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Systematics and Evolution of Kestrels

1.1

Chapter Summary
The family Falconidae constitutes a group of small to medium-sized diurnal
raptors whose monophyly is strongly supported. Kestrels are included in thesubfamily Falconinae. There are at least 13 species that belong to the kestrel
group, but recent genetic studies suggest that the number of kestrel species
might be larger, possibly 16. The paleontological and molecular evidence iscongruent in suggesting an evolutionary radiation of kestrels from theLate Miocene (4.0–9.8 million years ago) through the Early Pleistocene.
However, the geographic area where kestrels originated and dispersed from isunclear.

1.2

Diversiﬁcation of Falcons
The Falconidae is a monophyletic family of diurnal birds of prey that occupy awide variety of ecological niches and geographic regions (White et al., 1994).
Three subfamilies are currently recognised and their validity is supported byboth molecular and morphological data (Grifﬁths, 1999; Grifﬁths et al., 2004;
Fuchs et al., 2012, 2015): (i) Falconinae (falcons, falconets and kestrels),(ii) Herpetotherinae (forest falcons Micrastur sp. and laughing falcon
Herpetotheres cachinnans) and (iii) Polyborinae (caracaras) (Figure 1.1).
Dickinson (2003) has recognised 11 genera and 64 species of Falconidae, but
ﬁgures can vary slightly across authors.
Both the Herpetotherinae and the Polyborinae occur only in the New World,
while the Falconinae (the subfamily to which kestrels belong) are widespread
across both the New and Old World with 46 species, 40 of which belong to thegenus Falco (Fuchs et al., 2015). Molecular genetic estimates of diversiﬁcation
within Falconidae, based on fossil calibration using two Falco ancestors(Pedohierax and Thegornis) and on the analyses of DNA sequences from eight
loci, indicated that the diversiﬁcation started between 22.3 (95% conﬁdence
interval: 19.6–25.6) and 34.2 (95% conﬁdence interval: 26.2–43.2) million
years ago for the Falconidae; between 12.6 (95% conﬁdence interval: 10.6–
14.7) and 19.3 (95% conﬁdence interval: 14.4–24.6) million years ago for the© in this web service Cambridge University Press

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Figure 1.1 The family Falconidae includes three subfamilies: (a) Falconinae (Falco tinnunculus,
photograph by David Costantini); (b) Herpetotherinae (Herpetotheres cachinnans, photographby Andreas Trepte, via Wikimedia Commons); (c) Polyborinae (Caracara cheriway,
photography by www.naturespicsonline.com, via Wikimedia Commons).

Falconinae; and between 5.0 (95% conﬁdence interval: 4.0–6.1) and 7.7 (95%
conﬁdence interval: 5.6–9.8) million years ago for the genus Falco (Figure 1.2;
Fuchs et al., 2015).
Cenizo et al. (2016) proposed that the Lower Eocene Antarctoboenus carlinii, afossil species found on Seymour island (west Antarctica), could represent the mostancient falconiform described so far (Figure 1.3); this discovery would give support toa Neotropical or Austral origin of Falconidae (Ericson et al., 2006; Ericson, 2012;
Fuchs et al., 2015).
The diversiﬁcation of the genus Falco started during a period characterised byincreasing aridity and the spread of open savannahs (Cerling et al., 1997), which might
have favoured the diversiﬁcation of these open-land birds of prey (Cade & Digby,
1982). Molecular estimates of the diversiﬁcation of species belonging to the genus
Falco provided by Fuchs et al. (2015) are in agreement with the paleontologicalevidence, as fossils of several Falco paleospecies date from the Late Miocene to the
Early Pliocene period (e.g. Umanskajaa, 1981; Becker, 1987; Boev, 1999, 2011a,
2011b, 2011c; Li et al., 2014). However, most Falco paleospecies are known only
from fragmentary remains, which make phylogenetic inferences problematic. In conclusion, the recent molecular and paleontological data have supported the statement
made by Cade and Digby (1982):
The Late Miocene or Early Pliocene would seem to have been about the right time, just whenthings were starting to go well for another group of open-country inhabitants, the early hominids. It
is amusing and somehow prophetic to think that falcons and men both derive from the same
evolutionary stimulus – the creation of open grasslands and savannahs with new and unexploited
opportunities for both winged and bipedal hunters … Thus it appears that the association between
men and falcons is deep rooted indeed. What did ‘Lucy’ and her kin (Australopithecus afarensis)
experience when they looked up into the azure sky over the Afar Plains and saw hunting falcons?

