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Studies in Avian Biology No. 16:32-40, 1994.
A PHOTOGRAPHIC AND BEHAVIORAL GUIDE TO AGING
NESTLING NORTHERN GOSHAWKS
CLINT W. BOAL
Abstract. Observations of behavioral and morphological development of nestling Northern Goshawks
(Accipiter gentilis) were collected during three nesting seasons at 20 goshawk nests in northern Arizona.
A photographic record of a single nestling goshawk's development was made. I combined descriptions
of age-specific behaviors exhibited by nestlings with descriptions and photographs of nestling mor-
phological development to construct an aging guide for nestling Northern Goshawks. Adult goshawk
behaviors that relate to nestling age are also provided.
Key Words: Accipiter gentilis; ageguide; nestling development; nestling behavior; Northern Goshawk.
Accurate estimates of the ages of nestling rap-
tors are important for scheduling banding (Fyfe
and Olendorff 1976) and assessing productivity
(Moritsch 1983a, b; Steenhof 1987, Young and
Kochert 1987). Photographic guides to morpho-
logical changes that occur as nestlings age have
been developed for Prairie Falcons (Falco mex-
icanus), Red-tailed Hawks (Buteo jamaicensis),
and Ferruginous Hawks (B. regalis) (Moritsch
1983a, b; 1985), but are unavailable for most
Because the behavior of young raptors is cor-
related with age (e.g., Ellis 1979), descriptions of
~e-specific behaviors coupled with pictorial and
verbal descriptions of morphological changes
provide a more robust guide to aging nestling
raptors. In addition, the behavior of the adult
female changes with the age of nestlings and may
be useful in evaluating nestling ages.
Schnell (I958) provided a written description
of the development of nestling Northern Gos-
hawks (Accipiter gentilis) at a single nest. Obser-
vations of nestling development from several
nests may provide a better understanding of the
age ranges over which behavioral ontogeny oc-
curs. I provide a photographic and behavioral
guide to estimating the age of nestling Northern
STUDY AREA AND METHODS
The study was conducted on the North Kaibab Rang-
er District (NKRD), Kaibab National Forest, Coconi-
no County, Arizona. The NKRD is an area of ap-
proximately 259,000 ha located on the Kaibab Plateau
in northern Arizona. A detailed description of the study
area is provided in Boal and Mannan (this volume).
Nest observations were conducted at 20 Northern
Goshawk nests on the NKRD during the nesting sea-
sons of 1990, 1991, and 1992. Information on the
morphological and behavioral development of nestling
goshawks was collected during 1539 hours of obser-
= 76.8 hour/nest :t 19.3 [SD». These ob-
servations were made from blinds located on the ground
or in trees a mean distance of 53 m (:t 17.6) from the
nest trees. Observation periods began in the afternoon
and continued until sundown, resuming at dawn until
the time of initiation the previous day.
Photographs used in this guide are of the largest of
3 nestlings in a nest that was not part of my study, but
was being used in a concurrent telemetry study (Bright-
Smith and Mannan, this volume). I estimated the date
of hatching as 20 June, based on behaviors exhibited
by the nestlings and the adult female. I took photo-
graphs at 5-day intervals, starting with 5 days of age
and continuing until 20 days of age.
At 18 days the adult female disappeared from the
nesting area. Radio telemetry indicated the adult male
goshawk remained in the nest area during the following
TABLE I. GLOSSARY OF TERMS USED IN TEXT AND
The area of the invisible ear open-
ing just posterior to the eye
A young raptor that has left the
nest but not the nest tree
Small contour feathers of the
wings and tail
The top of the head
When a young bird leaves the nest
tree of its own volition for the
The back of the head below the
occipital portion of the skull
A growing feather that is encased
The outer flight feathers of the
Rapid movement and readjust-
ment of the head while visually
focusing on an object
Primary and secondary wing
Feathers located in the shoulder
Inner flight feathers of the wing
Wax-like keratinous material that
encases and protects newly de-
The feet and claws of a hawk
PHOTOGRAPHIC GUIDE TO AGING NESTLING GOSHA WKS-Boal 33
days, but nest observations revealed that he did not
care for the nestlings. At 20 days of age, the largest
nestling, a male, attacked his siblings. He killed and
cannibalized one sibling and forced the other to leave
the nest and fall to its death (Boal and Bacorn, in press).
