Body size, assortative mating and divorce rates in a little-known skua taxon, the Falkland Skua Stercorarius antarcticus antarcticus

Several studies have attempted to identify the selective pressures leading to reversed size dimorphism (RSD), a characteristic of skuas and of a wide range of bird species with a raptorial lifestyle. One of the prevailing hypotheses posits that females in species of aggressive and well-armed males should select males that are smaller than themselves, to avoid injuries during pair formation and mating. This hypothesis predicts assortative mating by size in such species, and in previous research, this was confirmed to occur in two skua species. This study presents, for the first time, biometric data for Falkland Skuas Stercorarius antarcticus antarcticus based on large sample sizes. Despite the clear RSD, we found no assortative mating by size, which could be related to the low divorce rates and few opportunities for mate selection in this population. Our results highlight the need for further work to find an explanatory hypothesis for the evolution and maintenance of reversed size dimorphism in skuas.


Introduction
Reversed sexual size dimorphism (RSD) has evolved in various bird families or subfamilies, such as falcons (Falconidae), hawks (Accipitridae), frigatebirds (Frigatidae) and skuas (Stercorariidae) with a raptorial lifestyle. Despite this common link between several of the otherwise disparate and unrelated groups exhibiting RSD, there has been surprisingly little consensus on the probable mechanisms leading to its evolution (Phillips et al. 2002). In fact, numerous hypotheses have been proposed for the evolution of RSD, mainly concerned with sexual selection, trophic niche divergence, or specialization in reproductive roles (Andersson and Norberg 1981;Mueller 1990;Hakkarainen and Korpimäki 1991), but much uncertainty remains regarding their acceptance or rejection. Catry et al. (1999) found evidence for assortative mating by size in two species of skuas. This might be due to females avoiding large males that, in this well-armed and aggressive species, could inflict injuries during pair formation (see Smith 1982). This mechanism might provide a selective pressure for RSD in skuas (Catry et al. 1999). However, one further study on another skua, the Brown Skuas (Stercorarius antarcticus lonnbergi), failed to find evidence for assortative mating by size (Phillips et al. 2002). The Falkland Skua (Stercorarius antarcticus antarcticus) is a seabird nearly endemic to the Falklands Islands and a scarcely studied subspecies compared with other taxa of the Stercorariidae family. Additionally, the only existing demographic data reported a significant population decline on New Island, one of the major strongholds of this seabird (Catry et al. 2011).
Body size is a fundamental trait of species and populations, and information on body size is critical for the study of a wide range of pure and applied ecological and evolutionary questions (e.g. Barth 1967;Barrett et al. 1985;Fairbairn and Shine 1993;Mancini et al. 2014;Heswall et al. 2021).
Published data on biometrics of Falkland Skuas are based on very small sample sizes (only 13 males and 20 females, Hamilton 1934 Most seabirds, including skuas, have high survival and low divorce rates, and a high incidence of non-breeding after mate loss Mercier et al. 2021). This suggests that there may be limited opportunities for mate choice, which may potentially constrain the possibility of development of assortative mating by size. Hence, when assessing assortative mating by size, it is relevant to also assess divorce rates of the study populations.
Our objective in this study is to report for the first time robust morphometric data of Falkland Skuas and assess whether assortative mating by size occurs in this population.
We also analyse divorce rates as supporting information to discuss our results.

