Adaptive plumage wear for increased crypsis in the plumage of Palearctic larks (Alaudidae).

Larks (family Alaudidae) are ground-dwelling passerines living in open habitats with sparse vegetation, i.e., deserts, grasslands, and agricultural fields. A majority of species occur in Africa and Eurasia. Lark species, about 100 in total, are mostly dull colored with a predominance of beige and light brown coloration, and/or streaked patterns due to melanin pigments, with an absence of bright and conspicuous carotenoids and/or structural blue or iridescence. There is no obvious sexual dichromatism in a majority of species (Cramp 1988), except a few species, including the Eremopterix genus and the Black Lark (Melanocorypha yeltoniensis), where males are much darker than the females, and the Eremophila species, in which males have longer head horns. Their general appearance has made larks a textbook example of cryptic species in the avian world (Cramp 1988). The reasons underlying their extreme dependence on camouflage seem to be that they are generally small and defenseless, and that they breed solitarily on open nests on the ground, this making them vulnerable to attacks by avian predators, as well as carnivore mammals and reptiles (Dean and Williams 2004). They are, however, very confident in their crypsis and, for instance, they would leave the nest only at the last moment when approached by a potential predator including humans (Cramp 1988). Most larks molt the juvenal plumage completely soon after they fledge (Cramp 1988), which is rather unusual among passerines (Kiat and Izhaki 2016), and are among the few songbirds in which adults undergo a single postbreeding molt, typically at the end of the summer (Svensson 1992). There are some exceptions to this rule, and, for instance, Vaurie (1951) suggests that two to five out of 27 Asian species of larks may undergo a double annual molt. Nonetheless, a majority of Palearctic larks (and perhaps African ones too, although precise data are lacking) initiate their aerial courtships in spring with a highly worn plumage, and they actually breed and raise their young in the most worn state of the plumage before the next molt. The Collins Bird Guide (Svensson et al. 1999), a comprehensive identification guide for the birds of the Western Palearctic, includes different illustrations for the autumn (fresh) and spring (worn) plumages of 15 out of the 20 lark species covered by the guide. Only 10 other species of different families are shown in fresh and worn plumages out of about 700 species covered in the guide. Most songbirds follow a different strategy from the larks and start breeding in rather prime plumage condition, many of them having previously undergone the prealternate molt, involved in the formation of a fresh, attractive plumage thought to be sexually selected for mate acquisition (Guallar and Figuerola 2016). The Thekla’s Lark (Galerida theklae) in the left panel of Fig. 1 was photographed on 25 February 2018 in central Spain. It was midway between its last full molt, which possibly had taken place in late August–September 2017, with the next one scheduled for late summer 2018. If the bird bred, it would do so no earlier than May 2018, and thus no less than 2–3 months after the picture was taken. The plumage of the bird is very worn, and this is particularly evident in the three tertial feathers covering the primaries and secondaries on both wings. These tertials have been reduced to one-half their original length, their margins are also abraded in a random fashion, and even the color of their outer margin is bleached in relation to the central vane of those feathers. In addition, there is evidence of feather wear in the tail feathers and upper tail coverts. By comparison, a Thekla’s Lark with a freshly molted plumage in September would show a light-colored fringe in all covert feathers and tertials (Fig. 1, right panel). To establish the feather wearing pattern of larks in low latitudes of the Western Palearctic, we examined 73 different specimens in the Bird Collection of Do~ nana Biological Station, collected in Spain or Morocco, and belonging to nine different lark species, including 27 adult Thekla’s Larks (Appendix S1: Table S1). We noted whether there was wearing of the tertials, the largest feathers visible on the back of larks, as well as the date of capture for every specimen. As a control, we checked feather wearing in four passerine species that share open