The surviving nestling was removed from the nest and
transferred to a wildlife rehabilitator for care. A pho-
tographic record was continued during captivity at 2-day
intervals from 25 days of age to 39 days of age. Pho-
tographs of this bird were combined with the obser-
vational data from the other nests to provide this guide.
Table I contains a glossary defining morphological
and behavioral terms used in describing ages of nest-
When a goshawk nest area is entered by hu-
mans during the first 5 days following hatching,
adult females show a strong tendency to remain
on their nests. Afterwards, females typically flush
from nests, perch in nearby trees while vocaliz-
ing, and possibly make low passes at intruders.
Adult male goshawks are rarely seen in the nest
area except during food deliveries. After deliv-
ering food to the female away from the nest,
males will often visit the nest briefly during the
first 10 days following hatching.
It is difficult to determine the presence of nest-
lings prior to approximately 4 days old. At about
4 days of age nestlings begin attempting to def-
ecate over the nest rim, and their presence is
evidenced by minute specks of white excrement
on the nest rim but not on the ground below the
nest (Fig. 1). At about 9 days white-wash globs
will be present on the branches and the ground
below the nest (Fig. 2).
Nestlings remain totally white in their first na-
tal down until 14-17 days when the gray second
natal down and flight pin feathers begin to appear
(Fig. 3). Scapular and covert feathers start emerg-
ing between 19 and 22 days (Fig. 4). Rapid and
noticeable feather development begins at 24-26
days. At this age, scapular and covert feathers
are visible, and the auricular area is covered with
small black feathers (Fig. 5). At 28-30 days, dark
feathers appear along the nape, but the crown is
still downy. Feathering is also apparent along the
FIGURE I (top left). Goshawks at 4-7 days post-hatching. Morphological Characteristics: Nestlings are small
(approximately 13 cm long) and covered in white natal down. Behavioral Characteristics: Nestlings lie prostrate
and usually out of view below the nest rim. Occasionally the tops of their heads may be seen, especially when
the female is feeding them. Nestlings are poorly coordinated and move by scooting motions with the body in
contact with the substrate. Nestlings may give whistle-like beg calls during feeding. Nestling hawks are able to
excrete over the nest rim starting at 4-5 days old. Small dime-sized specks of white excrement may be visible
on the sticks of the nest or against the tree trunk, but usually not on the ground. The adult female is almost
continuously present and always broods the nestlings at night. She tends to remain motionless on the nest rather
than flush and give the alarm call when the nest is approached.
FIGURE 2 (bottom left). Goshawk at 9-12 days post-hatching. Morphological Characteristics: Nestlings are
15-18 cm long and covered in white down. Behavioral Characteristics: Nestlings are most often lying in the
nest cup out of view. Primary movements are to position themselves to be fed or to defecate over the nest rim.
Movements are by scooting motions. Nestlings possess good head coordination when feeding, but are weak and
often use their wings for balance and support when moving. Nestlings are able to excrete with greater power.
Whitewash specks are numerous and obvious on the ground and branches below the nest. The adult female is
usually present and broods the nestlings at night. She tends to flush from the nest and give the alarm call when
the nest is approached.
FIGURE 3 (top right). Goshawk at 14-17 days post-hatching. Morphological Characteristics: Nestlings are
approximately 20-23 cm long and have molted into their second natal down, which has a gray, woolly appearance.
Auricular area is still downy. Pin 'feather development ofthe remiges and rectrices is apparent, especially when
the wings are extended upward. As much as 1 cm of the feather may have erupted from the pin feather sheaths.
Behavioral Characteristics: Nestlings are able to walk on their tarso-metatarsus while extending their wings for
balance. They may stand for brief moments and look about the nest area. Nestlings begin making preening
motions at their breast and wings.
FIGURE 4 (bottom right). Goshawk at 19-22 days post-hatching. Morphological Characteristics: Auricular
area behind and below the eye is developing as a dark patch of small feathers. Remiges and rectrices are erupting
from pin sheaths and contrast markedly with the body down. Greater coverts, upper tail coverts, and scapular
feathers are starting to emerge anc;lappear as dark dots against the body down. Close observations of the ventral
feather tracts reveal dark pins beneath the down. Behavioral Characteristics: Nestlings can walk on their feet
and usually do not use their wings for balance. They will stand for longer periods and preen. Nestlings rapidly
flap their wings for short periods (3-5 seconds), especially following feeding. Nestlings may attempt to peck their
own bites of food when the female is feeding them.
