Visually Sexing Loggerhead Shrike (Lanius Ludovicianus) Using Plumage Coloration and Pattern

Summary

We present a protocol to characterize the sex of loggerhead shrike visually based on the coloration and pattern of the sixth primary wing feather.

Abstract

The loggerhead shrike is a small sexually monomorphic passerine bird using grassland habitats across North America. Based on Breeding Bird Survey data, the species has undergone a drastic decline since the mid-1960s. The cause of decline is unknown, and research is actively underway to address this knowledge gap. These efforts are hindered by an inability to sex the species in hand, which to date was only possible using molecular markers. Here, we present a protocol to sex loggerhead shrikes by visually analyzing the coloration and pattern in the sixth primary feather. The application of the method will facilitate our ability to identify threats on a finer scale than has been possible to date and to address various ecological and evolutionary hypotheses. The methodology is simple and results reliable–we encourage including this method for research of both in situ and ex situ populations.

Introduction

The loggerhead shrike (Lanius ludovicianus) is a North American passerine with a broad geographical range encompassing most of North America and a variety of habitats that can generally be described as grassland¹. It is one of only two species of shrikes (Order Passeriformes) that occur in North America. Shrikes are best known for the unique raptor-like bill, which allows them to take vertebrate prey, and their unique behavior of impaling food items on thorns or other sharp objects. The loggerhead shrike is the only species of 'true shrike' (Family Laniidae) endemic to the continent. Shrikes breeding above 40°N are generally obligate migrants^1,2,3, with wintering grounds almost entirely encompassed within that of non-migratory conspecifics^1,4.

North American Breeding Bird Survey data⁵ for loggerhead shrike indicate a significant (3.18% yr^-1) range-wide population decline. The loggerhead shrike is one of Partners in Flight's 24 "Common Birds in Steep Decline"-i.e., those that have lost more than 50% of their populations over the past 40 years, but which lack other elevated vulnerability factors that would warrant higher "Watch List" status⁶. Habitat loss due to succession and human development likely contributed to the initial declines^4,7, but continued population declines are outpacing habitat loss in the breeding season, suggesting other limiting factors, in particular in areas where the species is an obligate migrant^4,8. Results of a population viability analysis conducted for the critically endangered population of loggerhead shrike in Ontario suggests that over-wintering success of birds in their first year of life is a driver of the population trends^9,10. Results further indicate that the conservation breeding effort, which is augmenting the wild population, has kept the species from extirpation in this area^9,10.

Understanding sex differences is an important component of both ecological and evolutionary hypotheses. The plumage of loggerhead shrikes (Laniusludovicianus) is sexually monochromatic and therefore individuals cannot reliably be sexed in the hand. However, based on a method applicable to the northern shrike (Lanius excubitor)^11,12, it has been shown to be possible to sex at least some populations of adult loggerhead shrikes using the coloration pattern in the sixth primary wing feather¹³. We have revised this methodology¹³ to include consideration of a second variable, specifically the extent of pigmentation in the rachis of the sixth primary, which allows reliable identification of sex in the majority of individuals in eastern populations, and tested its application (previously only applied to adult birds) to fledged young of the year. The method requires no specialized equipment or costly lab assays, and no measurements are required that would be subject to observer bias. Based on our results, the method is easily learned and, once mastered, highly accurate. Here, we present detailed instructions on how to sex shrike in the hand using our method and discuss the wider implications of including sex assessment in future research and conservation efforts for this unique and enigmatic species of conservation concern.

Protocol

The research protocol presented herein complies with African Lion Safari's Animal Care Committee guidelines.

1. Sexing Loggerhead Shrikes by Color and Patern of the Sixth Primary Wing Feather

NOTE: Shrikes can be sexed in hand based on the coloration and pattern in the sixth primary wing feather (P6). In brief, the technique requires the observer to visually extrapoloate a line along the lower edge of the primary wing coverts, and then to assess how far the brown extends in the rachis (shaft) through the white portion of the feather visible below the primary coverts.

1. Hold the bird firmly in the banders grip and carefully extend one wing to allow the sixth primary wing feather to be viewed (Figure 1). Do not over-extend as this can cause harm to the bird's musculature.
2. Locate the P6 feather: Loggerhead Shrikes have 10 primary feathers, with the last (10^th) being the most distal (outermost) feather and reduced in size compartive to the other 9 primary feathers. Unlike the secondary feathers, and with the exception of the reduced 10^th primary feather, all primaries have a degree of white coloration. It may be easier to count backwards from the 10^th primary than to locate the first primary and count forward to locate the P6.
3. Assess the brown in the rachis (hereafter shaft): Does it extend at least half way through the white, and touch, or nearly touch the distal end (furthest from the body and point of emergence) of the primary covert feathers as they lay naturally over the primary feathers? If "yes", the bird is female (Figure 2). If "no", then the bird is male (Figure 2).
4. The pattern and extent of coloration in the primary feathers varies among individuals. If unsure of whether the brown in the shaft extends at least half way through the white patch to the distal tips of the primary coverts, or if the brown is somewhat indistinct, use a secondary technique. Specifically, assess the symmetry of the brown to white transition in the feather vane on each side of the shaft (Figure 2). If the brown at the point where the coloration changes to white in the vane meets at the same spot on either side of the shaft, the bird is male. If there the line meets asymetrically at the shaft, creating a step or notch, the pattern indicates that the bird is a female (Figure 2).
5. If there is still uncertainty as to the individual's sex, examine the angle of the transition between the brown and the white coloration. If the brown in the feather vane has a steep angle where it meets the white in the vane at the shaft, forming an upside down "V", the pattern indicates that the bird is a female (Figure 2). If the pattern is more that of an "M", it indicates a male bird (Figure 2).

