Aggressive Behaviour by Australian Owlet-Nightjars Defending Fledglings

60 AUSTRALIAN FIELD ORNITHOLOGY 2007, 24, 60–63 Aggressive Behaviour by Australian Owlet-nightjars Defending Fledglings

LISA I. DOUCETTE Centre for Behavioural Ecology and Physiology, Zoology, University of New England, Armidale, New South Wales 2351 (Email: [email protected])

Summary I report observations of aggressive behaviour by Australian Owlet-nightjars Aegotheles cristatus defending newly fledged juveniles. Aggressive behaviour in Owlet-nightjars is uncommon, and an ‘attack’ on another species has not been described previously.

Introduction Australian Owlet-nightjars Aegotheles cristatus are generally passive birds, with weak feet and beaks and little means of defence. Aggressive responses are uncommon, even in adults with eggs or young that are cornered in their roosts (Brigham & Geiser 1997). Brigham & Geiser (1997) checked the nests of four incubating and brooding Owlet-nightjars in the wild on 27 occasions and did not report any kind of threat display. Individual Owlet-nightjars often flush off the nest when approached (25 occasions, three individuals, four nests), but individuals may occasionally remain on the nest in defence of young or eggs (two individuals) (Brigham & Geiser 1997; L. Doucette unpublished data). Owlet-nightjars in captivity have been driven from their nest-boxes by small parrots and finches (Carney in press). Herein I describe aggressive behaviour by Owlet-nightjars defending newly fledged chicks in Imbota Nature Reserve near Armidale, New South Wales (30°34′S, 151°43′E), in late December 2005.

Observations and Discussion From 2004 to 2006, I radio-tracked and observed 39 individual Owlet-nightjars and witnessed little aggressive behaviour. On at least eight occasions I recorded birds cornered in their roosts make threat displays, but more frequently the birds sat quietly. Threat displays included puffing of plumage, spreading of wings to the side, swaying back and forth, and gaping (Plate 14). On four occasions an individual ineffectually snapped at my hand with its beak as I reached into the roost. This response was also observed in older Owlet-nightjar chicks in the nest (L. Doucette unpublished data). An adult Owlet-nightjar roosting in a stump sometimes leaned to one side and spread one wing across its body to make it appear larger (recorded on two occasions when stump observed from above: Plate 10, front cover). Owlet- nightjars rely on their cryptic plumage for protection and seem to employ threat displays only as a last resort when they are cornered. I have never heard Owlet- nightjars hiss, even when handled. This vocalisation has been reported on only one occasion (Bryant 1934). I have heard them give the harsh, grating alarm call (Debus 1997) only when they are being handled, with the exception of an encounter with a Tawny Frogmouth Podargus strigoides that I describe on p. 62. On 19 November 2005 I lured an Owlet-nightjar into a mist net using playback calls and affixed a harness-mounted radio-transmitter before release. Owlet- VOL. 24 (2) JUNE 2007Aggressive Behaviour of Australian Owlet-nightjars 61

