Cavity roosting, philopatry, and cooperative breeding in the Green Woodhoopoe may reflect a physiological trait

-Cooperative breeding in birds generally is related to a critical resource. A critical limiting factor for the Green Woodhoopoe (Phoeniculus purpureus) is the availability of roost cavities. Although predation at roost holes is the major source of mortality, woodhoopoes invariably roost in cavities or under loose bark. This dependence on cavities appears to be related to the birds' inability to cope physiologically with low nighttime temperatures. This is the first evidence to suggest that a physiological limitation has played a major role in the evolution of philopatry and possibly cooperative breeding in an avian species. Received 2 April 1987, accepted 8 October 1987. THE prevalent view of the basis of cooperative breeding in birds is that it is related to some kind of resource (e.g. Brown 1974, Koenig and Pitelka 1981, Emlen 1982). In some cases the putative critical resource is territorial space (Woolfenden and Fitzpatrick 1984), while in others it may be costly self-constructed structures such as food storage sites (Stacey and Koenig 1984) or roost cavities excavated into living trees (Ligon 1970). Although the selective basis of the nondispersal element of avian cooperative breeding is unresolved (Stacey and Ligon 1987), the specific factors prohibiting dispersal or, alternatively, favoring group living vary from species to species, and nondispersal of young birds is a necessary precondition for the evolution of cooperative breeding. We studied the behavior and ecology of the cooperatively breeding Green Woodhoopoe (Phoeniculus purpureus) near Lake Naivasha in the Rift Valley of Kenya (0?40'S, 36?23'E) for several years, 1975-1984, and found that for these birds tree cavities suitable for roosting are a critical limiting factor (J. D. Ligon and S. H. Ligon 1978, 1982, 1988). We followed several hundred birds to their roosts, usually to capture them for individual marking (386 woodhoopoes banded). Without exception, the woodhoopoes roosted under cover. They almost always entered a tree cavity and usually roosted in groups; as many as eight birds have been recorded in a single cavity (J. D. Ligon and S. H. Ligon 1982). If, during the day, the roost cavity was occupied by another species, e.g. honeybees (Apis mellifera; S. H. Ligon and J. D. Ligon 1982), the returning woodhoopoes usually flew to a secondary cavity. In the absence of a backup roost hole, the birds crawled under loose bark to spend the night. There is no evidence that they ever roosted in the open. Moreover, because woodhoopoes do not excavate their own cavities and must compete for holes, often unsuccessfully, with numerous other cavity-dwelling animals (J. D. Ligon and S. H. Ligon 1982, 1988), high-quality (i.e. relatively predator-proof) roost sites are scarce. The severe interspecific competition for cavities (honeybees, rodents, other birds) means that safe cavities vary in number both spatially and temporally. Because of the frequent takeover of cavities by other species, a key element of territory quality is the number of potential backup or reserve roost sites. Although the woodhoopoes thus require cavities in which to roost, this behavior appears to be costly because the holes used often are in weak wood not chosen by other species, and the birds are vulnerable to arboreal nocturnal mammalian predators, such as large-spotted genets (Genetta tigrina) and possibly cats (Felis libyca and F. domesticus; Ligon and Ligon 1978). Driver ants (tribe Dorylini) also capture adult woodhoopoes in their roost holes. Primarily as a result of nocturnal predation at roost sites, Green Woodhoopoes suffer annual mortality rates about twice as high as some other wellstudied cooperatively breeding species, about 40% per year for males and 30% per year for females (Ligon 1981, 1983; Ligon and Ligon 1988). 123 The Auk 105: 123-127. January 1988 This content downloaded from 207.46.13.131 on Sat, 15 Oct 2016 04:40:30 UTC All use subject to http://about.jstor.org/terms 124 LIGON, CAREY, AND LIGON [Auk, Vol. 105 TABLE 1. Metabolic rates and body temperatures of Green Woodhoopoes at different ambient temperatures (T0). Male Male Fe1 2 male Body mass (g) 71.0 66.1 52.3 Daytime body temperature (0C) at Ta 270C 420 420 400 Metabolic rate (ml 02 g-' h-') At T0 30.50C 1.33 1.67 1.14 At T0 27?C 1.35 1.40 1.09 At T, 190Ca 1.92 2.54 1.31 Body temperature (?C) at T, 19?C, at 2400 360 350 330 Metabolic rates at 19?C were taken as the birds moved about. We were unable to obtain "resting" rates. In view of the high risk of mortality associated with roosting in cavities, the obvious question is why woodhoopoes do not roost among the thorny branches and twigs of the acacia trees (Acacia xanthophloea) on the study site, as do most other small birds, including some other group-living species of similar mass such as two species of Turdoides babblers (pers. obs.) and the Grey-backed Fiscal Shrike (Lanius excubitorius; Zack 1986). The low, 7-10?C, nighttime temperatures characteristic of the study site at 1,860 m above sea level, together with the extreme agitation shown by woodhoopoes when denied access to their roosts at dusk, suggested they might be physiologically incapable of coping successfully with low nocturnal ambient temperatures (Ligon and Ligon 1978). To address this possibility, we investigated thermoregulatory responses of three captive Green Woodhoopoes to low nocturnal ambient tempera-