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mintoni**ruficollis

*

*

*

*

*
Herpetotherinae

gilvicollis
buckleyi
mirandollei
semitorquatus

*

* *

*

Herpetotheres cachinnans**

Polyborinae

*
* *

*
*
*

*
*

*

*

*
*
*
Microhierax

*

*

*

*

*

*
*

*
*

*
Falconinae
*

*

*

*

*

*
*
*

*

*
*

erythrogenys
melanoleucos
caerulescens
latifrons
fringillarius
Polihierax semitorquatus
Polihierax insignis
severuseleonorae
subbuteo
cuyieri
subbuteoconcolor

hypoleucos
biarmicus
jugger* subniger

*
* *

*

Spiziapteryx circumcincta
Ibycter americanus
Daptrius ater
Milvago chimachima
Phalcoboenus australis
Phalcoboenus carunculatus
Phalcoboenus megalopterus
Phalcoboenus albogulans
Phalcoboenus megalopterus
Milvago chimango
Caracara plancus
Caracara cheriway** longipennis* rufigularis* deiroleucus** chicquera***

plumbeus

M
i
c
r
a
s
t
u
r

*

*
*

biarmicus
cherrugrusticolus
fasciinucha
peregrinus
pelegrinoides
mexicanus
novaezeelandiae
femoralis
berigoracolumbarius
aesalondickinsoni
ardosiaceus
vespertinus

F
a
I
c
o
n
i
n
a
e

*
*
* *
* amurensis*
sparverius
*

Falco

*

zoniventris
alopexrupicoloides
punctatus
araea
newtoni–/0.99** *** * rupicolus
moluccensis
tinnuncujus*
cenchroides** tinnunculus–/0.85**
Oligocene30

naumanni

Miocene
25

20

15

10

5

1 0 MyaFigure 1.2 Chronogram calculated using eight genetic markers and two fossil taxa (Pedohierax

and Thegornis) as calibration points. Asterisks indicate posterior probabilities and maximum
likelihood bootstrap support values higher than 0.95 and 70%, respectively. The different shadesof grey indicate different geological epochs. The different branch colours indicate the two partsof the tree with different diversiﬁcation rates. Reprinted from Fuchs et al. (2015) withpermission from Elsevier.

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Figure 1.3 Geographical range, temporal distribution, and phylogenetic afﬁnities of extant andfossil falconid birds. The temporal distribution of fossil taxa (dagger) is indicated by blacksquares. Grey bars indicate divergence times estimates for the primary lineages within the extant
Falconids according to Fuchs et al. (2015). Reprinted from Cenizo et al. (2016) with permission
from Springer Nature.

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Systematics and Diversiﬁcation of Kestrels

1.3

Systematics and Diversiﬁcation of Kestrels

1.3.1

Morphological and Behavioural Evidences

5

Kestrels belong to the subfamily Falconinae. It is traditionally recognised that at least
13 species belong to the kestrel group, but recent molecular analyses have suggested
that this number of species might be larger (Table 1.1).
Boyce and White (1987) suggested that there are 14 or 15 species of Falco that might
be considered as kestrels. The classic kestrel group includes a single New World species(the American kestrel, F. sparverius) and 12 Old World species. The majority of kestrels(10 of 13 species) have been classiﬁed within the subgenus Tinnunculus (Brown &Amadon, 1968; Cade & Digby, 1982). This subgenus includes those species characterised by a classic kestrel form and brown-rufous plumage colouration. The three
species characterised by grey plumage colouration – the grey kestrel (F. ardosiaceus),
Madagascar banded kestrel (F. zoniventris) and Dickinson’s kestrel (F. dickinsoni) – are
all from Africa and have been placed within the subgenus Dissodectes (Snow, 1978).
This distinction between Tinnunculus and Dissodectes kestrels has also been supportedby the electrophoretic patterns of feather proteins (Olsen et al., 1989).