STUDIES IN AVIAN BIOLOGY
PHOTOGRAPHIC GUIDE TO AGING NESTLING GOSHAWKS-Boal 35
36 STUDIES IN AVIAN BIOLOGY
FIGURE 5. Goshawk at 24-26 days post-hatching. Morphological Characteristics: Auricular area is now
covered with small black feathers. The head and neck are still downy, but sheathed feathers may be seen beneath
the down. Scapular feathers and wing coverts are visible and contrast against the natal down. Feathers of the
ventral tracts start emerging and under tail coverts may also appear. Nestlings are approximately 112adult size.
Behavioral Characteristics: Nestlings stand on the nest rim and observe the nest surroundings. They spend a
great deal of time preening and wing flapping. Nestlings start stretching their feet and legs and making fists of
the talons. They may start grabbing nest twigs with their feet. Nestlings may successfully feed themselves if the
food has no skin or has been opened up by an adult. The adult female is usually in the nest area but does not
brood or shelter nestlings at night except during wet or unseasonably cold weather.
FIGURE 6 (top right). Goshawk at 28-30 days post-hatching. Morphological Characteristics: Dark feathers
emerge along the nape, but the crown is still downy. Covert and scapular feathers fill in the upper wing and
back areas. Breast feathers are filling in along the ventral tracts but the middle of the breast and belly is still
downy. The under and upper tail coverts and legs are predominately downy, but a few feathers may be present.
Behavioral Characteristics: Nestlings spend much of their time preening, and begin scratching their heads with
their talons. Foot grabbing of nest twigs and other objects increases. Nestlings are very attentive to their
surroundings and pay attention to adults exchanging prey away from the nest. Rapid peering and sleeping while
standing are new behaviors exhibited during this stage. The adult female is rarely at the nest but is in the nest
area and becomes defensive when the nest area is entered.
FIGURE 7 (bottom right). Goshawk at 32-34 days post-hatching. Morphological Characteristics: Dark feathers
have emerged on the crown and are beginning to emerge at the comer of the mouth. The back and dorsal side
of wings are 90% feathered. The breast is filling in with feather growth but is still downy in the center and on
the belly. The undertail coverts have filled in and feathers will be emerging on the thighs. The underwing area
is still downy. Rectrices are about 213adult length. Behavioral Characteristics: Nestlings readily feed themselves
when the female is not present and may fight aggressively over food. Nestlings will vigorously beat their wings
while hopping and running across the nest. Nestlings may start branching at about 34-35 days.
PHOTOGRAPHIC GUIDE TO AGING NESTLING GOSHA WK$-Boal 37
STUDIES IN AVIAN BIOLOGY
PHOTOGRAPHIC GUIDE TO AGING NESTLING GOSHA WKB-Boal 39
sides of the breast (Fig. 6). At 32-34 days dark
feathers emerge on the crown, and feathering of
the legs is visible (Fig. 7). Body feathering is near-
ly completed at 36-38 days, but downy areas
persist on the sides of the neck (Fig. 8). At 40
days the only visible down is along the underside
of the wings (Fig. 9).
Moss (1979) described weight gain in nestling
sparrowhawks (A. nisus) as being initially slow
for 4-6 days, followed by a 10-day period of
rapid weight gain, and then a return to slow weight
gain. Though the periods will vary with different
species, this is the general pattern of growth in
nestling raptors. Food shortages can depress
weight gain in nestling raptors (Newton 1986).
Food deprivation during the 18-20 day age pe-
riod may have affected negatively the growth and
development of the photographed nestling.
