Although the shorebirds (Charadriiformes: Scolopaci and Charadrii) are among the most thoroughly studied bird orders in Central Europe, no precise key for the identification of their skulls was available. This paper seeks to remedy this lack by presenting a dichotomous identification key for 38 species of shorebirds from Germany. Photographs and biometric measurements of the skulls are included to support the identification process.
Although the shorebirds (Charadriiformes: Charadriidae and Scolopacidae) are a special focus of ornithologists, an identification key for the skulls of this group still does not exist for Central Europe. As part of a graduate thesis project at the University of Konstanz (Schäfer 2014), the skulls of 38 German shorebird species were measured and analyzed in terms of functional anatomy and feeding behavior. Based on the underlying data and measurements, a distinction can even be made between skulls of closely related species. In this paper, we present a dichotomous identification key. In addition, we include supportive morphometric data and illustrations.
The skull of the Charadriiformes is characterised by the well-developed rhynchokinesis of its bill ( Hoerschelmann 1970, Gussekloo et al. 2001). According to this the bill is schizorhinous or — rarely — secondary holorhinous (but in Pluvianidae and Burhinidae probably primary holorhinous) (Mickoleit 2004).
As mentioned in Ellrott & Schmitz (2010), the skull identification key presented in Brown et al. (2003) includes general skull types and the bill-cranium-ratio and was supplemented with illustrations and some basic morphometric data. Unfortunately, the number of specimens examined was not specified (except when only one specimen was available). The illustrations were not very detailed, not suited to comparing species, and some important species were missing.
Jansen & Van Gestel (2009) present the following measurements on their “skullsite”: total length, length, width and height of cranium, upper bill length, skull relation (total length/upper bill length). The photographs are clear and informative. Unfortunately, the measurements are taken from just one specimen in each case. In addition, despite the enormous number of species included, some Central European species are missing.
Because Germany has a good cross-section of the Central European fauna, we based our analysis on the species list in Barthel & Helbig (2005), but excluded very rare species in order to keep the identification key practical (see section 2 under “Species selection”).
2 Materials und Methods
Species selection is based on Barthel & Helbig (2005). From the 61 shorebird species listed in this paper, we included 38, because the others are classified as vagrants with an average of less than five records per year since 1980.
The skull material originates mainly from the Nikolaus Collection (Stuttgart State Museum of Natural History). We also used specimens from the Schmitz Collection (University of Konstanz) and the collection of the Osteology Department of the Baden-Württemberg State Office for Monuments and Antiquities (Landesdenkmalamt Baden-Württemberg — Arbeitsstelle Osteologie) in Konstanz (particularly for skulls of Numenius phaeopus). We requested thirteen other collections to send us specimens of shorebirds, however none were available.
Of the 38 species selected, we analyzed ten specimens each, all of which met the necessary condition, i. e. no damage that could prevent proper measuring. An effort was made to maintain a balanced sex ratio in selecting the skulls. In contrast to the Anseriformes (cf. Ellrott & Schmitz 2010), shorebird skulls are less common in collections. For the following species, less than ten specimens could be included: Burhinus oedicnemus (9), Numenius phaeopus (9), Tringa glareola (9), Himantopus himantopus (8), Calidris canutus (8), Actitis hypoleucos (7), Limicola falcinellus (6), Calidris temminckii (5), Lymnocryptes minimus (4), Calidris minuta (4), Tringa nebularia (4), Tringa ochropus (4), Phalaropus fulicarius (3), Tringa stagnatilis (3), Charadrius morinellus (2), Calidris melanotos (1), Phalaropus lobatus (1), Calidris maritima (1) (see Tab. 1 for the nomenclature).
For photographic documentation, we chose those individuals that were most representative (i. e. with measurements close to mean values).
