Sexual size dimorphism and morphological sex determination in the Yellow-browed Bunting (Emberiza chrysophrys)

Abstract

Background

Sex-differential migration patterns may coevolve with sexual size dimorphism, and reliable field sexing is necessary to understand this relationship. At stopover sites near the wintering grounds, Yellow-browed Buntings (Emberiza chrysophrys) exhibit relatively low levels of protandry and sexual size dimorphism among bunting species. Plumage-based criteria for this species are further complicated at autumn stopovers, where molt progression varies, and the difficulty of sexing and ageing can vary across sites and seasons. Although sexual size dimorphism (SSD) in this species has been reported, morphological sexing has not been validated with molecular methods, and large-scale ringing programs are unable to feasibly genotype all individuals. We therefore aimed to quantify SSD and develop practical morphometric discriminant functions using molecularly validated sex determinations, thereby enabling a better assessment of sex differential migration patterns and their relationship with morphology in this species.

Results

We measured 50 migratory first-year Yellow-browed Buntings captured during autumn stopover on Daecheong Island, South Korea, and confirmed sex through molecular assays. Among the 50 first-year birds (24 males, 26 females), sexual size dimorphism was strongest in wing length (Cohen’s d = 3.00) and tail length (d = 2.11). For discriminant function analysis, a simplified wing-only model (D₀) achieved 92.0% leave-one-out cross-validation accuracy, as did the wing-and-tail model (D₂) and the stepwise-selected model (D₃, wing-only). All models relied principally on wing length as the primary discriminator.

Conclusions

These morphometric discriminant functions enable accurate, rapid sexing of Yellow-browed Buntings in the field without genetic testing. They facilitate demographic monitoring and sex-specific ecological studies at stopover sites and can be applied to individuals with ambiguous field identification characteristics, thereby improving our understanding of sex differential migration patterns and their coevolutionary relationship with morphology in Yellow-browed Buntings.

Background

Sexual size dimorphism (SSD), defined as the difference in body size between sexes [1], is central to understanding sex differences in migration timing, routes, distances, and stopover strategies in birds [2–6]. However, quantifying SSD and applying this knowledge in the field requires reliable morphological sex determination [7], which remains essential for many ornithological research, especially migration studies. Reliable sex determination in the field serves as a crucial gateway to understanding population structure (sex ratios) and sex-specific ecological patterns in migratory birds, providing important insights for the conservation of declining species [8–14]. Field sexing difficulty depends on the extent of morphological sexual dimorphism and varies with bird age, season, and molt strategy (timing and location) [15]. Molecular methods offer high accuracy for sex determination but are costly, and given the large number of captures and the extensive, long-term nature of fieldwork across multiple regions, it is often impractical and inefficient to conduct molecular sex determination on all captured individuals in national or international migratory bird monitoring programs, such as bird banding schemes [16]. Therefore, the importance and need for morphometric discriminant functions validated by molecular genetics are underscored [15, 16].

This study focuses on Yellow-browed Buntings (Emberiza chrysophrys), which breed in eastern Siberia and are long-distance migrants, wintering in central and southern China [17, 18]. In South Korea, this species occurs as an uncommon passage migrant, with individuals regularly observed making stopovers along the southwestern coastal islands during spring and autumn migration seasons [19, 20]. In Yellow-browed Buntings, protandry in young birds has been reported at stopover sites near both the wintering and breeding ranges [20, 21], though the magnitude is relatively small. In contrast, adults near the breeding grounds exhibit greater protandry, suggesting protandry can vary with age [22]. Additionally, a multi-species comparative study reported that despite being a high-latitude breeding long-distance migrant, the Yellow-browed Bunting shows relatively low SSD based on PC1 (Total body length, wing length and tail length) compared to other species within the same genus (Emberiza) that originate from relatively lower latitudes.

In species exhibiting weak sexual dimorphism or in young birds, the misclassification rate of sex determination in the field can be relatively high. This can consequently affect age identification as well, ultimately accumulating uncertainty in current knowledge of demographic and ecological traits according to sex and age.

