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Bone intake by vultures in Namibia

Authors:
  • Environment & Wildlife Consulting, Namibia
  • Ecosystem Services

Abstract and Figures

The use of bones by vultures was assessed during early 2005 in the O tjiwarongo area in north-central Namibia. Bone fragments were utilized by all species, especially by the African White-backed Vulture Gyps africanus and the Lappet-faced Vulture Torgos tracheliotos. There was an overall increase in bone fragment consumption from May onwards (taken as the beginning of the breeding period). A rough estimate of bone fragment use for all vultures of 2.49 g/vulture (consumption/ total number of vultures observed) and 60.31 g/vulture (consumption/ individuals of vultures observed) was determined. The results suggest that bone fragments should be added as a supplement at vulture restaurants.
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September 2007 Vulture News 57
17
Sum ma ry
Th e use of bon es by vultures was assessed durin g early 2005 in the Otjiwarongo area
in north-cen tral Nam ibia. Bone fragments were utilized by all species, especially by
the African White-backed Vulture Gyps africanus and the Lappet-faced Vulture Torgos
tracheliotos. Th ere was an overall increase in bone fragment consumption from M ay
onwards (taken as the beginn ing of the breed ing period). A rough estimate of bo ne
fragment use for all vultures of 2.49 g/vulture (consum ption/ total number of vultures
observed) and 60.31 g/vulture (consump tion/ ind ividuals o f vultures observed) was
determined. The results suggest that bone fragm ents should be added as a supplem en t
at vultur e restauran ts.
Bone intake by vultures in Namibia
Lothar Me nge , Maria Diekmann, Peter Cunningham
& Dave Joube rt
Introduction
Vultures may develop a bon e deficiency
ca lled osteo d ystr o p h y or me tab olic
bone disease, a disease m ore com monly
associate d wit h h o r se s (Mö nn ig &
Veldman 1989), m ainly due to a lack of
calcium in th e diet (Mun dy et al. 1992,
Richardson et al. 1986). It is assum ed
th at prev ious ly ca r n ivo r e s suc h as
hyaenas broke up the long and big bones
of dead animals with vultures utilising the
resultant bone fragments. T hese bone
fragmen ts would then also be transported
to n ests d uring th e b reeding season and
fed to the chicks. Richard son et al. (1986)
indicate that osteod ystroph y in C ape
Vulture (Gyps coprotheres) chicks declined
from 17% to 2.5% wh en crushed bone
was supplied at a feeding site in a farming
are a wit h few or n o bo n e -cru shin g
predator s. According to Mundy et al.
(1992) bone fragm en ts is a vital source of
calcium for vulture chicks to stren gthen
their bones for flight. In the ab sence of
sufficient suitably sized bones or bone
fra gm ents, vu ltur e n estlings seem to
have a p oorly calcified soft skeleton ,
an d th e win g bones in par ticular can
b en d an d e ven b re ak, res ult in g in
osteodys tro ph y or met a b o li c bo n e
disease (Mundy et al. 1992). Recently
po pulation s of hyaenas and othe r big
carnivore numbers have declined mainly
due to h um an persecution , resulting in
a lack of readily available crush ed bo ne
for vultures.
A study by Dobbs & Benson (1984)
on the calcium requ ire ments of Cape
Vulture ch icks at th e Kransberg colony
in So uth A fr ic a in d ic ate th at bo n e
ab n o r m al it i e s (3. 5 %) a n d c a lci u m
requ irements (< 0.75%) were lo w er
Vulture News 57 September 2007
18
than for other similar studie s (Mundy
& Le dger 1976, Evan s & Pip er 1981)
an d unusu ally low for a car nivo rous
bird. They furtherm ore stated that bon e
ab nor m alities are a n atur al m ortality
factor in Cape Vultur e ch icks an d no t a
result of unnatural calcium deficiency.