Table 1.1 List of species that belong or have been suggested to belong to the kestrel group. 1 = Fuchs etal. (2015) have proposed that the subspecies rupicolus may warrant species status based on molecular
genetic analyses. 2 = Brown and Amadon (1968) and Fuchs et al. (2015) suggested that F. amurensis andF. vespertinus might be part of the kestrel group.
Common nameScientiﬁc nameAuthority

Common kestrel

Falco tinnunculus

Linnaeus, 1758

Lesser kestrel

Falco naumanni

Fleischer, 1818

Fox kestrel

Falco alopex

Heuglin, 1861

Seychelles kestrel

Falco araea

Oberholser, 1917

Grey kestrel

Falco ardosiaceus

Vieillot, 1823

Dickinson’s kestrel

Falco dickinsoni

Sclater, 1864

Moluccan or spotted kestrel

Falco moluccensis

Bonaparte, 1850

Madagascar kestrel

Falco newtoni

Gurney, 1863

Mauritius kestrel

Falco punctatus

Temminck, 1821

Greater kestrel

Falco rupicoloides

Smith, 1829

Madagascar banded kestrel

Falco zoniventris

Peters, 1854

Australian nankeen kestrel

Falco cenchroides

Vigors & Horsﬁeld, 1827

American kestrel

Falco sparverius

Linnaeus, 1758

Rock kestrel

Falco rupicolus1

Daudin, 1800

Red-footed falcon

Falco vespertinus2

Linnaeus, 1766

Amur falcon

Falco amurensis© in this web service Cambridge University Press

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Radde, 1863

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There are no clear diagnostic features that deﬁne what a kestrel is and clarify theevolutionary relationships among kestrel species. For example, while many kestrel
species perform hovering for hunting small mammals, this hunting technique appears
to be very uncommon in the grey kestrel, Madagascar kestrel (F. newtoni) or fox kestrel(F. alopex) (Gaymer, 1967; Boyce & White, 1987; Londei, 2002). However, we know
very little about the behaviour and the ecology of kestrels in Africa and Asia, which
limits the reliability of any conclusions. Sexual dimorphism in plumage is also notcommon across all kestrel species. For example, sexual dimorphism is apparent in thecommon kestrel (F. tinnunculus), the lesser kestrel (F. naumanni) and the Australiannankeen kestrel (F. cenchroides), while it is barely evident in other species. As for theplumage colouration in nestlings, while they usually resemble adult females, male andfemale American kestrel nestlings differ in plumage colouration (Village, 1990).
Phylogenetic analyses carried out using data sets of morphological (e.g. plumage
colouration, wing size) and behavioural (e.g. hovering) traits led Boyce and White(1987) to conclude that the fox kestrel might be the ancestor species of the current red
and grey kestrel species, mainly because it does not hover and has negligible age or
colour dimorphism between sexes. This conclusion was also based on the assumptionthat both the lack of hovering and the negligible colour dimorphism are primitive
characters, i.e. closer to the most ancestral kestrel species. This might be plausible if
the ancestor species were a raptor living in forests, where hovering would not havebeen an efﬁcient hunting strategy. However, the fox kestrel can hover (Londei, 2002)
and the phylogenetic value of hovering is unclear. Boyce and White (1987) also
concluded that the American kestrel might be the most recently evolved species ofkestrel because (i) it has strong adult sexual dimorphism; (ii) male and female nestlings
differ in colouration, with each sex resembling their respective adult sex rather than thefemale; and (iii) the species occupies an extensive geographic area with 17 subspecies(14 in Boyce & White, 1987) that have not diverged enough to be considered distinct
species. Finally, the analyses further suggested that the lesser kestrel might share adirect ancestor species with the Amur falcon (F. amurensis) and the red-footed falcon(F. vespertinus).
Reconstruction of phylogenetic trees based on morphological characters may bequestioned because two species might be similar to each other because of convergent
evolution; this occurs when species share a trait that is different from the trait inferredto have been present in their common ancestor. Thus, the same trait has evolved
independently in the two species in order to perform a similar function. Other problems
in using morphological traits may arise with insular species. For example, the negligible sexual dimorphism of the Mauritius kestrel (F. punctatus) might (i) represent theancestral condition, (ii) indicate the loss of dimorphism due to the insular isolation or(iii) suggest that barely dimorphic individuals were those that colonised the islands.
Also, compared to other kestrel species, the Mauritius kestrel has more rounded wings(like those of forest-dwelling raptors), probably because of its adaptations to hunt inforested habitats. Thus, molecular data (e.g. DNA sequences of marker genes) areneeded to elucidate evolutionary relationships because they are less biased by convergent evolution than are morphological traits.

