THE OCCURRENCE OF FAULT BARS IN THE PLUMAGE OF NESTLING OSPREYS
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261
THE OCCURRENCE OF FAULT BARS
IN THE PLUMAGE OF NESTLING OSPREYS
MARLENE M. MACHMERl, HANS ESSELINK2,
CHRISTOPH STEEGERI & RONALD C. YDENBERGI
ABSTRACT We document the occurrence of fault bars in a population of
nestling Ospreys Pandion haliaetus under natural conditions. Ospreys had
an average of 9.9 fault bars on their rectrices, however variation was large.
Fault bar formation declined linearly with age and increased symmetrically
from outer to inner rectrices. Fault bar incidence is consistent in all plumage
groups and those groups most essential for flight are least affected. We also
examine fault bar occurrence in relation to two hypotheses: food shortage
and handling effect. The former is investigated indirectly by comparing the
number and intensity of rectrix fault bars in nestlings to their feeding rank
and to their brood size. Neither variable has a significant effect on fault bar
severity. The possibility of weather-mediated food shortage is examined, but
no strong effect of weather is detected. The role of handling is assessed by
comparing the number and intensity of fault bars in nestlings experiencing
different numbers of nest visits. Nestlings visited repeatedly had more fault
bars providing support for the handling hypothesis. Analysis of the timing
of fault bar formation with respect to nest visits is suggestive as further evi-
dence for the role of handling.
1Behavioural Ecology Research Group, Department of Biological Sciences,
Simon Fraser University, Burnaby, British Columbia, V5A IS6. 2Zoological
Laboratory, State University Groningen, AA 9735 Haren, The Netherlands.
INTRODUCTION birds. The dark band laid down during the day is
associated with a high metabolic rate and a de-
Fault bars are narrow, translucent bands found in crease in metabolic activity during the night is man-
the plumage of many bird species (Fig. 1). They ifested as a lighter band (Wood 1950). Each pair of
are caused by defective barbule formation (King bands therefore represents a 24-hour period of
& Murphy 1984, Stiefel 1985) and are often points feather growth (Michener & Michener 1938).
of feather breakage (Beebe & Webster 1964, Haw- When subject to prolonged food shortage, a bird
field 1986, Newton 1986). Fault bars are familiar to will develop a single fault bar per feather over a
falconers and biologists who regularly handle 24-hour period (Grubb 1989). This bar is similarly
birds, but few studies have documented their oc- light in color and also appears to be laid down at
currence or investigated possible causal factors night (Riddle, 1908). In contrast to a fundamental
leading to their formation. Riddle (1907) was the bar, it contains aberrant barbules or lacks barbules
first researcher to describe and experimentally in- entirely (Stiefel 1985). Fault bars therefore appear
vestigate these bars. He discussed several irregu- to be a more extreme form offundamental bars and
larly-spaced feather defects which he referred to the distinction between them is based on severity.
collectively as 'fault bars'. He also described 'fun- Fault bars have been referred to as 'subordinate
damental bars', consisting of regularly-spaced al- bars' (Glegg 1944), 'growth bars' (Wood 1950),
ternating light and dark bands found in most feath- 'hunger traces' (Beebe & Webster 1964), 'hunger
ers (Riddle 1908). Fundamental bars are the result streaks' (Hamerstrom 1967) and 'feather marks'
of cyclic changes in the metabolic rate of diurnal (Slagsvold 1982) and some of these terms have also
Received 26 April 1991, accepted 29 October 1991. ARDEA 80: 261-272262 ARDEA 80 (2), 1992
cipiter nisus, nestlings often developed fault bars
on rainy days, when young were fed significantly
less often than normal (Newton 1986). Gray Jays
Perisoreus canadensis provided with supplemen-
tal food caches grew tail feathers with statistically
fewer fault bars than unsupplemented birds (Waite
1990). All ofthese results support the food shortage
hypothesis.