Sex identification in Yellow-browed Buntings relies primarily on crown pattern and facial coloration as the principal distinguishing characteristics [23, 24, 26]. At our autumn study site, sexing was often more challenging because many birds retained juvenile crown feathers or showed substantial loss and replacement of crown and facial feathers (personal observation). Even in spring, identification may not be straightforward, depending on the extent of the pre-breeding molt [26]. While molt information is fundamental to such sexing and ageing, comprehensive, flyway-wide documentation, including wintering sites, remains limited across East Asia [27, 28]. Moreover, identification in Emberiza buntings is frequently challenging and therefore benefits from combining multiple lines of evidence, including field information and molecular confirmation [27]. Although some previous literature has reported morphometric measurements by sex in this species, most studies combine data across age classes, seasons, and regions without molecular validation, limiting their reliability and applicability. Therefore, in addition to plumage-based characteristics, reliable molecularly validated morphometric criteria are needed to enhance sex determination accuracy.

In this study, we aim to provide reliable criteria that can be referenced for sex determination of first-year Yellow-browed Buntings at a stopover site during autumn near wintering grounds. This study aims to verify sexual size dimorphism in first-year birds through molecularly validated sex determination and morphometric measurements, and to provide practical discriminant functions readily applicable for field sexing during autumn migration. Furthermore, our results may contribute insights into the morphological basis underlying sex-differential migration in first-year Yellow-browed Buntings.

Methods

Sampling and measurements

This study was conducted on Daecheong Island (N 37° 50′, E 124° 42′), Ongjin County in South Korea (Fig. 1), which is a stopover site of migratory buntings, including the Yellow-browed Bunting, during the autumn migration season in 2022 and 2023.

Fig. 1.

Distribution map of the Yellow-browed Bunting (Emberiza chrysophrys) [18]. Yellow indicates the breeding range, and khaki indicates the wintering range. The study site is indicated by a black star

A total of 50 first-year Yellow-browed Buntings captured during autumn using mist nets were banded with metal rings and measured for morphological features following standard protocols [17, 23, 29]. Provisional field-based sex assessments were made using established plumage criteria (crown-stripe contrast and facial coloration) following Svensson [23] and Norevik et al. [26]. Given the reduced reliability of plumage-based sex identification during autumn migration (field-based morphological sex determination rates: 96.51% in spring vs. 52.53% in autumn, unpublished data), we molecularly validated the sex for our samples using a CHD-based assay [30]. Wing (maximum wing length) and tail length were measured using rulers to the nearest 0.1 mm, while tarsus, head to bill, bill to skull, and bill to nostril measurements were measured to the nearest 0.01 mm using calipers. Body mass was measured but not included in this study as it significantly varied by fat conditions during migration and stopover stages. All measurements were taken by a single researcher (Seyoung Park) to ensure measurement consistency. All fieldwork was approved by the Institutional Animal Care and Use Committee of Seoul National University (protocol no. SNU IACUC-221013-5) and conducted under permits from the local government (permission no. 2022-32, 2023-40 by the Ongjin County Office). All birds were safely released immediately after data collection.

Molecular sex determination

We collected one outermost tail feather from each captured bunting for molecular sexing. Samples were stored in ziplock bags at − 20 °C. For molecular sex determination, we extracted DNA from the quill roots of the stored tail feathers using Chelex^®-100 resin [31]. Following the method of Griffiths et al. [30], we amplified the CHD1 (Chromo-Helicase-DNA-binding protein 1) gene with P2 and P8 primers.

Statistical analysis

For each morphometric measurement of the 50 individuals collected in autumn, data normality and variance homogeneity were assessed using the Shapiro–Wilk and Levene’s tests. Sexual size dimorphism was quantified using Cohen’s d, calculated as the difference between male and female means divided by the pooled standard deviation [32]. Sex differences in each morphometric measurement were tested using Welch’s t-tests due to unequal variances observed in wing measurements.

We compared four LDA (Linear Discriminant Analysis) approaches for sex determination [33]: 1–2) univariate models using individual variables with significant SSD (Sexual Size Dimorphism) and high SDI (Sexual Dimorphism Index) values to assess individual variable contributions and facilitate practical field application, 3) a bivariate model combining wing length and tail length to examine complementary effects, and 4) a model incorporating variables selected through a stepwise procedure across all measurement variables. These models are hereafter referred to as D₀ (wing-only), D₁ (tail-only), D₂ (wing-and-tail), and D₃ (stepwise-selected).

