Section of Ecology and Systematics, Cornell University, Ithaca, New York 14853, USA Hornbills (Bucerotidae) are unique in their nesting habits; females seal themselves within the nest cavity, where they lay the clutch and remain with the grow- ing young for most or all of the nesting period. In some species the male helps with the sealing process from outside the nest cavity. The nestlings and female are fed by the male through a narrow vertical slit in the sealed nest opening (Kemp 1979 and references therein). Thus far, four species of hornbills have been found to have cooperative breeding systems with helpers at the nest; nonbreeding birds assist the male in feeding the nest mates (Stacey and Koenig 1990). These are the Bushy-crested Hornbill (Anorrhinus gal- eritus; Madge 1969), Southern Ground Hornbill (Bu- corvus cafer [=leadbeateri; see Browning 1992]; Kemp and Kemp 1980), White-crowned Hornbill (Aceros comatus; Leighton 1982), and Brown Hornbill (Ptilo- laemus tickelli; Poonswad et al. 1983). I report helpers at the nest in another hornbill species, the Rufous Hornbill (Buceros hydrocorax mindanensis) on Minda- nao Island in the Philippines. Each of the three subspecies of Rufous Hornbill (B. h. hydrocorax, Luzon and Marinduque; B. h. semigalea- tus, Bohol, Leyte, Panaon, and Samar; and B. h. min- danensis, Mindanao and Basilan [McGregor 1909]) has a distinct casque morphology and bill coloration (il- lustrated in Hachisuka 1934). In all subspecies the sexes are similar and all show the same pattern of plumage maturation (McGregor 1909). The juvenal plumage is distinct with white feathers on the head, neck, chest, and belly. Subsequently, the pale juvenal plumage is replaced by the dark black and chestnut body plumage of adults. The casque and bill develop more slowly; many birds have adult plumage color- ation but incomplete development of the casque and bill coloration. Maturation is probably complete in about four years (Kemp 1979). Mindanao Rufous Hornbills develop from a black bill with a low, humped casque in juvenals to an ivory bill with a prominent, red, anvil-shaped casque in mature adults (see Hachisuka 1934). ! use "immature" to designate birds that have adult plumage coloration, but which have not yet developed the definitive adult mor- phology and coloration of the bill and casque ("adult" refers exclusively to these birds). I observed breeding activity at two Rufous Hornbill nests on the island of Mindanao, Philippines: one at Mt. Apo National Park near Davao City, Davao del Sur Province (7øl'N, 125ø22'E) from 31 July to 25 Au- gust 1984; and the other at Lake Sebu, South Cotabato Province (6ø12'N, 124ø41'E) from 4 to 14 August 1985. Both sites are approximately 1,000 m in elevation. The primary forest vegetation is tropical lowland ever- green rain forest (Whitmore 1984) dominated by trees in the family Dipterocarpaceae (Lewis 1988). I also recorded Rufous Hornbill group sizes at the Paper Industries Corporation of the Philippines (PICOP) logging concession (7ø50'N, 126ø14'E) during two weeks in November and December 1986. The PICOP site is 600 m in elevation with vegetation of approx- imately 20-year-old selectively logged forest. The Lake Sebu nest was found on 4 August 1985. I opportunistically observed hornbill activities at and around the nest daily through 14 August; total ob- servation time was 12 h. The nest cavity was 30-m high in the trunk of a huge emergent dipterocarp (40-m high, 220-cm dbh). This hornbill group was in the initial phases of breeding activity and included four adults and one immature (black bill and casque; the undeveloped casque of this hornbill suggests that it was an immediate postjuvenal bird). I was not able to individually identify the adults. An adult hornbill, probably the female, spent 49% of the total observation time (5.88 h) in the nest cham- ber. This bird excavated the chamber by chiselling at the cavity walls and dumping out beak-fulls of wood chips. Other group members, including the imma- ture, often perched outside of the cavity entrance, facing the nest hole, and hammered at the outer rim of the cavity opening. The presumed female was fed by at least two adults seven times during the two days (10 and 11 August) that she spent the most time (104 and 127 min) within the cavity. Foods transferred