Adaptations in bill morphology, diet preferences and social structures: insights from some species in the Free State.
By Dawie de Swardt.
LBJ’s or “little brown jobs” in birding terms are cryptic or brownish birds and most of the time difficult to identify. Larks, together with other generally drab-coloured birds such as pipits, warblers and cisticolas; are small to medium-sized passerines with such cryptic plumages. They occur mostly in open, structurally simple habitats, ranging from desert and semi-deserts, the Karoo areas, to the moister transitional grasslands and savannah woodlands to the east. Many larks exhibit substrate colour-matching, the dorsal plumage in particular bearing a resemblance to the substrate of the environment it inhabits. This means that in some species, there may be several different colour morphs, adding an additional level of complication in distinguishing between some species. For example, Spike-heeled Larks with a very light and sometimes grey plumage most probably occur in the arid regions of the Northern Cape or Namibia, whereas as darker, rufous-coloured birds are found in the mesic grassland regions in the east of the species range.
However, larks’ most distinguishing feature is arguably their adaptations in bill morphology, which indicates their preferred diets and also their ecological adaptations in distribution, diversity and movement patterns. Richard Dean and the late Phil Hockey published a paper in the 1990s in the peer- reviewed journal Ostrich, on the ecological perspectives of lark distribution and diversity in the drier western areas of South Africa. Their main finding was that larks can be divided into resident and nomadic species, each with their own suite of particular adaptations as regards body size, diet, demography and social structure. They also found that the different movement patterns select for different adaptions to their bills. They concluded that different lark species can be classified as resident-insectivore, resident-granivore, nomad-insectivore and nomad-granivore.
The resident-insectivore and resident-granivore lark species
The resident insectivore and granivorous larks are represented by species which are larger and weigh more than 20 grams, their larger and/or longer bills adapted primarily to an insectivorous diet with fewer seeds in their diets. The plumage of resident lark species tends to be uniform, and closely matched to the substratum where they occur. These resident birds tend to occur in pairs or small family groups and exhibit very limited local movements. In this section, some resident insectivore and granivorous bird species will be discussed with emphasis on their occurrence within the Free State. Three resident-insectivore larks are the Eastern Clapper, Spike-heeled and Fawn-coloured Larks while the resident-granivore species are the Sabota and Large-billed Larks. The Rufous-naped Lark is also included in this article as it is a characteristic species of the Free State.
Rufous-naped Lark Mirafra africana
The Rufous-naped Lark is a larger (average weight around 40 grams in both sexes) grassland / savanna species with an eastern distribution range. They prefer areas with adequate perches such as rocks, termite mounds and any man-made song posts such as fence poles. They are mostly absent or rare in the western Free State and Northern Cape. This species is mostly resident and occurs singly or in pairs, with males known to keep their territories all year. The males vocalize their characteristic “tsee-tsee’oo” calls from a song post with audible wing clappings between songs, mainly during summer. They are large and robust in build, with a long, strong, slightly decurved bill adapted for insect diet. They have a bright crest, usually raised during singing. Their chests have clear diffuse spots with streaky patterns on the back and rufous wing feathers. They forage on the ground for food at the bases of grass tufts and also scratch the soil with their feet or bill. Their diet consists of a variety of insect species including larvae and adult beetles, stink bugs, caterpillars, grasshoppers and also spiders.
Eastern Clapper Lark Mirafra fasciolata
The Eastern Clapper Lark is a resident-insectivore, and can be identified by its bright rufous plumage. Most of the body and wing feathers are bright reddish with finely mottled breast and wing feathers which are tipped brownish. This medium-sized lark weighs about 30 grams and has a fairly short, stubby bill adapted for its insectivorous diet, consisting mainly of ants and harvester termites. Seeds also comprise a part of the diet, albeit a minor part. They are usually encountered as solitary birds or in pairs on the ground, but in summer the males’ distinctive wing clapping and whistling display flights reverberate over the grasslands. They also can be observed sitting on fence posts or bushes giving sparrow-like calls. This is mainly a resident species with some localized, nomadic movements amongst them. Their preferred habitat in the Free State is sparse to dense grassland areas with patches of karroid vegetation, scattered bushes and ant hills. They are common to abundant in all areas of the Free State.