Linear discriminant functions are presented using unstandardized canonical coefficients, with scores ≤ 0 indicating female and scores >0 indicating male. Each discriminant function was evaluated using three statistical measures: Wilks’ lambda (Λ) to test statistical significance of group separation, canonical correlation to quantify the strength of discrimination, and leave-one-out cross-validation (LOOCV) to assess classification accuracy. LOOCV was particularly suitable for this study because of the small sample size, as this approach trains the model on (n–1) observations at each iteration, providing low-bias estimates of validation error, which are critical for small samples [34–36]. We prespecified three candidate sex-only LDA models (wing; wing + tail; wing + tarsus), assumed equal priors and misclassification costs, and evaluated performance with leave-one-out cross-validation (LOOCV). To reduce misclassification near the decision boundary, we implemented a reject option (“gray zone”) whereby individuals with the highest posterior probability < 0.60 were labeled Unknown [37]. When models performed comparably, we favored the more parsimonious specification for field use. All analyses were conducted in R 4.3.1 [38] using the MASS package for linear discriminant analysis and stepwise variable selection via the stepAIC function [39].

Results

Sexual size dimorphism

Males (n = 24) had longer wings (78.9 ± 1.47 mm) and tails (64.5 ± 1.83 mm) than females (n = 26; 74.9 ± 1.21 mm and 60.6 ± 1.87 mm), with the greatest sexual size dimorphism in wing (d = 3.00) followed by tail (d = 2.11); tarsus and bill metrics (HB: head to bill, BS: bill to skull, BN: bill to nostril) showed no sexual differences, and coefficients of variation were low across traits (≤ 4.5%; Table 1). Despite clear sexual differences in wing and tail length, some overlap remained between the sexes in each trait, prompting discriminant function analysis for more reliable sex classification. For reference, our morphometric measurements are presented alongside other published data in Table 2.

Table 1.

Sex-specific means ± SD of morphometric measurements and sexual dimorphism indices in first-year Yellow-browed Buntings (Emberiza chrysophrys)

Variablea (unit: mm)	Male (n = 24)	Female (n = 26)	Welch’s t	p-value	SDI (d)b	CVc
	Mean ± SD	Mean ± SD
Wing (W)	78.9 ± 1.47	74.9 ± 1.21	10.51	< 0.001	3.00	3.2
Tail (T)	64.5 ± 1.83	60.6 ± 1.87	7.45	< 0.001	2.11	4.3
Tarsus (Ts)	19.3 ± 0.54	19.1 ± 0.55	1.34	0.186	0.38	2.9
Head to bill (HB)	30.7 ± 0.52	30.7 ± 0.54	0.38	0.705	0.11	1.7
Bill to skull (BS)	15.0 ± 0.59	14.8 ± 0.57	0.83	0.412	0.23	3.9
Bill to nostril (BN)	8.3 ± 0.37	8.4 ± 0.37	-1.07	0.292	0.30	4.5

^a W: Wing, T: Tail, Ts: Tarsus, HB: Head to bill, BS: Bill to skull, BN: Bill to nostril

^b SDI (d): Sexual Dimorphism Index, expressed as Cohen’s d

^c CV (%): Coefficient of Variation

Table 2.

Morphometric measurements of the Yellow-browed Bunting (Emberiza chrysophrys) from this study and literature

Variable		This study (24 males, 26 females)	Shirihai & Svensson [24]	Byers et al. [17]	Heim et al. [25] (15 males, 15 females)
Wing (W)	M	78.9 ± 1.47 (76.2–82.3)	ma 79.7 (75–84; nb 26)	77.5–84.0	80.4 ± 1.2
	F	74.9 ± 1.21 (72.1–77.9)	m 75.1 (71–78; n 20)	71.5–78.0	74.6 ± 2.0
Tail (T)	M	64.5 ± 1.83 (61.3–69.0)	m 62.6 (57–66; n 24)	58.0–66.0	64.1 ± 2.1
	F	60.6 ± 1.87 (57.0–65.0)	m 60.1 (55–64; n 18)	55.0–63.5	59.9 ± 2.4
Tarsus (Ts)	M	19.3 ± 0.54 (18.02–20.11)	m 19.2 (18–21; n 27)	19.2–21.5	20.1 ± 0.6
	F	19.1 ± 0.55 (18.10–20.24)		19.2–20.8	19.5 ± 0.4
Head to bill (HB)	M	30.7 ± 0.52 (29.92–31.99)	-	-	-
	F	30.7 ± 0.54 (29.95–31.58)	-	-	-
Bill to skull (BS)	M	15.0 ± 0.59 (14.14–16.60)	m 13.4 (12–15; n 33)	13.4–14.8	12.6 ± 0.5
	F	14.8 ± 0.57 (13.94–16.69)		13.4–15.0	12.2 ± 0.5
Bill to nostril (BN)	M	8.3 ± 0.37 (7.20–8.79)	-		-
	F	8.4 ± 0.37 (7.19–9.01)	-		-

