Mallard
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[Photo] en: Three mallards (Anas platyrhynchos) on ice. de: Drei Stockenten (Anas platyrhynchos) auf einem zugefrorenem See. Photographer: Francois Trazzi

The Mallard (Anas platyrhynchos[1]), also known as the wild duck, is a dabbling duck which breeds throughout the temperate and sub-tropical areas of North America, Europe and Asia. Probably the best-known of all ducks, it gave rise to most domestic ducks, apart from the Muscovy Duck.

It is strongly migratory in the northern parts of its breeding range, and winters farther south; it is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies. It also frequents Central America and the Caribbean, and has been introduced into Australia and New Zealand. It is now the most common duck in New Zealand.

In captivity, Mallards come in wild-type plumages, white, and other colours. Most of these colour variants are rare but increasing in domestic collections.

Description
The dabbling duck is 56???65 cm length, with an 81???98 cm wingspan, and weighs 750???1000 g. The breeding male is unmistakable with a green head, black rear end and a blue speculum edged with white, prominent in flight or at rest. Males also possess a yellow bill with a black tip, females dark brown.

The female Mallard is light brown like most female dabbling ducks. It can be distinguished from other ducks by the distinctive speculum. In non-breeding (eclipse) plumage, the drake looks more like the female.

The Mallard is a rare example of both Allen's Rule and Bergmann's Rule in birds. Bergmann's Rule, which states that polar forms tend to be larger than related ones from warmer climates, has numerous examples in birds. Allen's Rule says that appendages like ears tend to be smaller in polar forms to minimize heat loss, and larger in tropical and desert equivalents to facilitate heat diffusion, and that the polar taxa are stockier overall. Examples of this rule in birds are rare, as they lack external ears. However, the bill of ducks is very well supplied with blood vessels and is vulnerable to cold.

The size of the Mallard varies clinally, and birds from Greenland, although larger than birds further south, have smaller bills and are stockier. It is sometimes separated as subspecies Greenland Mallard (A. p. conboschas).

The Mallard inhabits most wetlands, including parks, small ponds and rivers, and usually feeds by dabbling for plant food or grazing; there are reports of it eating frogs.[2] It usually nests on a river bank, but not always near water. It is highly gregarious outside of the breeding season and will form large flocks.

A noisy species, the male has a nasal call, the female the "quack" always associated with ducks. (Rogers 2001)

A 29-year-old Mallard has been recorded.

Breeding behaviour
Mallards form pairs only until the female lays eggs, at which time she is left by the male. The clutch is 8???13 eggs, which are incubated for 27???28 days to hatching with 50???60 days to fledging. The ducklings are precocial, and can swim and feed themselves on insects as soon as they hatch, although they stay near the female for protection. Young ducklings are not naturally waterproof and rely on the mother to provide waterproofing. Mallards also have rates of male-male sexual activity that are unusually high for birds. In some cases, as many as 19% of pairs in a Mallard population are male-male homosexual (Bagemihl 1999).

When they pair off with mating partners, often one or several drakes will end up "left out". This group will sometimes target an isolated female duck ??? chasing, pestering and pecking at her until she weakens (a phenomenon referred to by researchers as rape flight), at which point each male will take turns copulating with the female. Male Mallards will also occasionally chase other males in the same way. (In one documented case, a male Mallard copulated with another male he was chasing after it had been killed when it flew into a glass window (Moeliker 2001[2]).


[edit] Hybridization and systematics
Mallards frequently interbreed with their closest relatives in the genus Anas, such as the American Black Duck, and also with species more distantly related, for example the Northern Pintail, leading to various hybrids that may be fully fertile. This is quite unusual among different species, and apparently has its reasons in the fact that the Mallard evolved very rapidly and not too long ago, during the Late Pleistocene only. The distinct lineages of this radiation are usually kept separate due to non-overlapping ranges and behavioral cues, but are still not fully genetically incompatible. Mallards and their domesticated conspecifics are, of course, also fully interfertile.