C o n su m p t i o n o f b o n e i s we l l
do cumented for the Bearded Vu ltu re
Gypaetus barbatus in South Africa (Huxley
& Nicholson 1963, Newm an 1969, Steyn
1982, Ma clea n 1993, Pickfo rd et al.
1994), but very little is known regar din g
bone use and / or ho w this affects the
bree d ing of Cap e V ulture , Afr ic an
Wh ite-backed Vulture (Gyps africanus)
(e x cep t Ke m p & Kem p 1975 ) a n d
Lappet-faced V ulture (Torgos tracheliotos)
fro m Nam ibia . M acle an (1993) an d
Mund y et al. (1992) sta ted th at b one
fragments are utilized by G. africanus,
m ain ly during th e bree d in g sea so n
and Mund y et al. (199 2) con firm e d
th at they do n ot usu ally su ffer from
metabolic bone disease. Variou s reasons
for th e cata strop h ic co llap se of the
Cape Vulture, a known bone consumer
(Mun dy et al. 1992), in Namib ia have
been cite d (C .J. Brown p ers. comm.,
R.E. Simmons pers. comm., Brown 1985,
Simmons & Bridgeford 1997, Bridgeford
2001, Hengari et al. 2004) although no
wo rk ha s de ter m in ed th e actua l use
of bone or lack thereo f for the species
locally. M un dy et al. (199 2) in clud e
food quality – i.e. “in particular, bones”
[or lack ther eof] – as having a potential
imp act on G . coprotheres. Acc ording
to Benson et al. (2004), Gyps vultures
comp ete with ma mmalian carn ivo re s
and other scavenging bird s for food and
bone and where th ese competitors are
ab sen t such as in farming areas, th ey
consume more bon e. Meat is often scarce
in farm ing areas thus requiring Gyps
vultures to feed more on availab le bone,
which is en er getically sim ilar to mea t
and nutritious, as an altern ative source
of food (Benson et al. 2004).
This pap er investigates the use of bone
fragmen ts supplied at a vulture restaurant
close to the last known remainin g free
ran ging G. coprotheres pop ulation in
Nam ibia. Th e aim of th is stud y was to
determ ine if free-ranging vultures would
eat crushed bone offered and to wh at
extent bone would be utilized. The study
also aim ed to deter m in e wh ether an
increase in b on e co nsump tion occurred
during the breeding season. If it could be
proven that vultures actively sought bone
fragm ents, it would suggest th at b on e
fra gm ent supplem entat ion at vultur e
restaurants is worthwh ile.
Study are a an d methods
Th is stud y was con du cted at a vulture
restaurant situated at REST (R are and
Endanger ed Sp ecies Trust) on th e farm
Uitsig, approxim ately 54 km north-east
of O tjiwaron go and 25 km north-west
of th e Water berg Plateau, in north-
central Namibia. R E ST is dedicated to
September 2007 Vulture News 57
19
the preservation of end an gered species
in Nam ibia with sp ecial em phasis on
res earc h an d lo gistica l support for
the local G . coprotheres po p ula t io n .
Th e general vegetatio n co nsists of an
ecotone between Thornbush Savanna,
mo stly bush encroach ed , and Moun tain
Savan na (Giess 1971), and is domin ated
by Acacia mellifera and Dichrostachys
cinerea trees an d shr ubs. The mean
annual rainfall lies between 450-500 mm
and sum mer and winter tem peratures
ra nge b etwe en 18-38º C an d 5-27º C,
resp ectively (Mend elsoh n et al. 2 002).
Weathered bones were cru shed with
th e resultan t bo ne fragm ents varyin g
from 2-10 cm in length and were classified
as follows: small = ±2 cm, medium = ±6
cm and large = ± 10 cm . 2 kg of b on e
fragments were placed on a clean r ock
slab approxim ately 2-3 m away from the
actual feeding site an d weighed on an
electro Samson Salter Brecknell digital
scale before and after feeding incidences.