Spike-heeled Lark Chersomanes albofasciata
The Spike-heeled Lark is another resident-insectivore lark. They can be distinguished by their long, slender and decurved bill (adaptation for digging for insects in the soil), their reddish plumage and their diagnostic, short, white-tipped tail. They also have a pale throat that contrasts strongly with the rest of the underparts. Another feature of this lark is its long hind claw or spike (hence its name) and can be sometimes be observed closely in the field by the observer. This medium-sized lark weighs between 22 and 28 grams and there is marked sexual dimorphism, particularly with as regards bill length. It has been suggested that the different bill lengths of the sexes allow males and females to exploit different dietary niches, and thus avoid competing for food. They forage for invertebrates – mostly ants and termites – with low proportions of seeds in their diet. This species is mostly resident with some local movements and usually occurs in small family groups of 3 – 5, but groups of up to 10 have been recorded. Nest helpers have been observed feeding nestlings or juvenile birds. In the Free State they occur in grassy to Karoo areas, giving preference to open patches with sparse vegetation cover. They also like to forage and dust bath in the loose soil on the edges of roads. They occur in nearly all areas of the Free State but are more abundant in the western areas. They are absent in the eastern Free State bordering Lesotho.
Fawn-coloured Lark Calendulauda africanoides
The last resident-insectivore lark discussed in this article is the Fawn-coloured Lark. As its name indicates, this species has sandy-brown plumage, with weak streaking, nearly white underparts and a yellowish wash on its flanks and breast, matching the sandy soil areas where they occur. They are medium-sized larks, weighing about 23 grams, with a small and sharp bill adapted for insect feeding. Their diet consists mainly of insects, including termites but also some seeds. In a study in the Karoo, of all seeds consumed, 50% were grass seeds. They are resident birds with some local movements and are usually seen singly or in pairs. They prefer sandy soils (mostly red) in bushy grasslands, Kalahari savannas and open woodland. They have a western distribution range in the Free State with their range extending eastwards to the Bloemfontein–Soutpan areas and northwards in the Kalahari thornveld areas of the Hoopstad–Bloemhof region.
Sabota Lark Calendulauda sabota
The Sabota Lark, a resident-granivore weighing about 25 grams, can be identified by its bold, blackish-streaked plumage with a striking white eyebrow. Its breast is lighter with a distinctive malar stripe at the base of the lower mandible. The bill morphology of this species is particularly interesting, with slender-billed and thicker-billed morphs recognized. Based on a study published in Ostrich in 2017 using specimens of the National Museum and other museums by Derek Engelbrecht and his team, it has been suggested that the larger thick-billed form with a more western distribution may represent a separate species from the slender-billed form which has a more eastern distribution. Both forms occur in the Free State, with the thick-billed occupying the southern and western parts and the slender-billed morph the northern and eastern grasslands. A detailed genetic study is in the pipeline on the Sabota Lark complex. Their diet consists mainly of invertebrates, mainly ants, but also seeds. In a dietary study conducted in Namibia, their diet consisted of 40% invertebrates and 60% seeds, and of the seeds, 69% were grass seeds. They are resident birds and occur singly or in pairs. Their preferred habitat is thornveld savanna and sparse woodland. In the Free State they have also been recorded in olive tree-dominated hills and Karoo areas. They have a more western distribution range in the Free State and are absent from the eastern parts of the province. They also occur in hilly areas in the Orange River valley of the southern Free State.