included fruits (9 of 10 visible items) and arthropods (1 item). On 4 and 8 August I observed copulation. The first copulation took place in the nest tree and the second in an adjacent tree. With the pair perched on the same branch facing the same direction, the male sidled up next to the female and lowered his head forward low under the female's chin, gently shaking his head. After 1 to 2 s, he hopped laterally over her to land on the same branch, where he repeated the behavior from the opposite side. With each successive hop the male appeared closer to mounting the female. After five or six of these hops, the male mounted the female for 1 to 2 s. In the first observation the male then performed three or four more of the precopulatory hopping displays and mounted the female again (with his tail to the right side of hers) for about 5 s. Just prior to the 8 August copulation, the female had al- lopreened with another of the adult hornbills. On 4 August three adults moved about the tree in a close group, while the immature bird lagged be- hind. On 13 August one of the adults chased and bill- fenced with the immature bird as the immature tried to approach the other three adults, which were perched close together. The Mt. Apo nest was discovered on 21 July 1984. I began observations on 31 July and watched the nest from a ground blind located 20 m from the nest tree for a total of 62 h on 9 days (5-8 h per day) from 4- 25 August. On 5 September a single nestling fledged. The nest was a cavity about 14-m high in a 20-m-high rotten tree trunk (102-cm dbh). I could see no seal at this nest; the female had apparently broken out of the nest by the time I began observations, and the cavity was either left unsealed or I was unable to discern remaining sealing material. Thus, I observed these hornbills during the final 45 days (nest discov- ery to fledging) of a presumed four-month nesting cycle (see Kemp 1979), during which time the female was out of the cavity and feeding the nestling. Three Rufous Hornbills tended this nest. Both adults, presumably the mated pair, had blue-gray eyes. These birds could be individually identified by dif- ferences in tail molt and bill coloration; one had a short central rectrix and a dirty-white bill tip (adult A), while the other had a complete tail with three new feathers that were lighter than the rest of the tail and an ivory-white bill tip (adult B). The other hornbill seen at the nest was an immature bird, rec- ognizable by a dark-gray bill tip and maroon casque. This bird's casque was a raised anvil-shaped protu- berance, but it was not as prominent as in the adults; its eyes were yellow-brown. During the nine days of observation I was able to recognize the individual feeding the nestling for 66 of 72 feeding visits: 22 (33%) by adult A; 29 (44%) by adult B; and 15 (23%) by the immature. The feeders tended to arrive and depart together when delivering food to the nest; group feeding visits were followed by long periods of inactivity at the nest. The feeders visited the nest singly, in rapid succession, perching on the bottom lip of the cavity entrance to regurgitate food to the nestling. Individual feeders usually stayed at the nest cavity for less than one minute. The mean number of hornbills tending the nest during a group visit was 1.8 ñ SD of 0.76 (n = 35). Rates of visitation to the nest by these three birds were notably constant. Intervals between feedings by the group (defined as a visit by any or all of the feeders) averaged 1.35 ñ 0.75 h and ranged from 0.25 to 3.00 h (n = 32). Known visit intervals were not significantly different be- tween individual birds (ANOVA, Fz35 = 0.04, P = 0.96). Of 83 individual items that I observed being transferred to the nestling, 78 were fruits and 5 were animal prey (2 cicadas identified). Repeated sightings of territorial Rufous Hornbill groups at the two nest sites and at the PICOP site enabled me to determine the number of individuals in these groups. Family group sizes ranged from three to seven birds (œ = 4.3, n = 7). Stott (1947) also ob- served groups of Rufous Hornbills in western Min- danao of from three to seven birds. Because hornbills in the genus Buceros usually produce a single young per nesting attempt and nest no more than once per year (Kemp 1979, Poonswad et al. 1986, this