Un consum ed b on es were retrieved on
average four d ays after placem en t, so as
not to disrup t other important vulture
observation s, wh ich are p art of RESTs
ongoing activities. Vu ltures feed ing on
the bones were observed between 07h00
and 14h 00 with the use of Bushnell 10x50
binoculars from a hide approximately 20
m from the feeding site. The observations
we re re p eat ed o n 22 occ asio ns (i.e.
feeding inciden ces) between 10 March
and 5 June 2005.
Th e followin g param eters: the mean
num ber of vultures visiting the restauran t,
mean mass of bone fragmen ts consum ed
per obser vation period, mean mass of
bone fragments co nsumed per day per
observation period, mean mass of bo ne
fragments consumed per visiting vulture,
and mea n m a ss of b one fra gm en t s
con sum ed p er bone consuming vulture
(i.e. bird s observed to be swallowing
bone) p e r ob se r vat io n period wer e
comp ared be twe en the non breeding
se a so n (b e fo r e May) a n d b reed ing
season (fro m May onwards [Mundy et
al. 1992]) using the Mann Wh itney test
(Statsoft 1999). A P – value of < 0.05 was
con sidered significan t.
Results
G. coprotheres was only ob served utilising
the bone chips on offer on one occasion
ou t of a total of 36 visits d urin g th e
observation period. G. africanus an d T.
tracheliotos in d ividu als we re obser ved
utilising bone ch ips on 77% (mean of
15.96 [SD= 16.89] individuals) an d 83%
(22.00 [SD = 19.19] ind ividuals) and 41%
(me an of 0.87 [SD= 1.69 ] in divid ua ls)
and 42 % (m e an of 1.33 [S D = 2.2 3 ]
individ uals) of the feed in g occasion s
during the entire period compared to the
breed ing season , respectively.
Sign ifica ntly m ore bone fra gm ents
were consumed by mass, per day an d
per vultur e dur ing th e ob ser vat io n
period between the non-breedin g and
breed ing season s with m ore b one being
con sum ed during the breed ing season
(P<0.05) (Table 1).
Vulture New s 57 September 2007
20
Table 1. The mass of bones consumed, mean nu mber of vultures, m ean mass of
bones consumed per day and per visiting vu lture and the mass of bones consum ed
per b one-consuming vulture at R EST.
Mass of bo nes
co nsum ed p er
ob serv ation
pe riod (g)
Mea n num be r
of vu lture s
vi siting pe r
ob serv ation
pe rio d
Mea n mass
of bo ne s
co nsum ed
pe r day per
ob serv ation
pe riod (g)
Mea n mass
of bo ne s
co nsum ed
pe r visiting
vulture (g)
Mea n mass
of bo ne s
pe r bo ne -
co nsum ing
vulture
Non -bree ding
season
700 466 225 1.65 49 .4
Breed ing
season
1308 356 48 1 10.25 61.1
Mea n 1017 409 358 6.14 55.52
Statistical
significance
(Man n
Wh itney test)
Significant Significant Significant Significant Not significant
Discussion
Th e data sh ow th at vultures utilise b on e
chips if offered and it can be assumed
that they require bon e chips in their diet.
Th e data suggest that this is especially
so dur in g the br eed in g season when
minerals n ece ssary for egg, bon e and
chick development are essential, although
there is no statistical difference between
the amount of bone consumed per bone-
consuming individual between breed ing
and non -breeding season (Table 1). Since
this was a pilot stu dy it is felt that some
of the differ ences ob ser ve d could be
ascrib ed to acclimation b y the vultures
during the non-breeding season when the
procedures were begun . It is suggested
that the stud y b e con tinued for an other
season. G. africanus, which makes up the
bulk of th e vultures obser ved , shows an
increa se in bon e chip utilisatio n (77%
non-breeding and 83% breeding season).
Th is is consisten t with th e fact that G.
africanus are known to breed locally. T his
also confirms pub lished breeding data
(Mun dy et al. 1992).
