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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-
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.
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-
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
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
280 Anderson et al.
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
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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... We extracted DNA samples from ptarmigan skeletal remains using a commercially available, silica-based DNA purification kit, following manufacturer's protocols (DNeasy Blood and Tissue Kit; (2019) and unpublished Peregrine Fund data. Mean clutch initiation is an estimate of the date of first laid egg and was calculated following Anderson et al. (2017). Ptarmigan breeding phenology from Kessel (1989) for both species. ...
... Internal heat loss due to convection increases substantially when downy feathers are wet, suggesting a greater cost of convection for wet nestlings (Nye 1964, Reid et al. 2002; such heat loss may reduce nestling survival during inclement weather, particularly storm events. This effect is likely amplified when nestlings are young, due to scarce down feathering, low body mass, and absence of physiological mechanisms required for thermoregulation (Fortin et al. 2000, Anderson et al. 2017). This period of vulnerability coincides with the Arctic spring, which is characterized by sporadic cold temperatures and precipitation that can result in the death of young nestlings (Anctil et al. 2014). ...
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Habitat suitability for breeding birds is defined at scales ranging from the landscape to individual nesting sites. Nesting site characteristics that govern exposure to inclement weather may affect breeding success, although attempts to understand this effect for Arctic breeding raptors have yielded ambiguous results. Further, breeding adults incur substantial costs from incubating eggs and brooding nestlings, and it is possible that greater site exposure results in increased nest attendance rates, increasing their cost of breeding. We quantified nesting site characteristics of Gyrfalcons (Falco rusticolus) and assessed how breeding parameters and nest attendance rates varied by protective site qualities on Alaska's Seward Peninsula, 2014–2019. The degree of physical exposure in the horizontal plane correlated negatively with the probability of hatching and fledging (provided hatch occurred), as well as overall productivity. The negative effect of horizontal exposure on the probability of fledging and productivity was greatest at nesting sites that were also more exposed in the vertical plane, although this interaction did not affect the probability of hatching. Early breeding pairs had higher productivity and tended to select more protected nesting sites. Additionally, nest attendance rates were higher in more horizontally exposed nesting sites, particularly when nestlings were approximately 2 wk old. The increased nest attendance and concurrent decreased productivity associated with greater nesting site exposure demonstrated that nesting site characteristics can have direct and indirect effects on Arctic breeding raptors and also highlight the importance of small-scale variables when evaluating habitat suitability.
We studied food habits of Gyrfalcons (Falco rusticolus) nesting in central west Greenland in 2000 and 2001 using three sources of data: time-lapse video (3 nests), prey remains (22 nests), and regurgitated pellets (19 nests). These sources provided different information describing the diet during the nesting period. Gyrfalcons relied heavily on Rock Ptarmigan (Lagopus mutus) and arctic hares (Lepus arcticus). Combined, these species contributed 79-91% of the total diet, depending on the data used. Passerines were the third most important group. Prey less common in the diet included waterfowl, arctic fox pups (Alopex lagopus), shorebirds, gulls, alcids, and falcons. All Rock Ptarmigan were adults, and all but one arctic hare were young of the year. Most passerines were fledglings. We observed two diet shifts, first from a preponderance of ptarmigan to hares in mid-June, and second to passerines in late June. The video-monitored Gyrfalcons consumed 94-110 kg of food per nest during the nestling period, higher than previously estimated. Using a combination of video, prey remains, and pellets was important to accurately document Gyrfalcon diet, and we strongly recommend using time-lapse video in future diet studies to identify biases in prey remains and pellet data.