Threat display of Australian Owlet-nightjar Aegotheles cristatus Plate 14 Photo: Lisa I. Doucette nightjars are sexually monomorphic; therefore, the sex of this bird was not known at the time but was later determined to be female based on DNA extracted from a feather sample. Several nights later, this bird was tracked to a nest containing four eggs. Two of these eggs hatched on or around 30 November, and the other two did not hatch. The two chicks fledged on the night of 20 December when they were at least 21 days old (the nestling period is recorded as 21–29 days: Higgins 1999). On 21 December, the night after the two chicks had fledged, I commenced radio-tracking the female Owlet-nightjar at 0356 h. Between 0434 h and 0444 h I heard a series of Owlet-nightjar cries by the male parent of the newly fledged chicks, followed by the yipping call of a Sugar Glider Petaurus breviceps. The noise continued for 4–5 minutes before I located the source. The Glider was flattened against the trunk of a tree, and on a branch ~1 m above it was the male Owlet- nightjar. When the Glider moved, the male Owlet-nightjar circled and flew at the Glider with his beak wide open, and snapped at it before beating it once with his wings. The male then circled the tree closely, staying within 1.5 m of the trunk, and struck out towards the Glider several more times with his beak wide open. The Glider froze but continued to broadcast its yipping call. The male then circled wider (coming in from 5–6 m away), flew in at the Glider with his beak wide open and, almost landing on the Glider, beat it with his wings twice before returning to the same perch. I scanned the surrounding trees with a spotlight and found that a AUSTRALIAN 62DOUCETTE FIELD ORNITHOLOGY tree ~1 m away from the one in which this encounter was occurring had a hollow suitable for an Owlet-nightjar roost. Perched in a tree ~5 m away was one of the newly fledged Owlet-nightjar chicks. Shortly after the encounter, the adult male flew off and the Glider scurried away. After several minutes, the fledgling followed the adult male, flying well only one day after fledging. The second fledgling was not seen in the area. On at least six occasions, Owlet-nightjars in Imbota Nature Reserve have been found in nest-hollows that have previously or later been occupied by Sugar Gliders (Brigham et al. 1998; N. Christian and L. Doucette unpublished data), indicating that the two species may compete for roost-hollows. Sugar Gliders are known to carry leaves and bark back to hollows to line and insulate their nests (Fleay 1947; Henry & Suckling 1984), and these nests may become a valuable resource for other animals, such as Owlet-nightjars, as they benefit from the added insulation. Brushtail Possums Trichosurus vulpecula and Squirrel Gliders Petaurus norfolcensis are known to prey on adult birds, eggs and chicks (Winter 1966; Brown et al. 1996), and Fleay (1947) noted that Sugar Gliders may kill and eat small birds. Sugar Gliders may pose a threat to Owlet-nightjar chicks, and thus the aggressive behaviour of the male Owlet-nightjar would seem justified. At 0125 h on 23 December 2005, three nights after the Owlet-nightjar chicks fledged, I was tracking the female parent again when she flew closely by my head. Adult Owlet-nightjars occasionally fly near to people in response to playbacks, but this was the first time that one had flown close to me with no playback. I found one of the fledglings high up on the trunk of a tree ~5 m away. When I shone a light on the fledgling the female parent flew in close and gave approximately ten shrill calls. Each time I shone the light on the fledgling, the calls intensified, and they gradually tapered off when I turned the light off or shone it away. As I moved towards the tree where the fledgling perched, the female parent came within ~4 m of me and the fledgling. At 0157 h the fledgling flew off and was lost from sight. On 27 December, I was mist-netting in this same area. At 2045 h, ~15 minutes after dark, I heard something hit the net and the screeching alarm call of an Owlet- nightjar (Debus 1997). I found the male parent Owlet-nightjar and a Tawny Frogmouth both caught in the net. This was such an unusual occurrence that I thought it possible that the male Owlet-nightjar was chasing the Frogmouth while defending his fledglings. However, this cannot be confirmed and the reverse could have been true. It is suggested that captive Owlet-nightjars not be kept in aviaries with Tawny Frogmouths (Carney in press). Presumably this is related to aggressive encounters between the two species. Owlet-nightjars lay eggs in October and November (Schodde & Mason 1980; Brigham & Geiser 1997), and most nestlings have fledged by the end of December. However, based on mist-net capture rates using call playback, it appears that Owlet- nightjar adults are at their most aggressive in January and February (L. Doucette unpublished data), suggesting that they are actively defending territories and young at this time. These observations are supported by data collected by Debus (1997), showing that spontaneous calling by Owlet-nightjars is two to four times more frequent in October and February than from March to September. Although one would expect calls to be more frequent during the breeding period in October, the high calling rate in February suggests that the fledglings are still dependent on their parents, and that the parents are still actively protecting the fledglings. Despite limited physical defensive capabilities, it would seem that Owlet-nightjars do make efforts to protect newly fledged young and are capable of performing effective VOL. 24 (2) JUNE 2007Aggressive Behaviour of Australian Owlet-nightjars 63 aggressive displays. The submissive response of the Sugar Glider to the male Owlet- nightjar’s aggressive behaviour in December 2005 demonstrates the potential of these birds to defend young.

Acknowledgements I thank Fred Harvey for assistance in the field and Fritz Geiser, Mark Brigham and Chris Pavey for comments on a draft. Permits for the study were issued by the University of New England Animal Ethics Committee, the NSW National Parks & Wildlife Service and the Australian Bird & Bat Banding Scheme. This study was facilitated by funding from the University of New England, and grants from the Royal Zoological Society of New South Wales and the Australian Bird Study Association.

References Brigham, R.M., Debus, S.J.S. & Geiser, F. (1998), ‘Cavity selection for roosting, and roosting ecology of forest-dwelling Australian Owlet-nightjars (Aegotheles cristatus)’, Australian Journal of Ecology 23, 424–429. Brigham, R.M. & Geiser, F. (1997), ‘Breeding biology of Australian Owlet-nightjars Aegotheles cristatus in eucalypt woodland’, Emu 97, 316–321. Brown, K., Moller, H. & Innes, J. (1996), ‘Sign left by Brushtail Possums after feeding on bird eggs and chicks’, New Zealand Journal of Ecology 20, 277–284. Bryant, J. (1934), ‘Bird notes from Toolern Vale’, Emu 34, 113–120. Carney, T. (in press), ‘Captive husbandry of aegothelids’, in Holland, G. (Ed.), Encyclopedia of Aviculture, Random House, Auckland. Debus, S.J.S. (1997), ‘Vocal behaviour of the Southern Boobook Ninox novaeseelandiae and other nocturnal birds’, pp. 71–85 in Czechura, G. & Debus, S. (Eds), Australian Raptor Studies II, Birds Australia Monograph 3, Birds Australia, Melbourne. Fleay, D. (1947), Gliders of the Gum Trees: The Most Beautiful and Enchanting Australian Marsupials, Bread and Cheese Club, Melbourne. Henry, S. & Suckling, G. (1984), ‘A review of the ecology of the Sugar Glider’, pp. 355–358 in Smith, A. & Hume, I. (Eds), Possums and Gliders, Australian Mammal Society, Sydney. Higgins, P. J. (1999), ‘Family Aegothelidae: Owlet-nightjars’, pp. 1036–1048 in Higgins, P.J. (Ed.), Handbook of Australian, New Zealand & Antarctic Birds, vol. 4, Oxford University Press, Melbourne. Schodde, R. & Mason, I. (1980), Nocturnal Birds of Australia, Lansdowne, Melbourne. Winter, J. (1966), ‘Bird predation by the Australian marsupial Squirrel Glider’, Journal of Mammalogy 47, 530.

Received 9 February 2007 !