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Why are female raptors bigger than males?

Why are female raptors bigger
In most birds and mammals male–male competition has led to males being larger
than females (‘Normal’ Sexual Dimorphism, NSD). However, in birds of prey the
opposite is true; most exhibit Reversed Sexual Dimorphism (RSD) with females
larger than males. Over 20 hypotheses have been proposed and debated in the
literature to explain RSD in raptors (Krüger 2005). In his analysis, Krüger said
these hypotheses fit into three broad theories: niche partitioning – sexes diverge in
size to reduce intersexual competition for prey; role differentiation – females
become larger to protect and efficiently incubate eggs, and/or males become
smaller for more efficient foraging or territory defence; and behavioural – larger
females dominate males (Figure 12.1), aid in the maintenance of the pair bond and
increase male food provisioning or larger females compete more effectively with
other females to win males. Krüger collected data on 237 Accipitridae (hawks), 61
Falconidae (falcons) and 212 Strigidae (owls), and determined a dimorphism index
from the wing length of males (mm) divided by wing length of females. He
performed comparative analyses, using both cross-taxa data and phylogenetically
independent contrasts, to investigate potential correlates of RSD. Using a set of
explanatory variables, covering morphology, life history and ecology, he analysed
26 predictor values for hawks and falcons and 22 for owls, with RSD as the
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218 Australian High Country Raptors
dependent variable. He found no evidence linking RSD to egg formation, egg size
or females competing for males (sexual selection). He did not discuss an additional
problem with the female–female competition hypothesis – males are often
observed competing for females, for example in hooting owls (Olsen 2011a) but
there is no spike in female–female conflict during the pre-breeding or breeding
season, as often seen with territorial males or territorial pairs. For example, when
easily observed migratory species like Black Kites arrive in Spain from Africa,
there is no spike in visible female–female fighting (Fabrizio Sergio, pers. comm.).
Instead, Krüger found evidence that RSD might have developed for hunting
large and agile prey, providing support for the intersexual-competition hypotheses
– sexes have evolved different sizes to reduce competition for prey, and the small
male hypotheses – males became smaller to be more efficient foragers. Krüger found
in his analysis of owls that Australia held the two extremes: Sooty Owl exhibits the
most RSD and Rufous Owls the least (because the Rufous Owl, as with the Powerful
Owl exhibits NSD, with males larger than females). He did not discuss this anomaly.
Costs and benefits of hooked beaks and curved talons
Raptors differ from other birds in having hooked beaks and deeply curved talons.
A factor never considered in previous studies of RSD is the cost and benefits of
Figure 12.1 Female Wedge-tailed Eagles are larger than males and may control food from them (Mark
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12 – Why are female raptors bigger than males? 219
hooked beaks and curved talons. By contrast, a factor that is commonly
emphasised in studies of RSD is the importance of ‘production feeding’ of female
raptors by males in the early breeding cycle (Newton 1986; Balgooyen 1976) and
the male’s role in feeding the female and nestlings early in the nestling period.
However, male corvids such as the Common Raven also feed females before egg
laying, and male ravens feed females and nestlings early in the nestling period
(Ratcliffe 1997). Male ravens are larger than females, so what makes raptors
different? Compared with similar-sized birds like ravens with straighter beaks and
weaker talons, the hooked beaks and deeply curved talons of raptors offer several
advantages, including the abilities to:
1 kill comparatively large prey;
2 dismantle large prey; and
3 carry relatively large prey balanced under the raptor instead of in the beak.
For example, male Southern Boobooks weighing ~270g routinely kill and carry
prey such as adult Crimson Rosellas weighing ~135g. In contrast, slightly heavier
(than a Boobook) Australian Magpies or similar-weight Pied Currawongs would
have trouble killing, dismantling and carrying such prey to their nestlings.
Vertebrate prey is rare in the breeding diets of magpies and currawongs compared
with the breeding diets of similar-sized Boobooks, Australian Hobbies or Collared
However, the hooked beaks and curved talons that characterise raptors have
certain disadvantages. These include:
1 Large prey, such as birds or mammals, are often rarer in a bird’s home range
than are smaller taxa such as insects or spiders (Eltonian Pyramid; Krebs
2008), so raptors need to travel further to find vertebrate prey, and therefore
use more energy to find vertebrate prey compared to finding invertebrate prey
(Storer 1966; Reynolds 1972; Mendelsohn 1986).