Large-billed Lark Galerida magnirostris
The Large-billed lark is also a resident-insectivore species and also has a thick and heavy bill with a yellow base at the lower mandible. They are also larger and weigh more than 40 grams. They have a greyish-brown or straw-coloured plumage with dark blotches on the breast extending as thinner streaks on the flanks. They also have strong facial patterns and have a smallish rounded crest. They generally have an insectivorous diet which includes ants, termites, larval beetles as well as adult beetle species. The main part of their granivorous diet consists of seeds of sedges, grasses, cereal, legumes and succulents. Large-billed Larks are mainly resident species with some local movements and are usually observed as solitary birds or in pairs. Family groups can consist of up to 4 individuals. This species occurs in semi-arid habitats like Karoo scrub and shrubby grasslands, as well as cultivated areas including ploughed, harvested and fallow fields. In the Free State, their distribution is mostly restricted to the southern and western parts and northwards as far as the Bultfontein region. They also occur in the higher- lying areas of Lesotho and the adjacent southern and eastern areas. Interestingly, the populations in the highlands of Lesotho have considerably smaller bills than populations in lower-lying areas.
The nomad-granivore lark species
The nomadic insectivore and granivore larks are the opposite of the resident lark species. They are smaller in body size, weigh less than 20 grams and have a primarily granivorous diet characterized by shorter, deeper bills adapted for crushing seeds. The plumage of nomadic lark species comprises a mixture of dark streaking or spots on the feathers which is an adaptation to their wider habitat usage as nomadic species. These nomadic larks, as the name suggests, are gregarious, occur mostly in flocks and exhibit extensive movements. In some areas these species also respond to substantial rainfall events and will move into areas where resources such as food and nesting material is abundant and they will then opportunistically breed in these areas. The following nomadic granivore bird species (Red-capped and Pink-billed Larks) will be discussed with emphasis on their occurrences and habitats within the Free State. Of the three sparrow-lark species, only the Chestnut-backed and Grey-backed Sparrow-Larks occur in the Free State and are nomadic, occurring mostly in flocks and subsisting on grains.
Red-capped Lark Calandrella cinerea
The Red-capped lark is a medium-sized, nomadic-granivore species with a mass of about 24 grams and a pipit-like appearance. They are always observed in small groups or larger flocks. This lark’s diagnostic features are its white belly, the rufous chest patches and a plain, bright rufous cap. Its short, thin bill is adapted for its granivorous diet. Diet studies, based on stomach contents, reveal that its diet is mainly seeds of grasses, sedges, shrubs and forbs. Invertebrates form a small part of the diet and include mainly ants and termites. Their movements are complex, with some populations being resident while suitable conditions prevail, and others exhibit local movements or nomadism. Still other populations, or parts of them, are believed to be migratory. They are usually observed in small groups (5 – 20 birds) or flocks up to 1000s. This species prefers flat, open habitats with bare soils and short grass or shrub cover. They are often seen in small groups along gravel roads. They are also attracted to burnt grasslands, overgrazed areas and ploughed fields. They occur in most parts of the Free State, but more abundantly in the central, eastern and north-eastern parts of the province.
Pink-billed Lark Spizocorys conirostris
The Pink-billed Lark is a smallish (mass 15-16 grams), granivorous species which is sometimes overlooked by birders because of their cryptic plumage and small size. This species presence is mostly revealed by their distinctive flight call when flying up from the ground in grasslands. They also regularly visit farm reservoirs and puddles to drink water. They have strong, conical bills with a pinkish colouration (hence the name) and mostly resemble a seed-eating finch or weaver. Their diet consists predominantly of grass seeds but insects also form a part of their diet. This species is resident in some areas, but mostly nomadic when in search of suitable grassland habitats. It is also known to follow rainfall events in arid areas. Flock sizes range between 5 and 20 birds or even more. Grassland, where the grass is not too tall, and with plenty of bare ground, is their preferred habitat with shrubby cover and also overgrazed areas with short grass. They sometimes forage on gravel road verges or tracks with grassy areas. The distribution range is centred on the moist grasslands of the Highveld and the eastern Free State. They also occur in isolated pockets in grassy areas in the Northern Cape, the central Karoo and in the southern and western Free State.