Abbreviations and measurements
The morphological terminology is based on Baumel & Witmer (1993), Joseph & Strauch (1978), King & McLelland (1978), and Hummel (2000). The skulls were measured as follows (see also Figs. 1a, 1b, Tab. 2):
Total length (= upper bill length plus cranial length, i. e. the total length was not measured separately)
Length of cranium: measured from a theoretical line between the occipital ends of the nostrils to the prominentia cerebellaris
Length of upper bill: measured from tip of upper bill to a theoretical line between the occipital ends of the nostrils
Height of cranium
Width of cranium: measured behind postorbital process
Diameter of orbita: measured from base of postorbital process to base of lacrimal
Salt gland depressions: present (“yes”) or absent (“no”)
Width of supraorbital isthmus: measured at narrowest extent
Upper bill tip: protruding (cf. Fig. 6) “yes” or “no”
Pits of Herbst corpuscles: present (“yes”) or absent (“no”)
Length of nostril opening
Length of premaxillar: measured from tip of upper bill to rostral end of nostril
Shape of upper bill (see lateral view): divided into three categories: “straight”, “curved down” or “curved up”
3 Identification key
The complete data set for all 38 species is presented in Tab. 2.
1 Upper bill length and cranial length almost equal (CL : UBL = max. 1 : 1.1) 2
— Upper bill longer than cranium (CL < UBL) 10
2 TL > 75 mm, no supraoccipital foramina, diameter of orbita > 22 mm (Fig. 48) (Burhinidae) only Burhinus oedicnemus
— TL < 75 mm, 2 supraoccipital foramina, diameter of orbita < 22 mm 3
— Orbital edges at the supraorbital isthmus concave (dorsal view) and dorsally conspicuously enlarged (Fig. 3), often with distinct depressions of the salt glands (Charadriidae) 4
4 Supraorbital isthmus with 2 distinct depressions of the salt glands (Fig. 4) 5
— Supraorbital isthmus without distinct depression of the salt glands (Figs. 3, 5) 9
5 TL < 60 mm 6
— TL > 60 mm (Fig. 16) (Vanellinae) only Vanellus vanellus
— Prefrontale and frontale fused (cf. Fig. 5) 7
7 TL > 48 mm (Fig. 14) Charadrius morinellus
— TL < 45 mm 8
8 Dorsal orbit edge conspicuously enlarged, depression of the salt glands distinct, upper bill tip relatively solid, CW > 15mm, OD > 12 mm (Fig. 19) Charadrius hiaticula
— Dorsal orbit edge slightly enlarged, depression of the salt glands less distinct, upper bill tip slender, CW < 15mm, OD < 12 mm (Fig. 18) Charadrius dubius
10 Ventral orbit edge closed, i. e. fusion between lacrimal and postorbital process (Fig. 6) 11
— Ventral orbit edge not closed (Fig. 7) 13
11 Cranium massive (CH > 22 mm, CW > 22 mm, SOW > 9 mm), rostral part of upper bill laterally not enlarged (Fig. 25) Scolopax rusticola
— Cranium smaller (CH < 17 mm, CW < 17 mm, SOW < 7 mm), rostral part of upper bill lateral slightly enlarged laterally 12
12 TL > 75 mm, UBL > 55 mm (Fig. 26) Gallinago gallinago
— TL < 75 mm, UBL < 55 mm (Fig. 27) Lymnocryptes minimus
13 TL > 100 mm 14
— TL < 100 mm 17
14 Rostral half of bill curved down (Numenius) 15
— Bill straight or slightly curved up (Limosa) 16
15 TL > 150 mm (Fig. 21) Numenius arquata
— TL < 150 mm (Fig. 22) Numenius phaeopus
17 CW < 22 mm 18
— CW > 22 mm (Fig. 45) (Haematopodidae) only Haematopus ostralegus
18 Lacrimal pin- or bow-shaped (Recurvirostridae) 19
— Lacrimal planar, 2-dimensional 20
19 Bill conspicuously up-curved, distinct processsus prefrontale (Fig. 46) Recurvirostra avosetta
— Bill straight, no distinct processsus prefrontale, prefrontale and frontale fused (Fig. 47) Himantopus himantopus
20 Basal end of the fronto-nasal hinge with lateral extensions (Figs. 10, 11b), bill tip short (PL < 0.25 UBL), distinct Herbst corpuscles 21
— Other combination of characters 30
— Nostrals consistently tapering towards bill tip, no distinct transversal dorsal indentation in center of frontal hinge (Fig. 11b) 22
22 CL > 23 mm, prominetia cerebellaris salient/prominent (Fig. 36) Philomachus pugnax
