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).

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 ...

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 ...