A factor that could have influenced
the bone consumption of vultures visiting
th e R EST feed in g site is the regula r
av ailability of mea t as suggested by
Benson et al. (2004). Small bones, often
consum ed by mam malian carnivores
elsewh ere, are potentially availab le at a
feedin g site avoided by such competitors
and consumed togeth er with th e meat,
th us n ot ma kin g it ne ce ssary for the
September 2007 Vulture News 57
21
vultures to consume the crushed bon e
on offer.
Breeding is usually between May an d
November in Namib ia (Tarb oton 2001,
Cunningham & Strauss 2004). The lack of
bone fragm ent utilisation by T. tracheliotos
during the study period (March to June)
migh t ind icate that breedin g had not yet
begun in nor th-central Namib ia.
Few deductions could b e m ade for
G. coprotheres due to the few num bers
involved (e.g. a maximum num ber of
five individuals were observed utilising
the restaura nt d uring one observation
pe riod). Six male vultu res have been
visually and genetically identified (D NA
con firmation ) at REST with two m ore
adults known, one of which is suspected
of being a female. Breeding was ob served
at the Waterberg Plateau as recently as
2000 when C. J. Brown (pers. comm .)
observed one breeding pair. During the
past four years juven ile birds have been
seen at REST wh ich indicates breed ing
by at least one pair althou gh these birds
could also have migrated from Botswana
or Sou th Afr ica. T h is last suspected
breed ing pair has not yet been observed
utilising bone fragments.
It is h owever po ssible that all the
vultures could have utilised bon es on
the 2n d or 3rd d ays after initial feed ing
wh e n ob ser vat io n s we r e no t tak in g
pla ce, but still frequented the feedin g
site i.e. roostin g in the vicinity. Bone
con sum ption figures per vu ltures would
certainly be in fluenced b y o ther factors
such as Marab ou Stork (Leptopti los
crumeniferus) (21 obse rvatio ns utilising
bone fragmen ts) and Black-backed Jackal
(Canis mesomelas) (not observed utilising
the bones on offer, but could occu r) or
oth er scavenger s.
The auth ors re c om m end th at, in
further studies, the use of bone fragments
by captive vultures is observed. In this
way, th e population und er observation
co uld be divided in to ex p erimen tal
and con trol groups. Th e stud y ten ds to
confir m the susp icion tha t pr ovid in g
bone fragments to supplement the diet at
vulture restauran ts especially durin g the
breeding season would be prud en t. How
re ad ily bone fra gm ents are availab le
in th e wild and ho w the lack thereof
may influence breedin g success of the
existin g G. coprotheres in the Waterberg
area requires further study.
Acknowledgem e nts
O ur sincer e ap preciation goe s to the
following people for their assistan ce an d
additional in form ation supplied durin g
th is pro ject: Peter Brid ge ford , C hris
Brown , Jörg Diekm an n, Jü rgen Menge
and R ob Simm ons.
Vulture New s 57 September 2007
22
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Keywo rds: Bone consum ption, vulture restaurant, Nam ibia.
Cape Vultur e Gyps coprotheres, White-backed V ulture Gyps
africanus, Lappet-faced V ulture Torgos tracheliotos.