2 Meat spoils quickly, so it is difficult to store for long periods in hot weather.
3 Because raptors travel greater distances to find larger prey, they leave nestlings
and eggs unprotected from enemies and inclement weather for longer periods
compared to birds that forage closer to the nest.
4 Having curved beaks and talons, raptors can only catch and carry one prey
item at a time, so are energetically inefficient when capturing and carrying
smaller invertebrate prey. If birds with straighter beaks, such as Eurasian
Blackbirds, magpies or currawongs find an abundant source of crickets, spiders
or worms, they can load several individuals into their beaks and carry these to
the nest, as can Puffins Fratercula arctica (Figure 12.2). In contrast, raptors can
only catch flying prey in their feet (not in their beaks) and carry one terrestrial
prey such as a cricket, spider or worm to the nest. For example, breeding Little
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220 Australian High Country Raptors
Owls in Eurasia take many worms (König and Weick 2008), but they only carry
one worm at a time to their nest (Iñigo Zuberogoitia, pers. comm.). Herons
(Ardea spp.) might be able to bring several fish at a time to nestlings, but
Ospreys usually carry only one. The only published exception I could find was
a report by Ellis and Nelson (2010) of Bald Eagles grabbing numbers of fish
fingerlings north of the Queen Charlotte Islands in Canada, behaviour that the
authors considered unique and uncharacteristic.
In Australia, all four Ninox owls, the Southern Boobook (Figure 12.3), Rufous
Owl, Powerful Owl and Barking Owl, take flying or arboreal insects near their
nests, and carry these invertebrates one at a time to nestlings; they travel
further to capture and carry vertebrate prey (Hollands 2008; Stanton 2011;
Olsen 2011a). This sets them apart from other owls weighing 300+g around
the world, none of which are known to regularly take aerial insects and deliver
these to the nestlings (Jeffrey Marks and Iñigo Zuberogoitia, pers. comm.). It
may be that curved beaks and talons incur benefits to insectivorous owls,
Figure 12.2 Unlike raptors, birds without hooked beaks, such as the Puffin, routinely carry several small
items in their beaks.
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12 – Why are female raptors bigger than males? 221
allowing them to easily dismantle hard-shelled invertebrates such as beetles,
but incur costs when transporting such prey, so owls hunt them close to the
nest and do not transport them long distances.
5 Curved beaks and talons allow raptors to capture more vertebrate prey but,
unlike invertebrate prey, larger prey species such as birds or mammals
tactically use cover to evade capture. This puts the pursuing raptor at
increased risk of collision or injury. A raptor with even a slightly injured
wing caused by hitting a branch or colliding with flocking birds will be
unable to forage and will die. In contrast to the claim that female raptors
need to avoid collision because of developing eggs (Walter 1979), males in
particular need to reduce the risk of collision because they hunt more
vertebrates than do females. Males need to reduce the number of dangerous
hunting forays (Olsen 1989).
The differences in male–female roles in breeding raptors reflect at least three
trade-offs related to costs and benefits of hooked beaks and curved talons:
1 Energetics – foraging for one large prey item at a time distant from the nest, v.
catching smaller prey items close to the nest.
2 Food storage – buffer against irregular food (days with no food) and
protecting the nest contents by remaining inactive near the nest v. searching
for irregular food sources away from the nest.
Figure 12.3 Fledgling Southern Boobook learning to hunt in the eucalypt canopy.
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222 Australian High Country Raptors
3 Risk to adults from collision, other injury, or predation – pursuing larger,
dangerous prey v. pursuing smaller, safer invertebrate prey.
These costs and benefits may drive the direction and degree of RSD in raptors.
Energetics (efficient foraging)
There is considerable evidence to support the prediction that smaller males in
some species are more energy-efficient foragers than their larger female partners
(Balgooyen 1976; Mendelsohn 1986; Storer 1966; Reynolds 1972). For example,
Mosher and Matray (1974) examined the energy requirements of Broad-winged
Hawks during the nesting season to test the hypothesis that a pair with RSD used
less energy than a hypothetical monomorphic pair. They found no significant
difference between male and female metabolic rates per gram for captive birds, that
is, heavier birds – females at the same level of activity – used more energy in direct
proportion to their weight. Because flight energy was estimated to be ~10 times
more expensive than resting energy, the role division between male and female
delivered ~20% savings on energy use during the breeding season when males were
active for about four hours per day and females remained relatively inactive.
