Breeding biology of the Kakerori (Pomarea dimidiata) on Rarotonga, Cook Islands By EDWARD K. SAUL'. HUGH A. ROBERTSON2' AND ANNA TIRAA1 'Takiturnu Conservation Area Project, PO. Box 81 7, Rarotonga, Cook Islands; ZScience & Research Unit, Department of Conservation, PO. Box lO-420, Welling- ton, New Zealand. ABSTRACT The breeding biology of Kakerori, or Rarotonga Flycatcher, (Pomarea dimidiata) ~~asstudied during ten years (1987-97) of experimental management aimed at saving this endangered monarch flycatcher from extinction. Kakerori remained territorial all year and were usually monogamous, Most birds kept the same mate from year to year, but pairs that bilrd to raise any young were more likely to divorce than successful pairs. Despite living in the tropics, Kakerori breeding was strictly seasonal. with eggs laid from early October to mid-February, and mostly in late October and early November. Nesting started earlier in years when October was very sunny Most pairs (74%) laid only one clutch, but some pairs laid up to four replacement clutches when nests failed.Three pairs (1%) successfully raised tnro broods in a season. Rat (Rattus spp.) predation was the principal cause of nest failure, especially of nests in pua (mpaea berteriana), the main fruiting tree used by rats during the Kakerori breeding season. Annual breeding productivity was initially poor (0.46 fledglings per breeding pair over two years) and the population was declining, but intensive management since 1989has led to a great increase in productivity (1.07 fledglings perb~edingpairover eight years) and the number ofKakerorihas increased from 29 birds in 1989 to a minimum of 153 birds in 1997. 'Their IUCN consenrationstatus can therefore be lowered from 'critically endangered' to 'endangered'. KEYWORDS: Kakerori, Pomarea dimidiata, breeding, consemtion, Cook Islands, monarch flycatcher. INTRODUCTION The Kakerori, or Rarotonga Flycatcher. (Pornarea dimidiata) is a small endan- gered monarch flycatcher, endemic to Rarotonga. Cook Islands. No published infor- mation is available about the breeding biology of the Kakerori or the other four species of Pomarea flycatchers in eastern Polynesia. Kakerori were verycommon throughout Rarotonga until the middle of the 19th century; however, their numbers declined rapidly and their range became restricted to the forested interior in the late 1880s (Gill 1885). By the early 1900s they were thought to be extinct, but were rediscovered in 1973, and the first nest of the species was found in 1983 (Robertson et dl. 1994). Rod Hay and Gerald McCormack colour- banded eight Kakerori in 1984. and searches from 1984 to 1987 showed that the small population of Kakerori was restricted to dense forest at medium altitude (100- 250 m) in rugged terrain on the wettest side of the island (Robertson et al. 1994). Kakerori are unusual among passerines in exhibiting an extreme form of delayed plumage maturation. The age of birds can be determined from bill and plumage colours for the first three years. until the definitive basic plumage is attained at the 'Authorfor correspondence Notornis 45: 255-269 (1998) @ 1998 Ornitboiogical Society o/New Zealand SAUL et al. start of the fourth year (Robertson et al. 1993). Birds of both sexes are, however, capable of breeding as yearlings. Males are larger than females, and although there is some overlap in measurements, most birds can be sexed from bill length and/or head+ bill length measurements (Robertson et al. 1993). In the field, size &fferences are usually indistinguishable, but the sex of birds can be determined from their behaviour during the breeding season. Since 1987, we have studied the breeding of Kakerori as part of a recovery programme aimed at saving Kakerori from extinction, by firstly determining why they were critically endangered, and then experimentallymanaging their main threats. We found early in our study that Kakerori were declining because ship rats (Rattus rattus) and possibly Pacific rats (Rattus exulans) preyed on eggs and nestlings. Our management since 1989 has been to improve breeding success of Kakerori by seasonal poisoning of rats in the 155 ha of forest occupied by the remnant Kakemri population, and by banding trees to prevent rats from reaching nests (Robertson et al. 1998). Our observations on the breeding success of Kakerori have been used as a measure of the effectiveness of our experimental management. Since 1989, the number of Kakerori has increased more than five-fold, from 29 birds to at least 153 birds in 1997. Because of the critical state of the Kakemri population we maintained a policy of minimal disturbance to nesting birds, so, as many of the nests were high and inaccessible, some of our data on breeding biology are incomplete. Also, as the population has grown, it has become increasingly dBcult to follow the breeding attempts of all pairs. METHODS Study area Rarotonga (21°14'S, 