Summary • Territory quality may affect individual fitness and contribute to density-dependent reproduction, with repercussions on population regulation. We investigated the probable causes and population consequences of spatio-temporal variations in territory quality, measured by occupancy, in eight black kite Milvus migrans Boddaert populations, one of them studied for 10 years (Lake Lugano) and the rest for 4–5 years. • Over a period of years, the occupation rate of territories varied from a random pattern. Some territories were preferred while others were avoided. On return from migration, males and females settled earlier on high-occupancy territories. • The positive association between territory occupancy and breeding performance held in all years of study at Lake Lugano, and in six of seven tested populations. As a result, high-occupancy territories contributed most of the young produced by each population. • The occupation rate of the overall 225 territories was related positively to food availability and negatively to mortality risk, measured as proximity to the nearest eagle owl Bubo bubo Linnaeus nest. • At the population level, spatial variation in mean occupancy was positively correlated with spatial variation in mean productivity, suggesting that mean occupancy could be used as a measure of overall habitat quality and population performance. • In the Lake Lugano area, a higher proportion of low quality territories was occupied in years of higher density and annual productivity was related negatively to its coefficient of variation. However, annual productivity was not related significantly to the proportion of low quality territories occupied, so support for the theory of site-dependent population regulation was only partial. • In a review of 22 studies of territory occupancy in 17 species, occupancy always deviated from a random pattern in species in which it was tested and was always correlated with productivity and/or with some other measure of territory quality. Our results confirm the importance of prioritizing conservation of high quality territories. Occupancy may be a reliable method of quality assessment, especially for populations in which not all territories are always occupied, or for species in which checking occupancy is easier than finding nests.
In a study area in south Scotland, Sparrowhawks did not occupy the available nesting places at random, but more often used those places where breeding success was highest (here called high-grade places). Most birds stayed on particular nesting places for only one year, but others stayed up to 8 years. Some birds moved from low- to high-grade places as they aged. Continued occupancy of certain places was thus produced by many different individuals occupying such places in rapid succession, but most staying for only one breeding season. On the most used (high-grade) nesting places pairs produced more than enough young per breeding attempt to offset the average annual mortality, but on the less used (low-grade) places they produced too few. Low-grade nesting places therefore acted as a sink, whose occupancy could be maintained only by continual immigration. Over the study area as a whole, the population was in balance, with reproduction matching mortality. Habitat changed over periods of 15–30 years, as woodland matured. Nesting places in young woods, with small densely-growing trees, showed the highest occupancy and nest success. Both aspects of performance declined as the woodland aged, and trees became larger and more widely spaced. Long-term stability in nest numbers and success in the study area as a whole was associated with a system of rotational forest management, which ensured a continuing availability of young woods. It is proposed that spatial variation in habitat quality is involved in the regulation of breeding numbers. Removal experiments confirmed the presence of non-breeders, which could attempt to breed when high-grade nesting habitat was made available to them, but otherwise remained as non-breeders despite the presence of vacant low-grade habitat. This situation, involving an interaction between habitat quality and bird quality, probably occurs in some other raptors too.
Inter-and intraspecific variation of breeding biology, movements, and genotype in Peregrine Falcon Falco peregrinus and Gyrfalcon F. rusticolus populations in Greenland
  • K K Burnham
Burnham, K. K. 2007. Inter-and intraspecific variation of breeding biology, movements, and genotype in Peregrine Falcon Falco peregrinus and Gyrfalcon F. rusticolus populations in Greenland. Dissertation. University of Oxford. Oxford, UK.
Guide to management of Peregrine Falcons at the eyrie. The Peregrine Fund
  • T J Cade
  • J H Enderson
Cade, T. J., and J. H. Enderson, editors. 1996. Guide to management of Peregrine Falcons at the eyrie. The Peregrine Fund, Boise, Idaho, USA.
Guide to management of Peregrine Falcons at the eyrie. The Peregrine Fund
  • N Clum
  • P Harrity
  • W Heck
Clum, N., P. Harrity, and W. Heck. 1996. Aging young peregines. Page 97 in T. J. Cade and J. H. Enderson, editors. Guide to management of Peregrine Falcons at the eyrie. The Peregrine Fund, Boise, Idaho, USA.
Photographic atlas of American Kestrel nestling development
  • J R Klucsarits
  • J J Rusbuldt
Klucsarits, J. R., and J. J. Rusbuldt. 2007. Photographic atlas of American Kestrel nestling development. Hawk Mountain Sanctuary.