Torpor Use and Behavioural
Ecology of Caprimulgids in the Arid Zone
Lisa I Doucette,
Zoology,
University of New England,
Armidale, NSW, 2351
Objectives and
Justification:
Caprimulgiformes are common and found
throughout Australia, yet our knowledge of these species is very limited. Most
of our understanding of the physiology and behavioural ecology of nightjars in
Australia is anecdotal and relies on casual observations of pairs or individuals
(Higgins, 1999), or consists of assumptions based on research of nightjar
species on other continents. This project aims to enhance our knowledge of these
distinctive nocturnal birds through physiological, behavioural and ecological
studies on two of these species, Australian owlet-nightjars (Aegotheles
cristatus) and
Spotted
Nightjars (Eurostopodus argus), in the harsh environment of Australia’s
arid zone. A. cristatus is the only one of the hundred or so species of
Caprimulgids that roosts in tree cavities and one of a few species of birds that
will roost in rock crevices, making their behavioural ecology exceptionally
unique. Several species of owlet-nightjars found throughout Australasia are rare
and threatened with extinction (e.g. A. savesi; Ekstrom et al., 2002). Studies
on A. cristatus may provide essential information on the ecological needs of
Caprimulgids, enabling the conservation of closely related species.
Nightjars differ from most other species of birds in that they have the ability
to use torpor, a periodic lowering of body temperature and metabolic rate, to
conserve energy. Small birds have high energy expenditure and are often
constrained by low energy availability. Torpor is an important adaptation,
presumably used to overcome periods of resource limitation, but little is known
about how it is expressed. Our study will provide important new data on torpor
patterns in relation to environmental conditions and resource limitations. These
data will be important for wildlife managers as they will allow prediction of
energy limitations in the wild and how free-ranging birds have adapted to low
food availability. Furthermore, information gathered on the use of torpor in
varied habitats can be used to predict the effect of global climate change on
animals in the wild. As global climate change intensifies, one likely result
will be more variable temperature and precipitation patterns in temperate areas.
Thus, understanding the thermoregulatory behaviour and environmental physiology
of animals in the wild is ultimately vital to predicting and addressing the
impact of environmental change on them.
Aims: 1) To quantify thermal biology and torpor use
of A. cristatus and E. argus and how it is affected by resource
and roost availability; 2) To determine the ecological factors influencing roost
site selection by A. cristatus; 3) To compare home range size of A.
cristatus between the arid zone and the eucalypt forests of north-eastern
NSW; 4) To study the diet of A. cristatus through the examination of
faeces samples and observations of nocturnal feeding behaviour.
Hypothesis: 1) The availability of resources,
including food supply and protective roost sites, affects the use of torpor
by Caprimulgids; 2) Roost sites will be selected based on their thermal properties
and the protection they offer from predators; 3) Home range size of A.
cristatus will be greater in the arid zone due to lower prey availability;
4) The diet of A. cristatus will consist of prey captured while foraging
on the ground, in contrast to the prey types captured by aerial foraging
species.
Background:
Torpor has come to be recognized as a
sophisticated adaptation by some endotherms to local environmental conditions
(Lyman et al., 1982; Geiser, 1998; McKechnie & Lovegrove, 2002). Foraging
strategy and the ability to enter torpor both influence the energetics and
behavior of insectivorous Caprimulgids. For the tawny frogmouth, it appears that
torpor is employed at night when energy costs for foraging are likely to exceed
energy gained by food uptake (Körtner & Geiser, 1999). Recently completed
studies on poorwills (Woods, 2002) and whip-poor-wills (Lane, 2004), suggest
that cold weather and the resulting decrease in insect abundance are important
environmental cues correlated with torpor use by these animals. Previous work
found that A. cristatus use torpor (Brigham et al., 2000), even though
they live in thermally moderate conditions, suggesting that torpor may be a
response to food shortages, which are independent of reduced temperatures.
Another resource that is integral to the survival of A. cristatus is the
availability of protective roosts. Torpid birds do not readily respond to
disturbances (Carpenter & Hixon, 1988). For example, poorwills cannot respond
behaviourally to an approaching predator (Bartholomew et al., 1957), thus they
possess no means to avoid predators if discovered. Most true nightjars (Caprimulgidae),
including E. argus, roost and nest on the ground and rely on cryptic
plumage to avoid detection (Wang & Brigham, 1997; Brigham et al., 2000). The
tawny frogmouth carefully selects roosting sites in trees in which the bark
resembles its cryptic coloration to reduce chances of predation (Körtner &
Geiser, 1999). A. cristatus is the only one of the hundred or so species
of Caprimulgidae that roosts in tree cavities (Brigham et al., 1998; Brigham et
al., 2000). Results of field research conducted in winter 2004 in central
Australia show that A. cristatus is equally as likely to roost in rock
crevices as in tree cavities, despite tree cavities being abundant and available
(Doucette, pers. observation). It is unknown why rock crevices are preferred
over tree roosts. Studies have shown predation rates on A. cristatus in a
woodland habitat in north-eastern NSW to be very high (Brigham et al., 1999).