159'46'W) is the highest (653 m) and largest (6700 ha) of the 15 islands in the Cook Islands group. Most of the rugged basaltic interior of Rarotonga is forested and uninhabited. Almost all of the approximately 9000 residents live on the narrow (< 1 km wide), fertile, coastal ringplain. Kakerori are largely restricted to 155 ha in the southeastern part of Rarotonga. This area, known as the Takitumu Conservation Area, comprises lower foothills and steep V-shaped valleys between 100 m and 250 m above sea level (Robertson et al. 194). Most birds are in the upper catchments of the Totokoitu, Turoa and Avana Streams pig. I), but a few are found in the lower parts of these valleys. This is the wetter side of the island, as it faces the south-easttrade winds. Annual rainfall averages 3250 mm in the study area compared with 2021 mm at Rarotonga Airport on the opposite side of the island (Thompson 1986); about two-thirds of the rain falls in the wet season (November through April). Seasonal and diurnal variation in temperatures are minor, with the winter being dry and cool (94 mm rainfall and 21.9"C in July) and the summer being wet and warm (253 mm rainfall and 25.8"C in January) at Rarotonga Airport (Thompson 1986). NOTORNIS 45 KAKERORI 257 Rarotonga FIGURE 1 - Map of Rarotonga, Cook Islands, showing the location of the study area. The vegetation has been described by Merlin (1985) and Sanders et al. (1995). The tall (up to 25 m) vegetation on the moderate to steep slopes is dominated by mato (Homdlium acuminatum), koka (Biscbofiajavanica), karaka (Elaeocarpus tonganus) and matira (Cyclophyllum barbaturn), with a dense understorey of the giant kingfern or ana'e (Angiopteris evecta), kavakava atua (Macropiper lati$olium), and kiekie (Ereycinetia wildenl).Near the valley floor the main trees are mato, pua (Fagraea bertaiam),and 'au (Hibiscustiliaceus) , with a few emergent i'i (Inocalpus fagiffer) and coconuts (Cocos nucifera), and a few scattered mountain lantern-tree or turina (Hernandia moerenhoutiana). On the ridges the vegetation is generally shorter (3-5 m tall) and dominated by pua, neinei (Richia speciosa), mato and rata (Metrosideros collzna). Observations Just before each breeding season, we mist-netted and colour-banded as many yearlings and unbanded older birds as possible. During the concurrent annual censuses, we roughly mapped the territory of each pair and home range of non-territorial birds by plotting locations of birds and their territorial disputes. The proportion of SAUL et al. NOTORNIS 45 individually colour-banded birds in the population has increased from 37% in 1987 to about 80%since 1993, and the number of known-age birds has risen from 42% in 1987 to over 92% in 1997. Through each breeding season, from October to February, we and staff of the Cook Islands Conservation Service and then Environment Service, searched for Kakerori nests. For each nesting pair, we kept detailed notes on the colour-band combinations, bill and plumage colours of the birds, the territory they were using, height of the nest up the tree and vertically to the ground, species of tree, position of the nest, and details on behaviour of the birds that could help to identify the stage of their nesting cycle. We did not disturb breeding birds except to protect their nests by placing aluminium collars around the trunk of the nest tree and nearby trees to prevent rats from reaching nests. In a few cases, however, the contents of nests close to the ground could be examined with a mirror attached to a long pole, or by climbing an adjacent tree. No attempt was made to climb nest trees, or to band nestlings, for fear of causing desertions. This meant that some of our observations @articularlyof the number of chicks present) are incomplete, and so figures given are minimum numbers of successful nests or young fledged. By 1996, there were 38 breeding pairs and we were unable to keep track of about a third of their nesting attempts, and so these were excluded from analysis of breeding success. Most nests were visited weekly during our rat poisoning rounds, but some nests were visited almost daily, while some remote nests were visited fortnightly or even less frequently. Nest construction, incubation spells, and feeding visits were timed at a few nests, especially during the early years when there were few nests to follow Unless otherwise stated, we give mean f standard deviation and range of measurements. RESULTS Social structure Typically, adult Kakerori remained in pairs and defended their territory all year. We noted four exceptions to this pattern, all involving additional birds helping to feed chicks or a change in partners. Two pairs had helpers at their nests; in one territory a male helper stayed for at least three successive years (from yearling to 3 years old), and in the other a male (yearling) and a female (2-year-old) helped to defend the territory and feed nestlings.