Representative Results

Male and female plumage is, overall, monomorphic in the loggerhead shrike. However, it has been established that sex can be discerned based on the pattern of coloration in the 6^th primary in the population that occurs on mainland California¹³ and northern shrike¹². We tested Sustaita et al.'s (2014) protocol¹³ to determine if it was applicable to northeastern populations of loggerhead shrike and to younger age cohorts. We developed a modified version of the protocol, with the goal of developing an accurate and easy to use method suitable for use in the field. We tested our modified protocol by developing a Citizen Scientist survey project. The survey was sent to African Lion Safari's (ALS) e-newsletter database, which consists of subscribers from Canada, the United States, and around the world. In addition, we also posted the survey on ALS's Facebook page, which reached 5,604 people. In total, 399 people reviewed the survey and 120 participated. Participants were asked to read a brief instruction on how to sex using feather pattern (similar to that presented here) and then to sex a series of 26 shrikes (n = 13 female and n = 13 male) based on a photograph showing the pattern in the 6^th primary. Photographs were presented in the same order for each viewer, with photos of males versus females randomly ordered. The gender of the shrikes had previously been confirmed using genetic sexing methods¹⁴. All photographs were of 2017 fledged Hatch Year birds produced at African Lion Safari, a conservation breeding facility in Ontario, Canada, and originating from the same founder lines, representing only one subspecies and one population in eastern Canada. Only photos from birds in which the flight feathers were fully emerged were included in the study. All photos were taken opportunistically as birds were being handled for routine care and management (e.g. for vaccination). Photos were selected based on their image quality - i.e. resolution, focus and if the presentation of the 6^th primary was unobstructed - rather than on how well they represented the ideal for each sex.

Respondents were asked to rate their prior knowledge of birds and whether they felt the scoring became easier as they reviewed photos. Only 4% of respondents rated themselves as experts regarding birds. The remaining respondents were fairly evenly split, rating themselves as either having no experience (43%) or as an amateur (46%).

Our Citizen Scientists averaged a 77% (range = 70% to 85%) correct assessment for females, and 77% for males (range = 67% to 86%) (Table 1). Scoring was consistent among photos suggesting that the patterns are likely fairly consistent within each sex. Seventy one percent of our volunteers responded that they felt scoring became easier as they went along. However, the average number of correctly scored photos was nearly identical for the first ten (78% scored correctly) versus second ten (77% were scored correctly) photos reviewed, suggesting that scoring itself did not become easier or more accurate with practice, but that the participants were more at ease with the methodology.

We tested the hypothesis that accuracy increases with training and experience. Ten individuals were trained in person on our method, rather than having them read instructions. Each individual was shown 5 pictures of shrikes, which were not included on the survey. The sexing method was reiterated verbally by the trainer for each photograph. The trainees were then asked to conduct the same assessment as our Citizen Scientists. All individuals trained one-one-one assessed 12 out of 13 photos of females and 11 of the 13 photos of males correctly. One trainee incorrectly scored a female bird's wing as being that of a male. Two trained individuals incorrectly identified 2 photos of a male wing as being female. Our results suggest that one-on-one verbal training, which in our case was accomplished using photographs, was highly effective and increased accuracy of designations compared to written instructions. Training could also be conducted in the field with a bird in hand. Regardless of the method, we recommend one-on-one individual training whenever possible over use of written instructions prior to data collection.

Our results indicate that with a modest amount of practice, which does not have to rely upon having birds in hand, sexing loggerhead shrike using the coloration and pattern of the 6^th primary provides a highly accurate method by which the sex of shrike can be discerned. However, future research is required to determine if this methodology works universally among loggerhead shrike populations and in other subspecies. We also suggest that future research assess the degree of difference in the P6 pattern and coloration between left and right wings, and in subsequent and repeated molts. We would also recommend that future research assess the repeatability of the method by the same observer. Regardless, given the results of our own and previous research, it would appear that this technique has the potential for broad-scale utility within the species.

Figure 1: Wing of loggerhead shrike extended in preparation of assessing the 6^th primary feather. Please click here to view a larger version of this figure.