Methods
Fieldwork was conducted on New Island (51° 43′ S, 61° 18′ W), one of the westernmost islands of the Falklands archipelago. Biometrical data were collected from 2008 to 2019 during the breeding season. Nesting generally starts in November with territory formation and egg laying and ends in February-March with the fledging of the chicks. Birds from different territories were captured by hand or with a hand-net while attending the nest. Five body measurements were taken: bill length (from the tip of the bill to the start of the feathering at the base of the culmen), gonys (bill height at the gonys), tarsus length, wing length (maximum flattened cord) and mass during incubation.
Egg dimensions (length and breadth) were recorded to the nearest 0.1 mm from 2008 to 2011. Indices of egg volume were calculated as length × breadth 2 (Furness 1987). The vast majority of first clutches in skuas are composed of two eggs (Furness 1987(Furness , 1996. These measurements were taken by different observers throughout the years, but all fieldworkers had previously jointly measured birds with Paulo Catry to ensure the consistency of methods. It is likely that measurement error differed slightly between observers, but it does not seem plausible that after joint training measurement error would be so large as to mask natural variation. Wing length was measured with a ruler to the nearest mm, mass was determined with a 2-kg Pesola spring-balance to the nearest 10 g and bill length, gonys, tarsus length and egg length and breadth were obtained with callipers, to the nearest 0.1 mm. For body mass, we only present data from birds weighed during incubation. All measured birds were ringed with a metal ring and a coded Darvic ring to allow easy identification from a distance without the need for recapture. Breeding pairs were recorded during long-term monitoring on New Island (2008-2019) through regular visits to the nesting territories during laying and incubation.
To determine the sex of the birds, we used blood samples, and observations of copulation or of courtship feeding. For sex determination by molecular methods, a small blood sample was obtained from the interdigital capillaries and stored in 98% ethanol. DNA was extracted from blood samples, and PCR amplification carried out with primers 2550F and 2718R (Fridolfsson and Ellegren 1999).
A principal components analysis for all measured individuals was carried out incorporating the morphometric variables bill length, gonys, tarsus length and wing length to produce single-factor scores (PC1 scores) representing body size (Rising and Somers 1989). Body mass was not used as a measure of size because it also reflects body condition.
Male and female morphometric and composite body size indices (PC1) were compared within pairs to test for evidence of assortative mating with respect to size.
Divorce rates were obtained from the number of divorced pairs as a proportion of the total number of breeding pairs per year. A divorce is said to have occurred when two birds that bred together in year A are alive in year A + 1 but no longer breeding together . Power calculations for correlation test were conducted using pwr package in R (https:// cran.r-proje ct. org/ web/ packa ges/ pwr/ pwr. pdf). Based by the rule of thumb suggested by Cohen (1992) r values of 0.1, 0.3 and 0.5 represent small, medium and large effects sizes, respectively. For this dataset, the population correlation coefficients are the effect size.

Results
Measurements of male and female Falkland Skuas are presented in Table 1. Although there was considerable overlap in the five characters analysed, females were significantly larger than males (p < 0.001 ANOVA test) in tarsus length, wing length and body mass during incubation.
Egg measurements from first clutches for which laying order is known are presented in Table 2. Eggs laid first were larger than second eggs in all measurements.
A dimorphism index was calculated at the population level and within each pair for all body size measurements and body mass ( Table 3).
The percentage of variance explained by PC1 was 44%. Correlations between all body measurements, body mass and PC1 score were calculated (Table 4). There were no significant correlations within-pairs (n = 81) in any univariate measurements or in PC1 (Table 5). The power analysis for a medium effect size (r = 0.3) indicates a power varying between 0.70 and 0.77 for the different variables assessed, while for a large effect size (r = 0.5), the power is above 0.99 for all attempted correlations.
Between 2009 and 2017, the overall divorce rate was 8.1% (n = 223 occasions when both members of a partnership survived the winter and were resighted again). No pairs reunified in subsequent breeding seasons after they had divorced. Of the partnerships that ended during the study, 72% did so as a result of the disappearance of one of the partners, and only 28% finished as a result of a divorce. Table 6 presents comparisons of dimorphism indices of various measurements between pairs that remained united throughout the study and pairs for which divorce was recorded. Pairs which divorced during this study had males with relatively larger bills, compared to females, but there were no differences in other measured variables.

