Cavity-nesting ducks: why woodpeckers matter Janet Kear Goosander Mergus merganser Rosemary Watts/Powell ABSTRACT This paper poses a number of related questions and suggests some answers.Why were there no resident tree-hole-nesting ducks in Britain until recently? Why is the Black Woodpecker Dryocopus martius not found in Britain? Could the supply of invertebrate food items, especially in winter, be responsible for the distribution of woodpeckers in western Europe? It is concluded that, in Europe, only the Black Woodpecker can construct holes large enough for ducks to nest in, and that this woodpecker does not occur in Britain & Ireland because of the absence of carpenter ants Campanotus. A plea is made for the global conservation of dead and dying timber, since it is vital for the biodiversity of plants, insects, woodpeckers and ducks. irdwatchers, at least in Britain, tend to surface-feeding ducks (Anatini), including think of ducks as nesting in the open and those which we used to call ‘perching ducks’, Bon the ground, but cavity-nesting is actu- and in the seaducks (Mergini). ally normal, if not obligatory, in almost one- Because ducks are incapable of making holes third of the 162 members of the wildfowl family for themselves, however, they must rely on (Anatidae). Geffen & Yom-Tov (2001) suggested natural agents for the construction of nest cavi- that hole-nesting has evolved independently at ties, and this dependence has profound implica- least three times within the group. I think that it tions for their lifestyles, breeding productivity is more likely to have arisen five or six times; and conservation. As Eberhard (2002) pointed indeed, hole-nesting could be the primitive out, because they are not excavators, cavity- condition. In any event, the habit occurs among adopting ducks have limited control over the whistling-ducks (Dendrocygnini), shelducks location of their nests, and rarely nest colo- (Tadornini), in two or three different groups of nially. The skills of a variety of other animals © British Birds 96 • May 2003 • 217-233 217 Cavity-nesting Ducks: why woodpeckers matter are employed, but the bird family which con- females, which alone incubate the eggs, are tributes most to the process of hole-making, as extremely similar in appearance and body pro- ‘primary cavity excavators’ (Aitken et al. 2002), portions (Madge & Burn 1988). Since it has is the woodpeckers (Picidae), of which there are been suggested, quite convincingly, that the size just over 200 species (Winkler et al. 1995). and shape of the American bird evolved to To give a few examples: Audubon (1835) enable it to sit in a woodpecker hole, it seemed observed the North American Wood Duck Aix sensible to look for an Asian woodpecker, about sponsa laying for three successive years in holes the same size as the Pileated, which may have made and abandoned by the large, and probably had a similar influence on the Mandarin. While now extinct, Ivory-billed Woodpecker it is not as rare as used to be assumed, the Man- Campephilus principalis.Indeed,all early Amer- darin is still uncommon. The forests of ican explorer-naturalists (who were mostly Manchuria, where it formerly bred, have been Europeans) noted with interest that the Wood almost totally destroyed, and its breeding Duck nested in woodpecker holes. Bellrose & biology is little studied (Shurtleff & Savage Holm (1994) actually speculated that the duck 1996). The only woodpecker which occurs in evolved its particular size and slim shape in Japan in sufficient numbers, and can produce order to take advantage of cavities created by a cavities large enough for the Mandarin, is the woodpecker which is slightly smaller than Black Woodpecker D. martius. As a breeder, the Ivory-billed but much commoner – the Pileated woodpecker is more or less confined to Woodpecker Dryocopus pileatus. They made the Hokkaido and northern Honshu – and so is the point that the range of the Wood Duck fits nesting Mandarin. Black Woodpeckers are plen- completely within that of the Pileated Wood- tiful among the steep mountains of eastern pecker. Hokkaido, where Mandarins and Goosanders The Wood Duck’s nearest relative, the Man- Mergus merganser also occur in the nesting darin Duck Aix galericulata, breeds in tem- season. As the female Goosander uses Black perate forests similar to those inhabited by the Woodpecker holes in Europe (and Pileated Wood Duck, but on the opposite side of the Woodpecker holes in America; Mallory & Metz Pacific Ocean. The males of these two beautiful 1999), it is likely that the Goosander and the ducks differ substantially in plumage and a little Mandarin adopt such cavities in Japan as well. in size – the Wood Duck being stouter – but the Perhaps at one time, the Mandarin nested in /Windrush A. & E. Morris 154. Female Wood Duck Aix sponsa investigating nest sites, Canada. 