The Mallard is considered an invasive species in New Zealand. There, and elsewhere, Mallards are spreading with increasing urbanization and hybridizing with local relatives (Rhymer & Simberloff 1996). Over time, a continuum of hybrids ranging between almost typical examples of either species will develop; the speciation process beginning to reverse itself (Rhymer 2006). This has created conservation concerns for relatives of the Mallard, such as the Hawaiian Duck (Griffin et al. 1989, Rhymer & Simberloff 1996), the New Zealand Grey Duck (Gillespie 1985, Rhymer et al. 1994, Rhymer & Simberloff 1996, Williams & Basse 2006), the American Black Duck (Johnsgard 1967, Avise et al. 1990, Rhymer & Simberloff 1996, Mank et al. 2004), the Florida Duck (Mazourek & Gray 1994, Rhymer & Simberloff 1996, McCracken et al. 2001), Meller's Duck (Young & Rhymer 1998), the Yellow-billed Duck (Rhymer 2006), and the Mexican Duck (Rhymer & Simberloff 1996, McCracken et al. 2001), in the latter case even leading to a dispute whether these birds should be considered a species (and thus entitled to more conservation research and funding) or included in the mallard (AOU 1983).

On the other hand, the Chinese Spotbill is currently introgressing into the mallard populations of the Primorsky Krai, possibly due to habitat changes from global warming (Kulikova et al. 2004). The Mariana Mallard was a population - in most respects a good species - apparently initially derived from Mallard × Pacific Black Duck hybrids (Yamashina 1948); unfortunately, it became extinct in the 1980s. In addition, feral domestic ducks interbreeding with Mallards have led to a size increase - especially in drakes - in most Mallards in urban areas. Rape flights between normal-sized females and such stronger males frequently end in the female being drowned by the males' combined weight.

It was generally assumed that as the spectacular nuptial plumage of Mallard drakes is obviously the result of sexual selection - most species in the mallard group being sexually monomorphic -, hybrid matings would preferentially take place between females of monomorphic relatives and Mallard drakes instead of the other way around. But this generalization has proven to be incorrect (Rhymer et al. 1994, Kulikova et al. 2004).

Note that it is not the hybridization itself that causes most conservation concerns. The Laysan Duck is an insular relative of the mallard with a very small and fluctuating population. Mallards sometimes arrive on its island home during migration, and can be expected to occasionally have remained and hybridized with Laysan Ducks as long as these species exist. But these hybrids are less well adapted to the peculiar ecological conditions of Laysan Island than the local ducks, and thus have lower fitness, and furthermore, there were - apart from a brief time in the early 20th century when the Laysan Duck was almost extinct - always much more Laysan Ducks than stray Mallards. Thus, in this case, the hybrid lineages would rapidly fail.

In the cases mentioned above, however, ecological changes and hunting have led to a decline of local species; for example, the New Zealand Gray Duck's population declined drastically due to overhunting in the mid-20th century (Williams & Basse 2006). In the Hawaiian Duck, it seems that hybrid offspring are less well-adapted to native habitat and that utilizing them in reintroduction projects makes these less than successful (Rhymer & Simberloff 1996; see also Kirby et al. 2004). In conclusion, the crucial point underlying the problems of Mallards "hybridizing away" relatives is far less a consequence of Mallards spreading, but of local ducks declining; allopatric speciation and isolating behavior have produced today's diversity of Mallard-like ducks despite the fact that in most if not all of these populations, hybridization must always have occurred to some extent.

The aforementioned confounds analysis of the evolution considerably. Analyses of good samples of mtDNA sequences give the confusing picture[3] one expects from a wide-ranging species that has evolved probably not much earlier than the Plio-/Pleistocene boundary, around 2 mya. Mallards appear to be closer to their Indo-Pacific relatives than to their American ones judging from biogeography. Considering mtDNA D-loop sequence data (Kulikova et al. 2005), they may have evolved more probably than not in the general area of Siberia; mallard bones rather abruptly appear in food remains of ancient humans and other deposits of fossil bones in Europe, without a good candidate for a local predecessor species. The large ice age paleosubspecies which made up at least the European and W Asian populations during the Pleistocene has been named Anas platyrhynchos palaeoboschas.

As expected, haplotypes typical of American mallard relatives and Spotbills can be found in Mallards around the Bering Sea (Kulikova et al. 2004, 2005). Interestingly, the Aleutian Islands turned out to hold a population of Mallards that appear to be evolving towards a good subspecies as gene flow with other populations is very limited (Kulikova et al. 2005). This unexpected result suggests that reevaluation of the Greenland, Iceland, and NE Canada populations according to molecular and morphological characters is warranted.

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