These energetic savings were part of the early models explaining RSD proposed
by Storer (1966) and Reynolds (1972), but these authors stressed niche separation
with male accipiters taking, on average, smaller, more abundant prey than females
did (Eltonian Pyramid again). This seems to hold for some other raptors, for
example Eurasian Sparrowhawks Accipiter nisus (Newton 1986), but not all. For
example, Balgooyen (1976) found no difference in size of prey taken by male and
female American Kestrels, and his re-analysis of Storer’s data showed no difference
in the size of prey taken by male and female accipiters, as Storer had claimed.
Sunde et al. (2003) showed that breeding male and female Tawny Owls also took
similar-sized prey, but males travelled further for food than females did, suggesting
that selection was for males to be lightweight, energetically efficient foragers rather
than specialists in smaller prey. There are many examples of this. Male and female
Peregrines hunt the same small seabird on Langara Island, Canada, the crepuscular
and nocturnal Ancient Murrelet (Nelson 1977). Furthermore, in certain raptor
species including Southern Boobooks, Mississippi Kites and American Kestrels,
males take larger vertebrate prey further from the nest and females hunt smaller
prey close to the nest (Balgooyen 1989; Glinski and Ohmart 1983; Botehlo et al.
1993; Bader and Bednarz 2011; Liébana et. al. 2009; Olsen 2011a, 2013). Foraging
patterns in these raptors represent a trade-off between the energetics argument and
nest defence argument, linked to costs and benefits of curved beaks and talons,
rather than the niche separation model (Figure 12.4).
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12 – Why are female raptors bigger than males? 223
Buffer against irregular food and protecting the nest contents v.
searching for irregular food sources away from the nest
A raptor’s foraging patterns reflect other trade-offs, for example, females buffering
against irregular food supply – days without food – by remaining inactive,
protecting the nest, rather than searching for irregular food sources. Several
studies support this proposition (Balgooyen 1976; Newton 1979; Mendelsohn
1986). If meat spoils quickly, and males cannot add weight to their bodies without
increasing their risk of collision and decreasing foraging efficiency, it is better that
females rather than males store that energy as fat. Females can also buffer against
food shortage by hunting more. Researchers develop experiments to test such
hypotheses around food shortage, by artificially increasing the number of nestlings
(adding chicks from one nest to another), or decreasing brood size (removing
chicks from a brood so adults do not have to hunt for them). Female flight time
increased when broods were increased in Eurasian Kestrels, and female hunting
ceased almost completely in reduced broods (Dijkstra et al. 1990). Female
American Kestrels spent more time hunting when broods were enlarged in a poor
Figure 12.4 In Southern Boobooks, as with most raptors, the smaller male feeds his larger mate.
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224 Australian High Country Raptors
vole year, but there was no clear pattern in a year of higher vole abundance (Gard
and Bird 1990). In one of the most elegant studies, four nests of Great Horned Owls
that originally held two young each were manipulated during high densities of
cyclic Snowshoe Hare Lepus americanus populations in south-western Yukon,
Canada (Rohner and Smith 1996). Broods of two nestlings older than 35 days were
temporarily enlarged by one young, and then by two young. Radio-tagged parent
females with enlarged broods did not respond to the addition of one young, but did
respond to the addition of two young by moving further from their nests at night,
apparently increasing their hunting effort. When extra food was supplied to the
broods enlarged by two, the females’ behaviour returned to normal, and they
stayed closer to the nest and guarded the young. Massemin et al. (2000) found that
smaller female Eurasian Kestrels in low vole years fledged more young than did
larger females because smaller females hunted more.
There are also examples where females responded by not hunting. Female
Ospreys in pairs with artificially increased broods did not hunt, but these
sedentary females lost three times as much weight as their actively foraging male
partners (Poole 1989). The females probably did not hunt because of the risk of nest
predation by species such as Bald Eagles (Alan Poole, pers. comm.). Olsen and
Tucker (2003) and Olsen (2011a) argued that field researchers may change female
raptor behaviour by repeated visits to a nest, for example, to measure nestlings, so
females become more reluctant to leave broods unprotected and refuse to hunt
because of perceived risk of predation.