study), these territorial groups are probably composed of the breeding pair and their young (Stacey and Koenig 1990) from previous years' nestings. The behavior of the Lake Sebu hornbills is com- parable to descriptions of early nesting activity for captive (Poulsen 1970) and wild (Poohswad et al. 1986) Great Hornbills (Buceros bicornis), as well as captive (Reilly 1988) and wild (Johns 1982) Rhinoceros Horn- bills (B. rhinoceros). The nest site is visited for several months prior to nesting and the female alone exca- vates the nest from within the nest cavity. The male begins feeding the female when she starts to seal the nest, although she still exits to forage on her own. Poohswad et al. (1986) also noted copulation during this period when the female emerged from excavating and plastering the nest. Nest-sealing behavior and materials have yet to be reported for Rufous Horn- bills. The use of living trees for nesting, in addition to the frequent hammering within the nest cavity and on the cavity rim by the Lake Sebu hornbills, suggests that Rufous Hornbills may use tree resin for nest sealing, as observed for Rhinoceros Hornbills (Hose in Shelford 1899, Johns 1982). Feeding of the nestling by both the male and female breeders at the Mt. Apo nest during the later stages of nesting is similar to reports for Rhinoceros Hornbills (Johns 1982, Reilly 1988) and Great Hornbills (Poohswad et al. 1983). Synchronized group feeding visits have also been observed in Brown Hornbills (Poohswad et al. 1983). This pattern of nest attendance by feeding groups may minimize the duration of disturbance at the nest, reducing the chance of revealing the nest to preda- tors. My observations document helpers at the nest in Mindanao Rufous Hornbills. One breeding group consisted of four adults and one immature; the other group consisted of two adults and one immature. Pre- sumably, nonbreeding birds were affiliated with a monogamous pair. Helpers assisted breeding adults in nest preparation as well as feeding the nestling. The immature helper at the Mt. Apo nest contributed a substantial amount of provisioning to the nestling. Cooperative breeding is likely to be common for Mindanao Rufous Hornbills because territories are usually occupied by three or more birds. The breeding systems of the other subspecies of the Rufous Horn- bill have yet to be described. Kemp (1979) noted the association between unique coloration of juvenal and immature birds compared to adults and cooperative breeding systems in hornbills, suggesting that all sub- species of Rufous Hornbill have helpers at the nest. The other three species in this genus have been re- ported to breed monogamously without helpers (Rhi- noceros Hornbill, Johns 1982, Leighton 1982; Hel- meted Hornbill [B. vigil], Leighton 1982; Great Hornbill, Poonswad et al. 1986), although helping behavior has been observed in Rhinoceros Hornbills (Hose in Shelford 1899, Reilly 1988). Because of the diversity of breeding systems within this genus, com- parative ecological work on these hornbills might be useful in investigating the causes of cooperative breeding. Acknowledgments.--I thank the Philippine Bureau of Forest Development for permission to conduct this work and for logistical support. The staff of the Phil- ippine Eagle Conservation Program at Mt. Apo Na- tional Park and the Santa Cruz Mission at Lake Sebu generously provided accommodations during my vis- its. The Paper Industries Corporation of the Philip- pines granted me permission to work on their prop- erty. ! especially thank Susing and Taba Babao for their competent and energetic assistance in the field. Jean Caleda, Marlo Caleda, Nina Ingle, Richard Lew- is, Hector Miranda, Jr., and Bill Wischusen helped me in various ways during this work. Mary and Mike Stephen and Lydia Ingle kindly opened their homes to me during my stays in Manila and Davao, respec- tively. This work was partially supported by a grant from the Chicago Zoological Society. Nina Ingle, Alan Kemp, Robert Kennedy, Walter Koenig, and Kevin McGowan provided helpful reviews of this paper. LITERATURE CITED BROWNING, M. g. 1992. Comments on the nomen- clature and dates of publication of some taxa in Bucerotidae. Bull. Br. Ornithol. Club 112:22-27. HACHISUKA, M. 1934. 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