^a m: mean value

^b n = sample size

Sex determination by discriminant function analysis

All four discriminant functions significantly separated the sexes (all Wilks’ Λ, p < 0.001; Table 3). The simplest wing-only model (D₀), the wing-and-tail model (D₂), and the model selected by stepwise LDA (D₃), all demonstrated the best performance, with the lowest Wilks’ Λ (0.009), the highest canonical correlation (r = 0.996), and the highest LOOCV accuracy (92.0%). By contrast, the tail-only model (D₁) performed the worst across all metrics (LOOCV = 84.0%), indicating weak discriminatory power. Adding tail length to wing length (D₂) did not improve performance relative to D₀ or D₃. The model selected by the stepwise LDA (D₃) identified wing length as the optimal variable, making it identical to D₀. In this context, wing length is the most robust single predictor; tail contributes little, and the additional predictor of tail provides no practical gain within the linear discriminant framework. LOOCV accuracies for females and males represent specificity (true-female rate) and sensitivity (true-male rate), respectively.

Table 3.

Four discriminant functions for sex determination in first-year Yellow-browed Buntings (Emberiza chrysophrys) in autumn. Accuracy values represent leave-one-out jackknife cross-validation (LOOCV)

Discriminant functiona	Wilks’ Λ	p-value	Canonical r	Accuracyb (correctly sexed ratio)
				Females	Males	All
D0 = 0.746 × W – 57.368	0.009	< 0.001	0.996	87.5%	96.2%	92.0%
D₁ = 0.540 × T – 33.759	0.018	< 0.001	0.991	83.3%	84.6%	84.0%
D₂ = 0.693 × W + 0.057 × T – 56.868	0.009	< 0.001	0.996	87.5%	96.2%	92.0%
D₃ = 0.746 × W – 57.368	0.009	< 0.001	0.996	87.5%	96.2%	92.0%

^a W: Wing, T: Tail

^b Males: sensitivity (true-male rate), Females: specificity (true‐female rate)

For practical field application, a single discriminant function was selected based on the simplified wing-only model (92.0% accuracy):

1

where D₀ >0 in males, indicating that males have a wing length larger than 76.91 mm. Model diagnostics indicated approximately normal discriminant scores and only mild heteroscedasticity with no material effect on classification; therefore, no transformation was required, and the 76.91 mm decision threshold is robust for field use.

We found that the wing-only discriminant function (D₀) was effective for sex determination, showing clear separation between males and females with minimal overlap in morphospace (Fig. 2). Analysis focused on 50 first-year birds using D₀. With a gray-zone threshold p∗ = 0.60, LOOCV withheld 2/50 birds (4.0%) as Unknown, yielding 96.0% coverage and 95.8% accuracy among the 48 classified birds. The male false-positive rate (FPR; males misclassified as females) was 4.55%. The gray zone corresponded to ∣D₀∣ < δ, i.e., wing = 76.91 ± 0.09 mm (between 76.82 and 77.00 mm were classified as ‘Unknown’). Females (Fig. 2a) clustered on the negative side of the discriminant axis (D₀ ≤ 0); a female was confidently misclassified into the positive range. Males (Fig. 2b) showed the converse pattern, concentrating on the positive side (D₀ >0) with one confidently misclassified near the boundary. All Unknown assignments (p* = 0.60) occurred exclusively in first-year males and fell within the central gray band (|D₀| ≤ 0.06, equivalent to wing = 76.91 ± 0.09 mm). The nearly bimodal separation with minimal overlap at D₀ = 0 confirms that wing length alone is an effective primary discriminator for sex in first-year birds.

Fig. 2.