Risk to adults from collision, other injury, or predation
As discussed previously, hunting raptors are at risk of collision. Though owls such
as Great Grey Owls Strix nebulosa and Short-eared Owls Asio flammeus take
similar prey (mainly rodents), the forest-dwelling Great Gray Owl is far more
dimorphic than the grassland-dwelling Short-eared Owl (König and Weick 2008).
This may be both because the male Great Gray Owl risks collision while dodging
between trees to search for and capture prey and due to the energetic costs of
carrying prey around and between trees. Australia’s two forest-dwelling Tyto owls,
the Masked and Sooty Owls, are perhaps the most dimorphic owls in the world
(Higgins 1999; Krüger 2005), but the grassland-dwelling Barn Owl and Grass Owl
are among the least dimorphic. As expected, swift-flying falcons or woodland
accipiters end up in rehabilitation centres out of proportion to their numbers in the
wild (Olsen 1989).
Ninox owls with ‘normal’ sexual dimorphism
Unlike other owls, Rufous, Barking and Powerful Owls have ‘Normal’ Sexual
Dimorphism (NSD). These owls take abundant prey in the forest canopy at two
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12 – Why are female raptors bigger than males? 225
size extremes in relation to their bodyweight: 1) heavy prey – large and medium-
sized, slow-moving, possums and gliders, and fruit bats, roosting birds; and 2) light
prey – flying and arboreal invertebrates. Rufous, Barking and Powerful Owls
seldom hunt the more intermediate-sized terrestrial mammals (Hollands 2008;
Stanton 2011; Olsen 2011a,b). In contrast to Australian Tyto species, Australian
Ninox do not have asymmetrical hearing so they are less effective at exploiting prey
on the dark forest floor. Ninox do listen for prey, but they hunt mainly by sight.
Females often hunt abundant insect prey near the nest; males often hunt arboreal
mammals away from the nest.
In most other raptor species studied, males arriving with prey either give this
prey to females, which feed or cache it, or males cache it themselves (Cade 1982;
Olsen 2011a,b). In contrast, male Powerful Owls often roost on prey for the day
(Figure 12.5) (McNabb 1996); females roost on prey after males relinquish it. This
Figure 12.5 Ninox species such as Powerful Owls have males larger than females and, unlike species with
RSD, they roost on prey for the day.
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226 Australian High Country Raptors
could be read as an ‘honest signal’ (Simmons 1988), as well as being a way for larger
males to cut their energetic costs and their risk of accidents by cutting the number
of hunting forays (Olsen et al. 2013c). Males, then, are the main source of food
storage in the Powerful, Rufous and Barking Owl pairs.
Relative abundances of arboreal marsupials and NSD in
Powerful Owls
Small animals are usually more abundant than large animals (Newton 1998; Krebs
2008) and ecologists can predict abundance of an animal from its weight. For
North American mammals, Krebs calculated a density of 60 individuals/km2 for
squirrels Tamiasciurus sp. (200g) compared with only 0.5 individual/km2 for Elk
Cervus elaphus (250kg) (Krebs 2008). However, it is not clear whether this
relationship holds for arboreal marsupials, the main prey of Powerful Owls in
eucalypt forests in south-eastern Australia. Could the relative abundance of their
main prey explain NSD in some Ninox owls? Powerful Owls select large arboreal
marsupials such as Common Ringtail Possum Pseudocheirus peregrinus, Common
Brushtail Possum Trichosurus vulpecula (Figure 12.6) and Greater Glider
Petauroides volans as their main prey (Fleay 1968) more often than they select
smaller arboreal marsupials such as Squirrel Glider Petaurus norfolcensis, Yellow-
bellied Glider P. australis, Sugar Glider P. breviceps and Feathertail Glider
Acrobates pygmaeus as their principal food source (see e.g. Kavanagh 2002a). If
these smaller arboreal marsupials are more abundant than larger ones, Powerful
Owls are taking proportionally fewer of them compared to larger arboreal prey in
the forest. However, if the opposite is true, i.e. larger arboreal marsupials are more
abundant than smaller ones (cf. the normal Eltonian Pyramid), Powerful Owl prey
selection is simply proportional to populations. This is suggested by several
researchers, including Seebeck (1976), Cooke et al. (2006), Tilley (1982) and
Lavazanian et al. (1994).
Common procedures for assessing the abundance of arboreal marsupials
involve systematic spotlighting, transects, and/or listening for calls (Lindenmayer
et al. 1999; Cooke et al. 2006; Wintle et al. 2005). Spotlight data of Cooke et al.