However, I estimate the nestling goshawk ex-
perienced food deprivation near the end of the
rapid weight gain period (Moss 1979). Potential
depression of the nestlings' development may
have been minimized by the timing of the food
shortage and the constant food supply provided
by the rehabilitator. Fault bars, a likely side effect
offood deprivation and stress, were not observed
in the nestlings' rectrices after feather growth was
complete (c. Van Cleeve, Icarus Foundation,
pers. comm.). The photographic record agrees
with the observational data of nestling devel-
opment collected at the other nests used in this
Reported fledging ages of nestling goshawks
are variable. Reynolds and Wight (1978) found
that nestling goshawks in Oregon fledged at 34-
37 days. Newton (1979) reported goshawks
fledged at 40-43 days, and Brown and Amadon
(1968) reported nestlings fledging as late as 45
days. Variation in fledging ages may be related
to nestling condition, geographical location, or
different definitions of branching and fledging. I
define 'branching' as when a nestling leaves the
nest but remains in the nest tree, and 'fledging'
as when a nestling first leaves the nest tree on its
own volition. Reynolds and Wight (1978) found
that male nestlings develop and fledge earlier than
females. In northern Arizona, male nestlings
usually fledged at about 38 days (range = 36-40
days), whereas female nestlings fledged at 39-42
I am grateful to B. Baker, B. Bibles, E. Bibles, R.
Brown,M. Collins,N. Karnovsky,R. W.Mannan, and
J. Dawson,S. Joy, M. Kochert, R. Lehman, D. Leslie,
R. W. Mannan, R. Reynolds, D. Smith, H. Snyder,
and K. Steenhofforprovidingthoughtfuland construc-
tive reviews of this manuscript. I also thank C. Van
Cleevefor caringforthe nestlingand assistingwith the
photographic record. This material was collecteddur-
inga studyfundedbya challenge-cost-shareagreement
betweenthe University ofArizona and the USDAFor-
provided by Kaibab Industries, the Arizona Wildlife
Federation, and the Arizona Falconer's Association.
BOAL,C. W., ANDJ. E. BACORN. In press. Siblicide
and cannibalism at Northern Goshawk nests. Auk.
BROWN,L., ANDD. AMADON. 1968. Eagles, hawks
and falcons of the world. Country Life Books, Lon-
ELLIS,D. H. 1979. Development of behavior in the
Golden Eagle. Wild!. Monogr. 70:1-94.
FYFE, R. W., AND R. R. OLENDORFF. 1976. Mini-
mizing the dangers of nesting studies to raptors and
other sensitive species. Can. Wild!. Servo Occas. Pap.
MORITSCH,M. Q. 1983a. Photographic guide for ag-
ing nestling Prairie Falcons. USDI Bur. Land Man-
age., Boise, ID.
MORITSCH,M. Q. 1983b. Photographic guide for ag-
ing nestling Red-tailed Hawks. USDI Bur. Land
Manage., Boise, ID.
MORITSCH,M. Q. 1985. Photographic guide for aging
nestling Ferruginous Hawks. USDI Bur. Land Man-
age., Boise, ID.
FIGURE 8 (top left). Goshawk at 36-38 days post-hatching. Morphological Characteristics: Nestling bodies
are approximately 90% feathered, but downy areas remain along the side of the neck, in the thighs, and in the
underwing covert area. Usually a tuft of down remains just above the cere on the otherwise feathered head.
Rectrices are approximately 3f4full length. Behavioral Characteristics: All nestlings will be branching and some
of the males may fledge from the nest tree. Nestlings/fledglings will fight aggressively over food at the nest but
will still accept being fed by the adult female. Nestlings/fledglings will food beg for long periods.
FIGURE 9 (bottom left). Goshawk at greater than 40 days post-hatching. Morphological Characteristics: Nest-
lings/fledglings appear fully feathered, but downy areas persist along underwing covets. This is only visible when
the wings are spread. The crop will also appear downy when the bird is gorged. Rectrices are still slightly short
of full length. Behavioral Characteristics: Males should be fledged and females will usually fledge by 42 days
old. Fledglings can often be located when they food beg from different locations in the nest stand. At about 45
days the adults will begin providing food at locations away from the nest.
STUDIES IN AVIAN BIOLOGY
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(Accipiter nisus). J. Zool., Lond. 187:297-314.
NEWTON,t. 1979. Population ecology ofraptors. Bu-
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NEWTON,I. 1986. The Sparrowhawk. T&AD Poyser
Ltd., Staffordshire, U.K.
REYNOLDS,R. T., ANDH. M. WIGHT. 1978. Distri-
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breeding in Oregon. Wilson Bull. 90:182-196.
SCHNELL,J. H. 1958. Nesting behavior and food hab-
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