A photographic and morphometric guide to aging Gyrfalcon nestlings

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Aphotographicand
morphometricguidetoaging
Gyrfalconnestlings
Chapter·January2017
DOI:10.4080/are.2017/app1
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265
A1.1 Introduction: the importance of estimating nesting phenology
Nesting phenology is an important parameter in raptor biology used to
quantify territory quality and for understanding factors that affect popu-
lation productivity and stability, such as weather and food supply (Newton
1991, 1998). The accurate estimation of nestling ages is an easy way to
establish nesting phenology, including dates for clutch initiation, hatching,
and fledging. Nestling ages are also useful to inform the actions of field
researchers, such as planning dates for banding of nestlings and collection
of prey remains (Booms and Fuller 2003, Marti et al. 2007, Varland et al.
2007). Photographic aging guides are a useful tool for aging raptor
nestlings, and within Falconidae have been written for American Kestrel
(Falco sparverius; Kluscarits and Rusbuldt 2007), Prairie Falcon (Falco mex-
icanus; Moritsch 1983), and Peregrine Falcon (Falco peregrinus; Clum et al.
1996), but not for Gyrfalcon.
The primary aim of this work is to assist researchers in determining the
age of Gyrfalcon nestlings from hatch to fledging on the basis of photo-
graphic images and morphological measurements, and to establish
important dates in nesting phenology. Such a tool provides a quick and
easily interpreted reference to aid in aging nestling Gyrfalcons. A photo-
graphic aging guide is especially useful for observers who cannot access a
nest either for lack of training or applicable research permits, or who
through prudence opt to observe a nest from a distance and reduce distur-
APPENDIX 1
A photographic and morphometric guide
to aging Gyrfalcon nestlings
David L. Anderson, Kurt K. Burnham,
Ólafur K. Nielsen, and Bryce W. Robinson
Anderson D. L., K. K. Burnham, Ó. K. Nielsen, and B. W. Robinson. 2017. A
photographic and morphometric guide to aging Gyrfalcon nestlings. Pages 265–282 in
D.L. Anderson, C.J.W. McClure, and A. Franke, editors. Applied raptor ecology:
essentials from Gyrfalcon research. The Peregrine Fund, Boise, Idaho, USA.
https://doi.org/10.4080/are.2017/app1

bance to the Gyrfalcons. A photographic guide can also help minimize dis-
turbance times if researchers take photos of nestlings, and then compare
these to the aging guide after leaving the nest area. When doing so, place-
ment of a ruler or object of known dimensions in the nest will help
approximate the size of the nestlings.
We caution against entering nests with nestlings aged <15 days old.
Nestlings at such a young age are susceptible to mortality from cold
weather, and young can perish if the female remains absent from the nest
long enough due to disturbance. Additionally, a female that is surprised
by a human observer and flushed from the nest may accidentally knock
small young from the nest. Also note that nestlings ≥ 35 days old may
fledge prematurely when disturbed.
We point out that individual variation in nestling development can
derive from intrinsic and extrinsic factors such as sex, diet, and level of
parental investment. Also, nestlings often hatch asynchronously, and the
ages of nestlings in the same nest can differ by multiple days. Therefore, a
photographic aging guide should serve as a best approximation for
nestling age and for calculating parameters such as nest initiation, hatch
date, and fledge date. Nestling ages can also be calculated from morpho-
metric measurements taken by researchers with appropriate permits and
scientific justification. We therefore include equations to calculate nestling
ages based on morphometrics.
A1.2 Methods: Gyrfalcon natural history
After a monogamous courtship period ending in late March to early
April (low Arctic) or late March to mid-May (high Arctic), female Gyrfal-
cons lay clutches of two to five eggs, with clutch sizes of three to four eggs
more common at lower latitudes, and four to five eggs more common in
the high Arctic (Burnham 2007). The egg-laying interval is approximately
60 hours (Platt 1977, Tømmeraas 1989), and it takes approximately 7.5
days to lay a clutch of four eggs. Observations on the commencement of
incubation and synchrony of hatching are inconsistent and appear to vary
with latitude. Platt (1977) in describing nesting behavior from Canada
“believed” that incubation commenced with the penultimate (next-to-last)
egg, but did not possess observational data in support of this claim. He
observed both parents “sitting” on the incomplete clutch, but eggs were
also left unattended for hours at temperatures as low as 5ºC. K. Burnham
(unpubl. data) reports that in Greenland incubation commences with the
penultimate egg and lasts 35 days, and hatching is partially synchronous,
with three eggs hatching at one time, and the fourth egg hatching two days
later. In contrast, Tømmeraas (1989) reported that incubation commences
asynchronously in Norway. He observed a female incubating her first egg
<50% of the day, increasing incubation to 70% of the day with the second
266 Anderson et al.