Distribution of discriminant scores (D₀) in (a) females and (b) males in first-year Yellow-browed Buntings (Emberiza chrysophrys). Base-filled bars represent molecularly confirmed sex. White overlays: LOOCV confident misclassifications (p_max ≥ p*); dashed outlines: LOOCV Unknowns (p_max < p*); gray shaded area: uncertain classification zone (|D₀| ≤ δ); dotted line: decision boundary (D₀ = 0). p_max = maximum posterior probability; p* = confidence threshold; δ = gray-zone half-width

Discussion

Sex identification of first-year Yellow-browed Buntings can be less straightforward at stopover sites near wintering grounds due to ongoing molt during migration or less pronounced sexual dimorphism in first-year birds; however, the size differences between sexes confirmed by molecular sexing demonstrated that wing length is the best predictor of sex. Our results indicate significant sexual size dimorphism in Yellow-browed Buntings, with wing length emerging as the most informative trait for sexing, followed by tail length, consistent with previous reports for this species [20]. Although our study focuses on first-year birds at autumn stopover sites, the pronounced wing sexual size dimorphism we documented represents a morphological foundation already established before spring migration, when protandry is expressed in this species [20, 21]. First-year birds retain their autumn flight feathers (wing and tail) without molt until the first breeding season, ensuring the underlying sexual size dimorphism persists during spring migration despite potential feather abrasion. This persistence of autumn-measured SSDW therefore provides insight into the mechanistic basis for sex-differential migration timing.

Sexual size dimorphism, particularly in wing morphology, is prevalent among migratory passerines and has been hypothesized to coevolve with protandry [4, 8, 40, 41]. In East Asian buntings, sex-differential arrival has been repeatedly documented, though its magnitude varies among molt and age, latitude classes [20, 21, 52, 54]. For Yellow-browed Buntings, young males arrive 1.3–1.5 days earlier than females at stopover sites, while adult males exhibit substantially greater protandry (approximately 7.6 days) near breeding [20, 21], suggesting potential age-dependent variation. In a multi-species comparison at a stopover site near wintering grounds, Yellow-browed Buntings showed relatively lower overall sexual size dimorphism compared to congeners, despite being a high-latitude breeding long-distance migrant [20]. Given the modest protandry in young Yellow-browed Buntings, one might predict correspondingly modest wing sexual size dimorphism based on coevolution frameworks [4, 8, 40, 41]. However, our results reveal exceptionally strong wing sexual size dimorphism (SSDW) in first-year birds, concentrated in flight-related traits rather than distributed across morphological features. This suggests that flight efficiency may be particularly important in this species [42], and that SSDW provides mechanistic potential for protandry that may be more fully expressed with age and proximity to breeding grounds.

This finding is consistent with protandry–SSDW coevolution frameworks but suggests that strong wing sexual size dimorphism provides mechanistic potential for protandry, while actual expression appears modulated by age and geographic context [8, 22, 53]. In first-year Yellow-browed Buntings, although SSDW is already pronounced, protandry remains modest at stopover sites. As birds mature and approach breeding grounds where territory competition intensifies, the morphological potential provided by SSDW may be more fully realized, as evidenced by the greater protandry observed in adults near breeding sites [21]. Future studies with sufficient adult sample sizes could test whether protandry–SSDW relationships vary with age and proximity to breeding areas.

Beyond evolutionary implications, our findings provide practical tools for field ornithology and conservation monitoring. Accurate sex determination is essential for understanding population structure, sex ratios, and sex-specific ecological patterns in migratory birds [8–14], yet molecular methods remain costly and impractical for large-scale, long-term monitoring programs such as bird banding schemes [30, 44]. Sex determination in Yellow-browed Buntings is particularly challenging during autumn migration due to subtle non-breeding plumage characteristics and ongoing molt, with field-based morphological sex determination rates dropping from 96.51% in spring to 52.53% in autumn (unpublished data). Our molecularly validated discriminant functions address this limitation by enabling rapid, accurate field sexing using readily measurable morphometric traits. The wing-only discriminant function achieved 92.0% cross-validated accuracy using a single measurement (wing length >76.91 mm indicates male). Although sex and age identification in Emberiza buntings should ideally integrate multiple lines of evidence, including plumage characteristics, behavior, and morphometrics [28], single-variable discriminant functions like D₀ offer significant advantages in field settings where measurement constraints or time limitations preclude comprehensive assessments.