Auth ors’ add resses: Loth ar Menge, P.O . Box 2155 0, W indho ek , Nam ibia;
e-mail address: jgmenge@m web .com .na;
M a ria D ie k ma n n, P.O . Bo x 178 , O tji w a r on g o ;
e-mail add ress: R E ST@principia.edu; P e te r
C u n n in gham , P/ Bag 133 88, Dep ar tm en t of Natu r e
C on se r vation, Po lyt e ch nic o f Nam ib ia , Wind h o ek;
e-mail ad dress: pcunn ingham @po lytech nic.edu.na;
D ave Jou b e rt, P/ Ba g 13388, De p art m e n t o f Na tu r e
C on se r vation, Po lyt e ch nic o f Nam ib ia , Wind h o ek;
e-mail address: djoubert@polytechn ic.ed u.n a
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Article
The natural removal of 89 ungulate carcasses by predators and scavengers was monitored in various wildlife reserves and ranching areas of South Africa. Carnivores responsible for the bone remains were determined. Spotted hyaenas and, to a lesser extent, Brown hyaenas, were the only carnivores that regularly chewed bones. When hyaenas were absent, months of weathering were required before the smaller bones became disarticulated and able to be removed by vultures. The bone-collecting behaviour and related aspects of breeding of two species of griffon vulture were studied at five different nesting colonies in southern Africa—one Cape vulture colony and one White-backed vulture colony in or near wildlife reserves, as well as two Cape vulture colonies and one White-backed vulture colony in ranching land. A total of 2825 bones was found in or below the vulture nests. These bones were categorized and measured. Hyaena-produced bone fragments were found only in the colonies in the wild areas—none of the 387 chicks examined here had osteodystrophy (metabolic bone disease). By contrast, in the ranching areas, vultures collected larger and less fragmented bones. Many Cape vulture chicks had osteodystrophy (130 of 1917 examined), as did two White-backed vulture chicks (of 196 examined). In 1977, artificial feeding stations, ‘restaurants’ where carcass skeletons were crushed, were established for Cape vultures. Since then, the incidence of osteodystrophy has declined from 17% to 2–5% in 1983. It is clear that bone fragments are an essential dietary requirement, providing calcium for correct skeletal growth of griffon vulture chicks.
Article
We compared bones and non-faunal items collected by Cape Vultures at the Blouberg and Kransberg colonies. Bones from the base of the nesting cliffs were on average longer than those from the crops and stomachs of birds. Bones from the Blouberg cliff base were on average shorter than those from the Kransberg. A larger proportion of bones from smaller animals was the reason for this. The smaller size of the crop material was due to a greater proportion of fragmented bone. Fragmentation made bones less identifiable to species. The proportion of fragmented material and the particular skeletal elements discovered at the two sites were very similar and did not influence this size difference. Material from these colonies was, for the most part, smaller than bones collected from other Cape and Whitebacked Vulture colonies in Zimbabwe, Botswana and South Africa. Higher proportions of bones from smaller animals and smaller skeletal elements collected were the reasons for the smaller average size. In wildlife reserves, Gyps vultures compete with large mammalian carnivores and other scavenging birds for food, including bone. Where these competitors are absent or rare (i.e.farming areas), Gyps vultures eat more bone. Small bones (i. e. carpals, tarsals, phalanges, etc.), quickly eaten by spotted hyenas in game reserves, are collected in large numbers by Gyps vultures in farming areas, where competition is reduced. As a food, bone is almost as good nutritionally and energetically as meat. Where meat is scarce (e.g.farming areas), Gyps vultures collect more bone as an alternative food source. In areas of high human density, vultures eat more human-made material. Substitution or confusion of one item for another (e.g.human-made items for bone/food) will occur more regularly as the replacement item becomes more prevalent in the environment. Most of the non-faunal pieces did not resemble bone and were probably not confused for that item. Glass was the most common human-made substance found in vulture crops and stomachs, and rocks the most common overall. Grass and sticks were collected from nestling crops and stomachs but rarely from adults. When food is scarce, vulture nestlings feed on non-food items, particularly nesting material. The increase in collection and eating of bone and non-food items is a result of the shift in Gyps vulture's diet where meat is scarce and alternative foods are sought.
More vulture deaths in Namibia
  • P Bridgeford
Bridgeford, P. 2001. More vulture deaths in Namibia. Vulture News 44: 22-26.
Lappet-faced vulture Torgos tracheliotus breeding in southern Namibia
  • P L Cunningham
  • N Strauss
Cunningham, P.L. & Strauss, N. 2004. Lappet-faced vulture Torgos tracheliotus breeding in southern Namibia. Lanioturdus 37(3/4): 28-29.