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1.3.2

7

Molecular Evidence
Finer subdivisions within the Tinnunculus kestrels have been contentious for years.
Groombridge et al. (2002) constructed a molecular phylogeny of Tinnunculus kestrels
using the mitochondrial cytochrome b DNA sequence. All analytical approaches usedby Groombridge et al. (2002) produced phylogenetic trees of broadly similar topology,
but with inconsistent positions for the Mauritius kestrel and the greater kestrel (F.
rupicoloides). The molecular analyses provided strong support for a common ancestor
shared between the Madagascar kestrel and the Seychelles kestrel (F. araea). The
molecular data indicated that the kestrels that colonised the Seychelles likely camefrom Madagascar between 0.3 and 1.0 million years ago, probably favoured by thelower sea level due to glaciation (Groombridge et al., 2002). Given this scenario, it has
been suggested that the Aldabran kestrel (F. newtoni aldabranus), a subspecies of the
Madagascar kestrel (Benson, 1967; Benson & Penny, 1971), might be a possible relictof such dispersal (Groombridge et al., 2002). It has also been hypothesised that thecolonisation of Mauritius occurred via Madagascar less than 3 million years ago(Groombridge et al., 2002). The combination of molecular and geological data led tothe hypothesis that the colonisation of Mauritius by kestrels occurred about 1.9–2.6
million years ago, a period characterised by an interruption in the volcanic activity on
Mauritius (Groombridge et al., 2002). As compared to the Madagascar kestrel, both the
Seychelles kestrel and the Mauritius kestrel show morphological adaptations forforest-dwelling. Such adaptations might have evolved on the islands after the dispersion from Madagascar ended (Groombridge et al., 2002). Alternatively, it might be that
individual Madagascar kestrels with a more forest-kestrel form might have been thepioneers of such colonisation.
Molecular data produced by Groombridge et al. (2002) suggest a close afﬁnity
between the common kestrel and the Australian nankeen kestrel, supporting a recent
divergence, probably due to Pleistocene glacial events that pushed the common kestrel
stock southwards from Asia, as previously hypothesised by Boyce and White (1987)
on the basis of morphological traits. Further work based on the mitochondrial cytochrome b gene clustered the red-footed falcon, the Amur falcon, Dickinson’s kestrel
and the American kestrel separately from the kestrel group (Wink & Sauer-Gürth,2004).
More recent molecular analyses made by Fuchs et al. (2015) did not provide support
for the traditional hypothesis proposing (i) the American kestrel as closely related to
the Old World kestrels and (ii) the Madagascar banded kestrel as closely related to thegrey kestrel and Dickinson’s kestrel. Rather, Fuchs et al. (2015) hypothesised (i) the
existence of a group including the Old World kestrels together with the Madagascar
banded kestrel and (ii) that the American kestrel would be more closely related to otherfalcons (e.g. red-footed falcon) than to the Old World kestrels, as previously suggested
by DNA/DNA hybridisation studies (Sibley & Ahlquist, 1990) and by analyses of
mitochondrial cytochrome b DNA sequence (Groombridge et al., 2002). These results
might indicate that either the kestrel group should include more than the 13 speciesclassically recognised or that the position of the American kestrel within the kestrel© in this web service Cambridge University Press

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group might need to be reconsidered. Analyses of molecular genetic markers made by
Fuchs et al. (2015) also suggested that the Amur falcon and the red-footed falcon might
be part of the kestrel group, as previously proposed by Brown and Amadon (1968).
Finally, Fuchs et al. (2015) found that the fox kestrel might not be the sister species of
the greater kestrel as traditionally thought. Rather, both the fox kestrel and the greater
kestrel would form a paraphyletic group (i.e. a group which does not include alldescendants of the same ancestor), with the greater kestrel being sister to all otherkestrels included in the F. tinnunculus group.

1.3.3

Paleontological Evidence
There is scarce information about the fossil history of raptors belonging to the genus
Falco, particularly because of poor preservation of bone remains. Two close relative
species of the common kestrel are the Late Pliocene F. bakalovi, whose remains were
found near the town of Varshets in northwest Bulgaria (Boev, 1999, 2011a, 2011b), andthe Late Miocene F. bulgaricus, whose remains were found near the town ofHadzhidimovo in southwest Bulgaria (Boev, 2011c). As with other Falco fossils,
both F. bakalovi and F. bulgaricus have been described on the basis of a limited
number of isolated bones, which makes their description and systematic classiﬁcation
problematic. One of the best-preserved specimens of a fossil falconid so far recovered
has been classiﬁed as F. hezhengensis (Figure 1.4) by Li et al. (2014).
This falconid species was found in the Late Miocene deposits of Linxia Basinin north-western China. The analysis of 66 osteological characters led the authors
to place this new Falco paleospecies as the sister group of the common kestrel andthe greater kestrel (Li et al., 2014). This new paleospecies has also provided
paleontological evidence in favour of an earlier divergence of kestrels from theperegrine falcon (F. peregrinus) in the Late Miocene. Although the paleontological evidence is congruent with the molecular data in suggesting a radiation of
kestrels from the Late Miocene through the Early Pleistocene, the timing andlocation of the radiation are still ambiguous because of the paucity of wellpreserved fossils over a wide geographic area. It has been suggested that thepresence of the majority of kestrel taxa on the African continent could be evidence
for an African origin of the kestrel group. However, the absence of a PrePleistocene kestrel fossil record from Africa has meant that assumptions of kestrel
divergence within and from the African continent have not been easy to conﬁrm.
Li et al. (2014) suggested that F. hezhengensis might represent an early dispersal
event after the origin of kestrels occurred or, alternatively, it might be interpreted
as evidence in favour of an Eurasian origin of kestrels.