(2006) and Tilley (1982) showed that larger arboreal marsupials were more
abundant than smaller ones in the areas where they studied Powerful Owls. Table
12.1 contains published data from Kavanagh and Bamkin (1995), Bennett et al.
(1991), Ward (2000), Tilley (1982), James (1980), Lindenmayer et al. (1999), and
unpublished data from spotlighting counts conducted in Namadgi National Park
and Tidbinbilla Nature Reserve by the Australian Capital Territory Parks and
Wildlife Service from 1974 to 2009 (Murray Evans, pers. comm.) of counts of
smaller arboreal marsupials (averaging 10g to 600g – Sugar Gliders, Feathertail
Gliders, Yellow-bellied Gliders) versus larger arboreal marsupial mammals
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12 – Why are female raptors bigger than males? 227
(averaging 600g to 4000g – Common Ringtail Possums, Greater Gliders,
Common Brushtail Possums, Mountain Brushtail Possums Trichosurus caninus) in
south-eastern Australian forests.
Larger arboreal mammals were detected more often than smaller ones, with
the difference between the frequency of small versus large highly significant
(χ2=1822.5; P < 0.0001). Arboreal marsupials such as Common Ringtail Possums,
Brushtail Possums and Greater Gliders were more common than smaller species
such as Sugar Gliders (Figure 12.7) and Feathertail Gliders, and more common in
the diets of Powerful Owls. This suggests that Powerful Owls are indeed taking
larger arboreal marsupials in proportion to their abundance, though this needs
further study.
Compared to most owls, Powerful Owls have proportionally small heads, as do
forest-dwelling accipiters such as goshawks and sparrowhawks. This may relate to
Figure 12.6 Common Brushtail Possums are at the heavy end of the spectrum of marsupials taken by
Powerful Owls, but owls often take juveniles after they leave the pouch and ride on the mother’s back.
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228 Australian High Country Raptors
the Powerful Owl’s accipiter-like pursuit of quarry in the forest canopy, rather than
ground prey. With small heads, Powerful Owls lack the spacing and asymmetrical
ears found in Masked and Sooty Owls with broad heads that allow triangulation of
sounds on terrestrial rodents such as rats, and marsupials such as bandicoots.
Though Common Ringtail Possums (Figure 12.8) are the most common prey
reported in many dietary studies of Powerful Owls, one would assume that
breeding pairs cannot survive solely on Ringtails. For example, if Common
Ringtail Possums are an optimal prey size for both Powerful Owls and Sooty Owls,
Powerful Owls would have to take prey the size of Ringtail Possums and larger.
Sooty Owls, with RSD, could exploit prey the size of Ringtail Possums and smaller
because they can take terrestrial mammals which probably fit an Eltonian Pyramid
in abundance. It is interesting to speculate about why Australia has no quick-
moving arboreal rodent species such as squirrels Sciurus. Perhaps eucalypt-feeders
are comparatively slow moving because of problems with digestion, and slow-
moving diurnals would be easy prey for diurnal raptors.
Table 12.1 Published spotlight counts for eight species of arboreal marsupials in seven studies
Approximate mass of each species based on Van Dyck and Strahan (2008) and Menkhorst and Knight (2001).
Species and
approximate mass
et al.
et al. 1999 Total
n = 11
n = 87
0 0 0 0 10%
n = 4
n = 102
n = 12
n = 477
n = 3
n = 126
n = 6
n = 182
n = 10
n = 816
Greater Glider
n = 70
n = 433
n = 37
0% 22.6%
n = 12
n = 313
n = 15
n = 880
900 g
n = 13
n = 294
n = 19
n = 442
n = 16
n = 133
n = 11
n = 928
n = 67
n = 20
0 0 0 0 0 2.8%
n = 87
Squirrel Glider
n = 21
0 0 0 0 2.5%
n = 1
n = 22
Sugar Glider
125 g
n = 110
n = 59
n = 1
n = 13
n = 17
n = 19
n = 3
n = 222
n = 1
n = 13
n = 1
n = 1
n = 2
n = 4
n = 3
n = 25
Total no. 284 1404 61 582 53 651 47 3082
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12 – Why are female raptors bigger than males? 229
Figure 12.7 Sugar Glider.
Figure 12.8 Common Ringtail Possum (Julian Robinson).
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