Chestnut-backed Sparrow-Lark Eremopterix leucotis
Chestnut-backed Sparrow-Larks are smallish, sexually dimorphic, sparrow-like larks. Males can be identified by their rich chestnut brown upperparts and wings, with a pale grey rump. They have an attractive black head, with a white ear patch, and black underparts. Females are mottled buff-brown with black plumage on the lower breast and belly, and dark streaked. Their bill is short and stout and well-adapted to their granivorous diet. As such their diet consists mostly of small grass seeds. During their breeding season, insects also form part of their diet. This perhaps increases the protein content of their diet and mostly feed the nestlings insects. The nomadic movements of this species are poorly understood, and unexpected irruptions in large numbers can occur depending on rainfall events. They usually occur in small flocks or larger flocks up to 50 birds or more. They are sparsely recorded in overgrazed areas, bare areas in savanna and grassland, including agriculture fields, pans, air strips, and also on gravel road verges. In the Free State, they occur mostly in the central and north-eastern parts and sporadically in Karoo areas of the southern and western Free State.
Grey-backed Sparrow-Lark Eremopterix verticalis
The Grey-backed Sparrow-Lark looks similar to the Chestnut-backed Sparrow-Lark, where the males in this species have a grey back instead of a chestnut back. The Grey-backed Sparrow-Lark male also has a white cap and a diffuse white bar across the nape which links to its white cheek patch. Both sexes are pale to dark grey or brownish above. Females have a sooty belly patch and a slightly streaked breast. Their body mass (18 grams) and sparrow- like bill is also similar to other sparrow-lark species. Their diet consists mainly of grass and forb seeds, but insects (mainly in females) are also part of their diet. In Namibia, a diet study on this species showed that 91% of the diet comprised of seeds. They are locally common to abundant nomads in some areas and a partial migrant in others, mostly determined by rainfall seasons. Similarly to Chestnut-backed Sparrow-Larks, they are gregarious, occurring in small groups (5 – 6 birds) or flocks up to 1000s of birds in some areas. They also prefer overgrazed, bare areas in open savanna or sparse grassland and Karoo veld. In some areas they are also attracted to agricultural fields and the edges of gravel roads. They mostly occur in the southern and western Free State with north-east distribution range limit around the Bloemfontein area.
Conclusions
It can be seen that some lark species can be either resident at a certain locality or have some nomadic behaviour and may thus be observed only sporadically. The resident larks are more insectivorous in their diet, and coexist at low densities by exploiting diverse prey resources including ants. The resident birds also have larger bills, adapted for an insect diet, and tend to be bigger. The more nomadic granivorous species, which mostly can be observed in small groups or flocks, can coexist by moving to areas where rain has fallen and where nesting habitat and superabundant seeds are temporally available.
So next time when you observe a lark species in the field, think about these fascinating facts of larks and their adaptations.
References
Barnes K 2009. Angels in camouflage: An appreciation of larks. Africa – Birds & Birding 14 (4): 50-55.
Chittenden H, Allan DG & Weiersbye I 2012. Robert’s geographic variation of southern African birds. The John Voelcker Bird Book Fund, Cape Town.
Dean WRJ & Hockey PAR 1989. An ecological perspective of larks (Alaudidae) distribution and diversity in the southwest-arid zone of Africa. Ostrich 60: 27-34.
Hockey PAR, Dean WRJ, Ryan PG (eds). 2005. Roberts Birds of Southern Africa. 7th Ed. Cape Town: The Trustees of the John Voelcker Bird Book Fund.
Marr SM, Mashao ML & Engelbrecht GD 2017. Morphological variation in the Sabota Lark Calendulauda sabota in southern Africa, Ostrich 88: 67-71.
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