94, U.S. Federal Fish and Wildlife Permit MB773715-0 and Pennsylvania Special Use Permit 34796.

Data accessibility

Data available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.n8pk0p32t [31]. Additional material is available in the electronic supplementary material [32].

Declaration of AI use

I have not used AI-assisted technologies in creating this article.

Authors' contributions

R.L.M.: conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, project administration.

Conflict of interest declaration

The author declares no competing interests.

Funding

Financial support was provided by Allegheny College through the Academic Support Committee, the Provost's Office and the Biology Department.