Thus, the availability and quality of protective cavity roosts may be essential
for the survival of A. cristatus, as its only means of avoiding predation
while in torpor. It is also hypothesized that rock crevices may provide thermal
advantages over tree roosts.
The resources available within a specific area, such as those needed for food
gathering, mating, and caring for offspring, generally determine home range
size. Both prey availability and roost sites are more abundant in the Eucalypt
forests of the north-eastern NSW than the sparsely vegetated, dry, arid zone
(Brigham et al. 1998; 1999; pers. observation). The size of home ranges of A.
cristatus in the arid zone determined during winter 2004 were larger than
predicted, in some cases exceeding a 1.5 km radius, and it is expected they
would greatly surpass home range size in the eucalypt forests of NSW.
Most species of Caprimulgiformes are aerial foragers, pursuing flying insects in
sustained flight (Debus, 1994). A study based on observations of foraging
behaviour in a eucalypt forest in north-eastern NSW concluded that A.
cristatus were almost exclusively sally-type foragers and rarely foraged on
the ground (Brigham et al., 1999). However, analysis of faeces (Doucette et al.,
unpubl. data) and stomach contents (Jones, 2004) indicate that a large portion
of their diet consists of ground-dwelling ants and spiders.
Methods:
I will investigate the use of torpor by
A. cristatus and E. argus at Ormiston Gorge, West MacDonnell
Ranges National Park, central Australia, where arid conditions reduce insect
abundance while ambient temperatures remain warm. A. cristatus will be
caught by broadcasting taped A. cristatus calls to lure individuals into
mist nets. E. argus will be caught by flushing birds into mist nets or
spotlighting birds at night and capturing them with use of large hand nets.
Birds will be outfitted with 2.2 g external temperature-sensitive radio
transmitters, attached by using a back-pack-style harness made of elastic
thread, to measure skin temperature (0º to 40º C) for determination of the
degree, depth, frequency and duration of torpor (Brigham et al, 2000). The
transmitters emit pulses based on skin temperature that are recorded on
custom-made data loggers (Körtner & Geiser, 1998). Results will be compared to
the density of prey in the respective area and to ambient temperatures. Pitfall
and light traps will be used to measure insect abundance in different habitats.
Data on roost sites will be collected by tracking radio-tagged A. cristatus
to their roosts each morning, and recording the position of the roost using
a GPS. At the conclusion of the study, parameters of the roosts, including tree
species/ rock type, roost height, altitude, cavity entrance aspect, diameter of
cavity entrance, etc. will be assessed (Brigham et al., 1998). The ambient
temperature inside the roost will be recorded every 10 minutes by placing a
small (1.7 cm) temperature logger (Ibutton) inside the roost cavity. Home range
of A. cristatus will be determined by tracking individual birds for a
minimum of 20 hours over at least 5 nights. The position of the bird will be
recorded on a GPS every 15 minutes. Any faeces naturally produced by the birds
during capture and attachment of the radio-transmitter will be analyzed for diet
determination. Additionally, roost sites will be checked regularly for faeces
samples. To observe foraging behaviour of A. cristatus, self-contained
chemi-luminescent capsules (light-tags) will be affixed to the posterior dorsal
surface of the tail using Supa glue (Brigham et al., 1999). This will allow us
to visually track the bird for a period of from 3 to 12 hours and record
behavioural observations.
Detailed Methods Relevant to Banding: The transmitters are attached to the bird
using a backpack style harness made with elastic thread. The thread breaks
easily if the bird should become caught-up and will rot and fall off within a
few weeks to months. Last year we found that the transmitters typically came off
the bird in 3 to 4 weeks, staying on for as little as 48 hours and as long as 70
days. This creates a problem in that we cannot keep an ongoing record for an
individual bird. When we catch an owlet-nightjar or a spotted nightjar we have
no way of knowing whether we have caught a previously tracked bird again or a
new bird. Both owlet-nightjars and spotted nightjars typically have over-lapping
territories, switch roost sites daily, and two birds (presumably mates) may use
the same roost site on different days. By banding the birds we would be able to
determine if we have recaptured a previously studied bird within a single field
season (6 months) and in successive years. This would allow us to be able to
determine more precisely the density of owlet-nightjars and spotted nightjars in
an area and whether roost sites and territories are reused from year to year.
Additionally, the value of all data collected is greatly increased by knowing
exactly how many individuals were measured and for what length of time.
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