egg. Platt (1976) reported an incubation period of 35 days, although with-
out specifics, and Woodin (1980) reported an incubation period of 35
days for the final egg from a clutch of four eggs in Iceland. We have heard
anecdotal evidence of completely synchronous hatching, but published
records (Woodin 1980) and photographic evidence from Alaska (B. Robin-
son, unpubl. data) show asynchronous hatching at intervals of 8 to 24
hours, which corroborates the observation of asynchronous initiation of
incubation. In this manual, we follow data supported by observations
(Woodin 1980, Tømmeraas 1989, K. Burnham unpubl. data) and assume
asynchronous incubation, an incubation period of 35 days for the final
egg, and asynchronous hatching. Once hatched, the nestling period lasts
approximately 45 days for males and 49 days for females until fledging
(Wynne-Edwards 1952, Cade 1960).
A1.3 Photographic record
Our description of Gyrfalcon nestling development is derived from pho-
tos taken by KKB of a single female produced and raised in captivity at The
Peregrine Fund in Boise, Idaho, USA from hatch to age 35 days. In each
photo a ruler aids in the measurement and description of feather develop-
ment.
We further describe Gyrfalcon nestling development from photographs
obtained at a single nest in western Alaska. We used a Reconyx PC-800
camera mounted on a cliff face adjacent to the nest to record the entire
nestling period from before hatch to fledging, May to July 2015, one
female and two male nestlings. Because we observed the time of hatching
for all three nestlings, we provide their exact ages in hours for the first two
days of life, and give their ages in round days thereafter.
A1.4 Aging via morphometrics
Gyrfalcon nestlings can be aged from measurements taken of mass,
length of the seventh primary, and length of the central rectrix. We
obtained Equations 2 and 3 used to age nestlings from measurements of
body mass and length of the seventh primary, respectively, from Poole
(1989). We (OKN) derive Equation 4 to age nestlings by the length of the
central rectrix from measurements of 38 known-age nestlings in northeast
Iceland from 1982 to 1996. We measured the central rectrix to the nearest
mm with a ruler from the lip of the feather papilla to the feather tip along
the straightened rachis when nestlings were 15 to 40 days old. Known age
was regressed on central rectrix length to obtain Equation 4. We did not
distinguish between males and females in the analysis. Analyses were done
using STATISTICA and results were highly significant (R2= 0.953, p<0.000).
Appendix 1 | A photographic and morphometric guide 267

A1.5 Discussion: how to interpret the data
The estimation of nestling ages is a means to help us understand the
nesting biology of birds, and not an end unto itself. Nestling ages inform
us of important events in nesting phenology, chief of which is clutch ini-
tiation date. Clutch initiation date is a measure of territory quality
(Newton 1991, Sergio and Newton 2003) and can be influenced by
weather and prey availability during courtship. Nestling ages can also be
used to estimate hatch date, fledge date, and to inform researchers who
need to return and band nestlings at the appropriate age. To estimate nest
initiation date from nestling ages, use Equation 1.
Equation 1 – Clutch initiation date (CID)
Clutch initiation date can be estimated from the age of the oldest nestling.
Eq. 1) CID = JD – Age – 35 – OI
where
CID = date first egg laid;
JD = Julian date of the nest observation;
Age = age of the oldest nestling;
35 = the incubation period starting with the penultimate egg (Woodin
1980); and
OI = Onset of Incubation. Assumptions:
1) Egg laying interval of 60 hours, therefore: 2.5 days are required
to lay a clutch of 2 eggs, 5 days for 3 eggs, 7.5 days for 4 eggs,
and 9 days for 5 eggs.
2) Asynchronous initiation of incubation begins with first egg, full
incubation begins with the penultimate egg. Therefore, OI = egg-
laying interval – 2.5 days.
Example: On June 15 (Julian date 166) a nest contains four nestlings, the
oldest of which is 20 days old.
CID = 166 – 20 – 35 – (9–2.5) = 104.5 or ~ 105
The estimated Julian date for clutch initiation is 105, or April 15.
268 Anderson et al.