A second hypothesis to explain fault bar occur-
rence was put forth by King & Murphy (1984),
who felt that handling was the main causal factor
in captive birds. While studying White-crowned
Sparrows Zonotrichia leucophrys gambelii they
found that, taking feather growth rates into
account, distances between fault bars correspond-
ed to intervals at which birds had been handled
(King & Murphy 1984, Murphy et. al. 1988). King
& Murphy suggested that some form of stress is
similarly responsible for these bars in free-living
birds.
Almost all published investigations of fault
bars to date have been undertaken on captive birds
and little is known about the degree to which these
bars are manifested under natural conditions. An
opportunity to study fault bars in a natural setting
Fig. 1. Magnification (xl0) of a fault bar to the left of presented itself in 1986, when we noted that nes-
the rachis. Note defective barbule formation. tling Ospreys Pandion haliaetus in the Kootenay
region of British Columbia regularly had fault bars
been applied to fundamental bars. This lack of con- in their plumage. In 1987 we sought to document
sistent terminology has led to some confusion in the occurrence ofthis phenomenon in the Kootenay
the literature. Osprey population and to examine its occurrence
Two general hypotheses regarding fault bar in relation to the above hypotheses.
causation have emerged from investigations to Certain aspects of Osprey reproductive biology
date. The first attributes fault bar formation to poor facilitated indirect investigation of the effect of
nutrition during the period of feather growth. Rid- food shortage on fault bar formation. Ospreys ex-
dle (1908) was able to induce bar formation in Ring hibit asynchronous hatching, which results in nest-
Doves Streptopelia risorius by restricting their lings graded in size and competitive ability, leading
food intake over a 24 hour period, as did Melius in tum to the development of a feeding hierarchy
(1975,reportedinKing & Murphy 1984) with Ring- (Poole 1982, 1984, Hagan 1986, Jamieson et. al.
necked Pheasants Phasianus colchicus. Slagsvold 1983). Older siblings have priority of access to
(1982) found that the number offault bars in Hood- food, and younger siblings are often selectively
ed Crows Corvus corone cornix was inversely cor- eliminated through starvation or active siblicide
related to their body size and abdominal fat content. (Forbes 1989). If fault bars are in any way related
He also reported that fault bars were particularly to food shortage, one would expect a gradation in
common in undernourished and albinoid individu- their occurrence, with lowest ranking chicks mani-
als (Slagsvold own obs.). In Sparrowhawks Ac- festing the most bars.Machmer el 01.: PLUMAGE IN OSPREY NESTLINGS 263
In addition to rank, a nestling's food intake de- METHODS
pends on the number of siblings it must share food
with, assuming that parental food delivery is rela- The study was conducted from June to August,
tively constant. Stinson (1978) and Jamieson et. al. 1987, near Creston and Nelson, British Columbia.
(1983) found no differences in food delivery rates The study areas and the Osprey population breed-
to broods of different sizes, suggesting that nest- ing there are described in detail by Steeger (1989).
lings from large broods receive less food, on aver- A subset ofnestlings (n =66) was visitedona week-
age. The inverse relationship between brood size ly, or in some cases, on a biweekly basis, at which
and fledging mass (Stinson 1977), and the fact that time measurements of the length of the third pri-
nestlings from large broods grow significantly mary, tail and wing length, culmen and mass were
more slowly in some locations (Poole 1982) is fur- obtained. Using chicks of known age (n = 29), a
ther evidence that chicks from large broods are regression of primary length (PL, mm) on age (AG,
more likely to suffer food stress. If fault bars are a days) was established and used to estimate the
reflection of food stress, nestlings from large hatching date of all nestlings (AG = 13.1 + 0.14 .
broods should experience greater fault bar severity. PL; ]'2 = 0.97).