Discussion
The current study found that females of Falkland Skuas are larger in body size than males, confirming a reversed sexual dimorphism (RSD) in this taxon (Furness 1996). Comparison of our measurements with published data on other large skuas (formerly classified as Catharacta, and including also Stercorarius antarcticus lonnbergi, S. a. hamiltoni, S. skua, S. maccormicki, S. chilensis) reveals that the Falkland Skuas is the smallest taxon (Online Resources in Tables 1-4) (Furness 1987; Catry et al. 1999). We note that body mass measurements may vary from one season to another, as well as within a breeding season, and hence, the results from this variable need to be taken cautiously. On the other hand, there are inevitable differences between measurements of different fieldworkers in different studies and even within our study. We do not believe, however, that these are likely to significantly affect our conclusions. The mean egg volume was the third smallest after eggs from South Polar Skuas and Great Skuas (Online Resources in Table 5) (Furness 1987;. According to Bergmann's rule, taxa living in colder regions have larger body sizes than those in warm regions. In this sense, the small size of the antarcticus subspecies of Brown Skuas fits well with the theory, when compared with lonnbergi (who lives mostly south of the Antarctic convergence). However, this does neither explain the difference between hamiltoni and antarcticus nor the fact that South Polar Skuas, one of the smallest species of the large skuas, lives in colder environments (Antarctica) than the other skuas.
In Falkland Skuas, the first egg is on average larger than second egg, a trait commonly found amongst seabird species, that could be explained as a mechanism to facilitate brood reduction or to reduce hatching asynchrony (Slagsvold et al. 1984;. We found no evidence for assortative mating by size in Falkland Skuas, which goes against the current hypothesis linking sexual selection to the evolution of RSD. Similarly, Phillips et al. (2002) were unable to demonstrate assortative mating by size in Brown Skuas in South Georgia (correlation between PC1 score, representing body size, within pairs was r = − 0.02, n = 101, ns). Hence, the positive correlations between PC1 scores within pairs for Arctic Skuas S. parasiticus (r = 0.28, n = 67, p = 0.027) and Great Skuas (r = 0.22, n = 61, p = 0.085) described by Catry et al. (1999) do not seem to apply to skuas in general.
Dimorphism indices for wing length in Falkland Skuas have similar values to other skuas and jaegers, and the small differences found do not seem to reflect different foraging modes (see Catry et al. 1999). However, Falkland Skuas have a dimorphism index for body mass greater than the other large skua taxa (5.3 on average). This is analogous to the degree of dimorphism observed in jaegers (6.6 on average) that are generally more aerial foragers (e.g. Reinhardt et al. 2000). These interspecific comparisons make it difficult to support the hypothesis that small male size has evolved for efficient foraging during breeding (Andersson and Norberg 1981;Mueller 1990;Hakkarainen and Korpimäki 1991).
Divorce rates in Falkland Skuas (8.1%) were higher but similar compared to other skuas. For Arctic Skuas a 7% divorce rate was reported (Phillips 1995), 6.4% in Great Skuas , and in South Polar Skuas divorce rates ranged from 1.5% (Wood 1971) to 0.0% (Mercier et al. 2021). Birds that seldom divorce in populations with high survival rates (as is typical of skuas) may have few opportunities for being selective when there are limited options amongst widowed or new recruits. This may constrain the possibility for assortative mating to appear. Except for one measurement (bill length) pairs that divorced during this study were not more or less dimorphic than pairs that remained united, indicating that dimorphism may have little or no role in pair formation and maintenance.
In conclusion, our study shows that the smallest of all large skua taxa does not display assortative mating by size, questioning the generality of the findings by Catry et al. (1999), and leaving the evolution of RSD in skuas without an explanatory hypothesis that is well supported from the currently existing data.
Acknowledgements To all who helped with fieldwork along the years, particularly Ian Strange, Georgina Strange, Miguel Lecoq, José Pedro Granadeiro, Letizia Campioni, Francesco Ventura and Amanda Kuepfer. The New Island Conservation Trust, and more recently Falklands Conservation supported field studies on New Island through the supply of research facilities. We also acknowledge helpful comments and suggestions provided by the referee Jeroen C. S. Creuwels and one anonymous referee.
Author contributions PC designed and coordinated the research and carried out fieldwork. CP analysed the data. CP and PC jointly wrote the paper.
Funding This work was funded by the Fundação para a Ciência e a Tecnologia (FCT, Portugal) through the projects: UIDB/04292/2020 and UIDP/04292/2020, granted to MARE. The Falkland Islands Government provided formal permits and funding through the Environmental Studies Budget.
Data availability Data are available upon request.
Code availability Not applicable.