A1.6 Aging nestlings via morphometric measurements
Equation 2 – Body mass
Body mass can be used to estimate ages for small nestlings aged 11 days
or younger (Poole 1989).
Eq. 2) NA = –0.000069 * WT2+ 0.057 *WT – 1.2
where NA is nestling age and WT is body mass (g).
Equation 3 – Length of seventh primary
Length of seventh primary and Equation 3 can be used to estimate the age
of nestlings older than c. 11 days (Poole 1989).
Eq. 3) NA = 0.15 *PL + 11.7
where NA is the nestling age in days and PL is the length in mm of primary
number seven. The seventh primary, counted from the carpal joint out-
wards, is measured with a ruler from the point of insertion in the body to
the feather tip ventrally along the straightened rachis.
Equation 4 – Length of central rectrix
Length of central rectrix in mm and Equation 4 can be used to estimate
the age of nestlings older than c. 13 days (Nielsen unpubl. data).
Eq. 4) NA = 0.1886 *CR + 13.649
where NA is the nestling age in days and CR is the length in mm of the
central rectrix measured with a ruler from the lip of the feather papilla to
the feather tip along the straightened rachis.
Appendix 1 | A photographic and morphometric guide 269

270 Anderson et al.
Hatch to 12 hours
Natal down is moist, matted, and unfluffed when the nestling hatches, but
dries within approximately four hours. Chick capable of lifting head, but
weakly, and most likely to be observed in a prostrate position unless being
fed. Eyes closed, or if open, slit-like. Presence of egg shells and unhatched
eggs may indicate recent hatching, but egg shells may be removed from nest
by parents within hours of hatching, and infertile or dead eggs can be present
for weeks (B. Robinson, unpubl. data).

Appendix 1 | A photographic and morphometric guide 271
Ages 6, 38, and 47 hours
Ages 0, 10 hours

272 Anderson et al.
Age 2–4 days
Natal down is white and fluffy. Nestling able to sit in upright posture,
although unstable and not strong. Eyes are open and less slit-like, but not yet
fully open.
Age 3 days

Appendix 1 | A photographic and morphometric guide 273
Age 7–8 days
Natal down covers most of the back and dorsal surface of the wings, but is
sparse on the breast and abdomen. Eyes are now fully open. Posture is
increasingly upright.
Age 8 days

274 Anderson et al.
Age 10–12 days
Second natal down is emerging, giving the overall down covering a lumpy
appearance. Down remains relatively sparse with approximately 25% of skin
visible and bases of down feathers visible. Unsplit sheaths of rectrices emerge.
Age 11 days

Appendix 1 | A photographic and morphometric guide 275
Age 15–17 days
Second natal down is increasingly dense and now covering 90% of the nestling
with little bare skin visible. Bases of individual down feathers no longer visible.
Primary sheaths dark and plainly visible, length 2–3 cm. Primaries and greater
primary coverts begin to break the sheath at approximately 17 days. Rectrix
sheaths visible, length to 2 cm.
Age 16 days

276 Anderson et al.
Age 19–21 days
Primaries and greater primary coverts have extended from the sheaths, and
barring on feathers is plainly visible. Rectrices extend from sheaths by ≥2 cm.
Auricular skin begins to darken, as does the down on crown of the head.
Age 20 days

Appendix 1 | A photographic and morphometric guide 277
Age 24–26 days
Note the presence of scapular contour feathers. Auricular skin continues to
darken in color and is increasingly feathered, and down on head turning
increasingly gray. Rectrices to 10 cm in length.
Age 26 days

278 Anderson et al.
Age 29–31 days
Feather development is rapid and nestlings are increasingly covered with
contour feathers. Visible down on the back may be as low as 20% of total
coverage, and down coverage on breast and abdomen approximately 50%.
Contour feathers on the head are lengthening and feather patterning on head
becomes evident. Feathers on the face nearly cover the auricular skin.
Age 30 days

Appendix 1 | A photographic and morphometric guide 279
Age 34–35 days
By 35 days the majority of the nestling’s body is covered in contour feathers,
with visible down approximately 25% of total coverage. Primaries,
secondaries, and rectrices are still blood-rooted and growing.
Age 35 days

Age 40–42 days
Nestlings at this stage are nearly fully feathered, with down visible only in
small patches.
Age 46–48 days
Nestlings have attained full juvenal plumage and exercise vigorously prior to
fledging.
280 Anderson et al.

Glossary
Auricular – The area on the side of the bird’s head behind the eye where
the ear opening is located.
Coverts – Small contour feathers that cover the bases of flight feathers.
Those on the upper (dorsal) surface of the body are called upper wing
and upper tail coverts.
Crown – The upper part of the head.
Nape – The back of the neck.
Natal down – The layer of down feathers that is present on a bird when it
hatches.
Second natal down – The layer of down feathers that a nestling grows over
its first weeks of life.
Primary – The outer flight feathers of the wing that are attached to the
carpal and metacarpal bones of the wing tip. On Gyrfalcons there are
10 primaries on each wing.
Rectrix (pl. rectrices) – The flight feathers of the tail. Gyrfalcons have 12
rectrices, six on either side of the tail.
Scapular – The area over the shoulders and along each side of the back.
Secondary – Flight feathers of the wing that are found proximal to the pri-
maries, and which attach to the ulna. In Gyrfalcons there are 16.
Sheath – The wax-like keratinous material that encases and protects newly
developing feathers as they emerge from the follicle.
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