218 British Birds 96 • May 2003 • 217-233 Cavity-nesting ducks: why woodpeckers matter Gordon Langsbury 155. Male and female Mandarin Ducks Aix galericulata,Virginia Water, Surrey. In Japan, the current breeding range of the Mandarin closely matches that of the Black Woodpecker Dryocopus martius (plate 156), which is found chiefly in Hokkaido and northern Honshu. Black Woodpecker holes in other parts of the times used (Sharrock 1976; Mallory & Metz latter’s extensive range, so that the Wood 1999). Wood Ducks and Mandarin Ducks, on the Duck/Pileated Woodpecker association in North other hand, are obligate tree hole-nesters, not America was mirrored by one involving Man- laying at all unless an elevated cavity can be darins and Black Woodpeckers in Asia. found. By no means all will use woodpecker A number of holarctic seaducks are cavity- holes, but many prefer them if they are available. nesters, and many likewise depend on wood- In North America, Pileated Woodpecker nests peckers. Nearly 100% of Buffleheads Bucephala constituted only 5% of cavities deemed ‘suitable’ albeola use holes made by a smaller woodpecker, by researchers in one study of over 300 such the Northern Flicker Colaptes auratus.Smew holes; nonetheless, 20% of cavities actually used Mergellus albellus, Hooded Merganser Lophodytes by Wood Ducks were old woodpecker nests cucullatus and Common Goldeneye B. clangula (Soulliere 1990). Why should they seek out a also use cavities made by large woodpeckers, as woodpecker hole rather than one which has does Barrow’s Goldeneye B. islandica in North merely rotted? By choosing a site in which to America (Evans et al. 2002). The population of excavate their nest, woodpeckers are maximising about 2,000 Barrow’s Goldeneyes in Iceland uses their chances of being free of interference from crevices in the lava rock, however, since neither their own kind, free from last year’s nest parasites trees nor woodpeckers are available (Kear in (20-50% of Black Woodpeckers may reuse last press). year’s hole if it was successful, but Pileated The requirement for a degree of adaptability Woodpeckers seldom do; Bull & Meslow 1977; and accommodation by cavity-nesting ducks is Winkler et al. 1995), and, most of all, free from obvious. The Goosander will occasionally nest egg-eaters such as climbing snakes and among boulders, in a hole in the ground or a mammals. In Europe and Asia, mammalian cavity in the bank, so treeless areas are some- predators include Pine Martens Martes martes, British Birds 96 • May 2003 • 217-233 219 Cavity-nesting Ducks: why woodpeckers matter produce live duck- lings at the end of 30 days of incubation. Cavity capacity may seem a tight fit for a duck and her eggs, as most female ducks are twice as large as a wood- pecker. The size of the cavity excavated by the woodpecker, however, as opposed to that of the entry hole, is determined by the size of four or five full-grown fledglings because, unlike ducks, young woodpeckers grow up within the cavity. Nest-hole capacity will, therefore, be greater than that needed for an adult woodpecker plus eggs. The Pileated Woodpecker nest has a rounder entrance than that of the Black Wood- /Windrush pecker: 10.8 cm × 8.7 cm was the average of 13 entrances (Haramis Pentti Johansson Pentti 1990), and the 156. Black Woodpecker Dryocopus martius, Finland, February 1987. Wood Duck seems quite capable of Japanese Martens M. melampus, Stone Martens getting through a hole of that size. Black Wood- M. foina and Raccoon Dogs Nyctereutes procy- pecker entrance holes, at 13 cm × 8.5 cm, are onoides, while in North America, Common Rac- more oval (Winkler et al. 1995), and a female coons Procyon lotor and the large Fox Squirrel Smew or Mandarin must turn through 90° to Sciurus niger are potential predators. Wood- enter. Cavity depth and width are less easy to peckers intend their nests to be, so far as pos- measure, but the average of 13 Pileated Wood- sible, out of the reach of predators, with few pecker nests was 56 cm deep by 23 cm wide perches or ledges to give a foothold, and with a (Bull & Meslow 1977) – long enough to make it small entrance, so that an enemy has difficulty hard for a Raccoon to reach the bottom with its gaining access. Since woodpeckers are usually arm, and quite wide. territorial, their holes are dispersed through the Even those few birds, including woodpeckers, forest, and often concealed under dense canopy which make burrows in the ground, can be cover (Evans et al. 2002). This may not be the useful to ducks. In South America, for instance, case with natural cavities; so, from the duck’s ground-burrowing woodpeckers (probably point of view, old woodpecker holes are more Campo Flicker Colaptes campestris) sometimes likely to be cryptic and impregnable, and to provide a home for the Speckled Teal Anas flavi- 220 British Birds 96 • May 2003 • 217-233 Cavity-nesting ducks: why woodpeckers matter rostris (Nores & Yzurieta 1980).