1.4

Diversiﬁcation and Geographic Distribution of Kestrels
The kestrel group is truly cosmopolitan, occupying wide geographical areas anddifferent environments. Most kestrels are open-land predators, and avoid deserts,© in this web service Cambridge University Press

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Figure 1.4 Comparison of the skull and distal tibiotarsus of the holotype Falco hezhengensis withother Falconidae species: (a) F. tinnuculus (USNM 610371); (b, e) F. hezhengensis (IVPP
V14586); (c) Polihierax insignis (USNM 490664); (d, f) Microhierax erythrogenys (USNM
613695); (g) P. semitorquatus (USNM 621024); (h) F. rupicoloides (USNM 430626); (i)Spiziapteryx circumcincta (USNM 319445); (j) Micrastur ruﬁcollis (USNM 621387); (k)Herpetotheres cachinnans (USNM 346714); and (l) Caracara plancus (USNM 614583). The
vertical arrows indicate the tomial notch on the premaxilla that is present in Falconinae; thehorizontal arrows indicate a third opening into the extensor groove located medial to thesupratendinal bridge that is present in Falconinae and Polyborinae but not in Herpetotherinae.
Anatomical abbreviations: cqot, cotyla quadrati otici; cqsq, cotyla quadrati squamosa; dep,
depression; ma, mandible; na, naris; ntm, notched tomial margin; prc, processus coronoideus;
prlma, processus lateralis mandibulae; probq, processus orbitalis quadrati; protq, processus
oticus quadrati; sb, septal bar. Reprinted from Li et al. (2014) with permission from theAmerican Ornithological Society.

dense forests and the Arctic–Antarctic poles. Some kestrel species show large geographic variation in both morphology and genetics, which led to the recognition ofseveral subspecies. The higher number of species and subspecies near the equator
might be due to greater speciation favoured by a more sedentary style of local birds ascompared to kestrels at higher latitudes, whose migratory habits limit isolation among
populations. There are, however, a number of factors that have likely contributed to thediversiﬁcation of kestrels, such as physical barriers and geographic isolation (Village,1990). We currently lack studies that elucidate the roles that different barriers had in
driving the diversiﬁcation of kestrels.

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Figure 1.5 Distribution map of the common kestrel (Falco tinnunculus). From BirdLife

International and Handbook of the Birds of the World (2017). Reproduced with permission from
BirdLife International. A black and white version of this ﬁgure will appear in some formats. For
the colour version, refer to the plate section.

The common (also known as Eurasian) kestrel is widespread across Europe, Africa
and Asia (Figure 1.5), but sporadic observations of free-ranging kestrels have beenrecorded in the New World (Campbell, 1985; Pranty et al., 2004).
It is classically recognised that there are 11 subspecies of common kestrel (Village,1990). However, recent molecular analyses led Fuchs et al. (2015) to propose that thesubspecies rupicolus (Table 1.1) may warrant species status (IOC World Bird List: Gill
& Donsker, 2018). The species or subspecies rupicolus (illustrated in Figure 1.6b)occurs from north-western Angola and southern Democratic Republic of Congo tosouthern Tanzania and South Africa. The subspecies belonging to the tinnunculus
group differ in colouration, body size and distribution (Figures 1.6a,b). The nominal
subspecies tinnunculus breeds from Europe and north Africa east to Siberia, Bhutan
and western China. It is partially migratory, wintering in the regions from south tocentral Africa, India and south-eastern Asia (Figure 1.6a; Village, 1990; BirdLife
International and Handbook of the Birds of the World, 2017). Four subspecies areendemic to the Atlantic islands of Macaronesia. The subspecies neglectus inhabits thenorth Cape Verde islands and the subspecies alexandri inhabits the south-east Cape© in this web service Cambridge University Press

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