We recommend incorporating a gray zone around the decision boundary (approximately 76.82 and 77.00 mm for D₀) to classify borderline individuals as ‘Unknown,’ thereby reducing misclassification risk while maintaining high confidence for clearly assignable individuals [37, 45]. This conservative approach is valuable for demographic analyses and sex differential migration studies, where classification errors can propagate into biased estimates of sex ratios, arrival timing differences, and population trends [8–14, 40, 43, 46]. The practical value of field-deployable sexing methods extends beyond basic demographic monitoring. Sex-resolved data can reveal differential habitat use [12–14] and stopover strategies that inform conservation actions, as demonstrated in American Redstarts (Setophaga ruticilla), where habitat quality affects migration timing and survival [11]. These applications are especially urgent given documented population declines across East Asian bunting species [50, 51], including severe collapses in some congeners due to habitat loss and illegal trapping [27, 47, 48]. While the Yellow-browed Bunting is not yet as severely impacted, it is exposed to the broader threats facing migratory birds along the East Asian Flyway [27, 48]. Many bunting species are illegally trapped and sold across East Asia, with large multispecies seizures documented [48]. Although direct, species-specific evidence for the Yellow-browed Bunting is limited, genus-wide patterns and a report indicate exposure risk along the same flyway [49]. Beyond standard banding studies, our morphometric discriminant functions can serve an additional practical purpose: sexing confiscated birds during law enforcement operations. When illegally trapped buntings are recovered during the non-breeding season, morphometric methods provide a means to extract demographic information that would otherwise be lost, potentially informing population models and enforcement strategies.

Several limitations warrant consideration. First, our sample consisted of first-year birds captured in autumn, and age-related differences in wing length as well as potential seasonal variation due to feather abrasion mean that discriminant thresholds may require adjustment when applied to adults or birds from other seasons. Second, geographic variation in body size could affect discriminant accuracy across different portions of the species’ range, necessitating validation with additional samples. Third, independent validation with external datasets would strengthen confidence in broader applicability.

Future research can extend this approach to adults, expand sampling across the wintering range, and integrate morphometric sexing with tracking data (e.g., departure decisions) to test whether SSDW predicts individual variation in migration timing. Additionally, applying similar approaches to other subtly dimorphic Emberiza species could facilitate comparative analyses of how SSDW and migration strategies co-vary across the genus.

Conclusions

Our study demonstrates that simple morphometric measurements, particularly wing length, serve as highly reliable tools for sexing first-year Yellow-browed Buntings at stopover sites during autumn near their wintering grounds. The wing-only model achieved 92.0% accuracy in practical ecological applications. By validating these functions against molecular sexing, we offer field researchers an accessible alternative to expensive genetic assays, particularly valuable for birds in non-breeding plumage and at stopover sites where rapid sample processing is required. We developed these morphometric criteria to provide researchers with more intuitive and field-applicable sex determination techniques, enabling accurate assessment of sex-specific migration patterns in Yellow-browed Buntings and related Emberiza species, and supporting conservation monitoring of declining long-distance migrants along the East Asian Flyway.

Author contributions

SP, CYC designed the study. SP, HYN, SS, HK, SH conducted the field investigation. SP performed data analysis, prepared visualizations, and drafted the manuscript. CYC and HYN acquired funding and reviewed and edited the manuscript. All authors read and approved the final manuscript.

Funding

This study was supported by the National Research Foundation (NRF-2018R1D1A1B07050135 awarded to CYC; NRF-2022R1I1A1A01069923 awarded to HYN) funded by the Korean Government (Ministry of Education), and by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) through the Animal Disease Management Technology Development Program, funded by the Ministry of Agriculture, Food and Rural Affairs (MAFRA) [Grant number 122062–2].

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

All fieldwork was approved by the Institutional Animal Care and Use Committee of Seoul National University (protocol no. SNU IACUC-221013-5) and conducted under permits from the local government (permission no. 2022-32, 2023-40 by the Ongjin County Office). All methods were carried out in accordance with relevant guidelines and regulations.

Consent for publication

Not applicable.

Consent to participate

Not applicable.

Competing interests

The authors declare no competing interests.