— CL < 23 mm, prominetia cerebellaris not salient/prominent (Figs. 28–35) (Calidris) 23
23 TL < 40 mm 24
— TL > 40 mm 25
— Bill tip not widened, consistently tapering towards bill tip (Fig. 31) Calidris temminckii
25 Bill straight (Figs. 28, 29, 35) 26
— Bill slightly curved down (Figs. 32–34). [The bill tip distal of the rostral bending-zone can be shaped up or down; therefore it is helpful to take the lower bill shape also into consideration] 28
26 CW > 15 mm, CH > 14 mm, OD > 11 mm (Fig. 28) Calidris canutus
— CW < 15 mm, CH < 14 mm, OD < 10 mm 27
27 UBL > 35 mm (Fig. 35) Calidris alpina
— UBL < 32 mm (Fig. 29) Calidris alba
28 NL < 30 mm (Fig. 32) Calidris melanotos
— NL > 32 mm 29
30 TL > 50 mm, bill tip elongated and solid (PL > 11 mm) (Tringa) 31
— TL < 50 mm, bill tip usually short, if elongated upper bill conspicuously dorso-ventrally flattened 36
31 TL > 81 mm 32
— TL < 74 mm 33
32 Rostral fourth of bill tip (PL) conspicuously curved down, PL > 0.6 UBL (Fig. 39) Tringa erythropus
— Rostral bill slightly curved up, PL ± 0.5 UBL (Fig. 42) Tringa nebularia
33 TL > 66 mm 34
— TL < 63 mm 35
34 CW > 16 mm, dorsal edge of lower bill extremely straight (Fig. 40) Tringa totanus
— CW < 15 mm, dorsal edge of lower bill slightly up-curved (Fig. 41) Tringa stagnatilis
36 Bill broad, conspicuously flattened (Fig. 50) Phalaropus fulicarius
— Bill narrow, not flattened 37
37 Skull tiny (TL < 43 mm, CL < 17 mm), bill tip extremely pointed (Fig. 49) Phalaropus lobatus
— Skull larger (TL > 46 mm, CL > 18 mm), bill tip not extremely pointed (Fig. 51) Actitis hypoleucos
If indicated for a given specimen, an effort was made to maintain a balanced sex ratio in selecting the skulls. One noticeable sexual dimorphism in skulls has been identified for the genus Numenius (cf. Glutz von Blotzheim et al. 1986). In this case, the crania of the females are larger on average (CW and CH), and they tend to have longer bills (UBL) than the males. However, because the smallest skull of the larger species (Numenius arquata) was still larger than the largest skull of the smaller species (Numenius phaeopus), this sexual dimorphism was irrelevant in terms of identification. A sexual dimorphism has likewise been reported for the related genus Limosa (Glutz von Blotzheim et al. 1986), but is less pronounced. Although the skulls of Limosa limosa and L. lapponica overlap morphometrically, other characteristics can be used to distinguish between these two species.
Species in the genera Calidris and Charadrius are not easily distinguishable. The smaller Calidris species, as well as Charadrius dubius and Ch. hiaticula can best be distinguished using a combination of morphometric data. Skulls of Ch. dubius are generally larger than those of Ch. hiaticula, but a more precise distinction still is difficult.
Lateral extensions on the basal end of the fronto-nasal hinge were identified as taxonomically relevant structures. However, their morphological function has yet to be determined. This characteristic appears in all the Calidris species included in this study, but also in Philomachus and Limicola. The idea that Limicola is closely related to Calidris is further evidenced by the similarities in the behaviour and habits of the two genera. Our morphological analysis of the skulls supports the theory (Glutz von Blotzheim et al. 1999, Joseph & Strauch 1978) that even though Philomachus is similar in its habits to the shanks and tattlers (Tringa), it more probably is related to Calidris.
We are grateful to Dr. Doris Mörike and Christiane Zeitler (Stuttgart State Museum of Natural History), Dr. Elisabeth Stefan (Osteology Department of the Baden-Württemberg State Office for Monuments and Antiquities, Konstanz) for providing specimen. Dr. Bernd Leisler (Max Planck Institute for Ornithology, Radolfzell) helped us to define measurement methods and terminology. Finally, we would like to thank Esther Gollan (medicalart, Aachen) for the illustrations Figs. 1a and 1b.