Figure 2: Example of sixth primary feather coloration and pattern in female (A and B) versus male (C and D) loggerhead shrike. The dark pigmentation in the rachis is touching, or nearly touching the distal tip of the primary coverts, the brown feather vane coloration is asymmetrical on either side of the shaft where it transitions to white, and there is a steep "V" angle at transition in females. The dark pigmentation in the rachis is no more than half way to the distal tips of the primary coverts and the brown in the feather vane on either side of the shaft at the transition to white is symmetrical, and forms a shallow "M" angle at the transition point in males. A solid black line has been superimposed on the pictures to demonstrate the distal tip of the primaries. In photos B and C, a blue line has been superimposed on the photos to demonstrate the "V" versus "M" angles. The fourth primary (P4) and sixth primary (P6) are both labeled to indicate the order in which primary feathers are numbered. Please click here to view a larger version of this figure.

Ratio of correct responses	Females	Males
60% to 69%	0	1
70% to 79%	7	7
80% to 89%	6	5
90% to 100%	0	0
Total pictures	13	13

Table 1: Ratio of correct responses by Citizen Scientists reviewing photographs of female (n = 13) and male (n = 13) loggerhead shrike wings to determine sex based on the color and pattern in the 6^th primary feather.

Discussion

Herein, we describe a simple and efficient method whereby loggerhead shrike can be sexed based only on visual cues, and provide an assessment of the method's accuracy. Our simple method is easily and quickly undertaken, with results indicating a high accuracy rate that increases with a small amount of training. Our results support those of previous work¹³ that indicated the method originally developed for use in northern shrike¹² had utility for sexing adult loggerhead shrikes in mainland California. We have extended this research to demonstrate that the technique also works elsewhere in the species' range, in a different subspecies¹⁵, and for young of the year with fully emerged flight feathers. We have simplified the approach¹³ to focus only on the pattern of a primary wing feather, as our goal was to determine if the method could be used as a user-friendly and easily learned technique to sex shrike in hand in the field, which has previously only been possible for a short period of time during the breeding season¹¹.

The ability to determine sex in loggerhead shrike will facilitate examination of a greatly expanded set of ecological and evolutionary hypotheses for the species. Sex-biased differences in demographic and life-history traits can impact the effectiveness of conservation actions, and data on these biases is needed to adapt management programs to best meet the species' needs. Research on other avian species indicates that demographic and life history traits, including molt^16,17, social structure and reproduction¹⁸, dispersal and gene flow^19,20, mortality^21,22,23, migration²⁴ and carry-over effects²⁵, body condition²⁶, habitat choice and use^27,28,29,30, stress-induced pathways^31,32, parisitization^33,34, and response to various environmental stressors³⁵ can vary based on sex. Sex-biases in populations can have important implications for population demography, breeding, and even social biology³⁶. Work on the population of loggerhead shrike in Midewin National Tallgrass Prairie in northern Illinois suggests that male-biased mortality may be negatively impacting the population's trend³⁷. With the advances in the use of exogenous markers, such as stable isotopes and nuclear genetic microsatellites³⁸, migrant shrikes can be discerned from residents. The ability to easily sex shrike will facilitate research into the cause of sex biased mortality and other demographic factors that could be driving the population trends in this species.

Conservation breeding has proven a critical management tool for the population of loggerhead shrikes in Ontario^9,10. The augmentation of the wild population using young of the year bred in human care has kept this population from extirpation, essentially "buying time" for research to better understand the cause of decline. To date, more than 1000 young birds have been released into the wild in Ontario since the release program was initiated in the year 2000, with young migrating and returning to breed in subsequent years^39,40. Each year, a few individuals are held back to ensure future breeding can occur. Ideally, the ratio of males to females in the ex situ population would be equal. A rapid and inexpensive method would help to ensure this goal of the management of the ex situ population is met while releasing as many young as possible. With more than 100 young birds released in most years⁴¹, the cost of DNA sexing is cost prohibitive. As a result, it is not possible to determine if return rates for each sex are proportional to that released, or if sex-biased mortality is occurring. Again, an inexpensive and easily implemented methodology by which young of the year can be sexed would allow us to better quanitfy the impact of the conservation breeding on the species in Ontario and help to broaden the scope of possible research to include, for example, sexual size dimorphism and nestling survival seasonal brood sex ratios, etc.

While molecular sexing methods are available that are a widely applicable for use with birds^14,42, and have proven effective with loggerhead shrikes^38,42, they require a tissue sample from which to extract DNA, specialized equipment, and expertise and are not as cost effective as our method. Other sex-determination methods using external characteristics such as vent sexing are less effective for loggerhead shrike, which only exhibit a cloacal protuberance for short periods of time (Chabot, unpublished data). Morphometrics suitable for sexing the closely related northern shrike (L. excubitor)¹² have not been as effective in loggerhead shrikes^43,44,45. Our accurate and easily implemented method of sex determination has broad utility, not only for informing comparative demographic and life history studies, but also for research and conservation activities that require sex-specific data.