Prevailing weather conditions are intimately Once feathers erupted, weekly visits included
related to the foraging success of many bird species a detailed examination of all 12 rectrices. The lo-
and may therefore influence food intake of altricial cation of each fault bar with respect to the proximal
young (Birkhead 1976, Stinson 1980, Dunn 1975, end of the feather, flush with the body, was mea-
Newton 1986). In Ospreys, high windspeeds and sured to the nearest mm and called bar distance.
choppy water surface conditions increase the ener- The length of each individual rectrix (right 1-6, left
gy and time required to capture each prey (Mach- 1-6) containing a bar was measured and called
mer & Ydenberg 1990). However studies mea- feather length. Additionally, all bars received an
suring the amount of food delivered to the nest intensity score of 1-4 according to the scheme in
show little or no effect of weather (Green 1976, Table I. A second group of nestlings (n = 45) was
Stinson 1978, Stinson et. al. 1987). Indirect evi- visited on a single occasion at about six weeks of
dence to support a weather-mediated decline in age for banding, body measurement and fault bar
nestling food intake is provided by Poole (1982, examination, as described above.
1984), who found a substantial increase in chick Nestlings were placed into groups that had re-
mortality and severely curtailed growth rates in ceived no nest visits, several (l - 3) or many (4 or
surviving young, during storms. Presumably Os- more) nest visits, prior to the final visit required
preys are able to compensate for reduced hunting for banding and measurement. The three groups
success during poor weather by hunting longer, but were compared using three fault bar criteria: the
are unable to sustain the necessary effort during number of bars on the rectrices (NOBARS) , the
prolonged or severe weather. We examined the pat- average rectrix bar intensity per nestling (lNT; ac-
tern of fault bar formation in nestling Ospreys in cording to Table 1) and the number of bars divided
relation to weather. by the tail length (ADINOBARS). A one way anal-
If handling results in fault bar formation, one ysis of variance (ANOYA) was performed between
would expect a gradation in bar severity, with nest- the three nestling groups for each of the bar criteria.
lings handled more often exhibiting the most ex- For bar criteria not normally distributed, Kruskal-
treme bars. Similarly, with respect to the timing of Wallis tests were employed. Nestlings from broods
bar formation, the dates on which chicks were of one, two and three chicks were compared in one
handled should correspond to the positions of fault way ANOYAs for each of the bar criteria. Nestlings
bars on the feathers, when growth rates are taken were assigned a rank (A, B or C) only when all the
into account. Both of the latter predictions were body measurements of an individual nestling were
also investigated. above or below that of its nestmate(s), with no over-264 ARDEA 80 (2), 1992
Table 1. Fault bar intensity scoring system. bars up the feather margin was computed and
grouped according to inner, middle and outer
Bar description Bar score rectrices, and a one way ANOVA perfonned be-
tween groups. The rectrix eruption age was pre-
(i) Bar width> 1.5 mm, or: 4
dicted by a regression of tail length (TL) on age
(ii) Intermediate bar width, occurring on
(AG), using only repeated measurements on nest-
both sides of rachis and severed at bar
lings whose hatching date was known to within one
Intermediate bar width day. An average rectrix eruption age of 15.7 days
( >0.5 mm but < 1.5 mm) 3 was detennined (AG = 15.7 + 0.18· TL; r2 = 0.96).
(i) occurring on both sides of rachis, or: We examined the relationship between inclem-
(ii) occurring on one side of rachis ent weather and the timing of fault bar fonnation.
and severed at bar Meteorological records (average daily windspeed
and precipitation) were obtained from an automatic
Intermediate bar width occurring on one side weather station at Redfish Creek, east of Nelson,
of rachis only 2 and from Creston.
The role of handling stress in the timing offault
Bar width < 0.5 mm
bar fonnation was investigated by computing the
number ofbars fonned by each nestling on the days
it was visited, as well as the three days preceding
lap. We compared fault bars in A, Band C chicks and following that visit.
in one way ANOVAs for the three bar criteria de- The pattern of fault bar distribution on other
scribed above. body feathers was investigated at the time of band-
The date offonnation was estimated for all rect- ing in 99 nestlings. Plumage groups examined were
rix fault bars (n = 1098) according to the follow- primaries, secondaries, alula, greater primary co-
ing equation: verts, greater secondary coverts, median upper-
wing coverts, lesser upperwing coverts, scapulars,
DATE = HD + ARE + (FL-BD)/GR upper tail coverts, under tail coverts, back and
head. Each ofthe 12 plumage groups on one side
where DATE is date of bar fonnation Gulian days of the bird were scanned briefly, and the number
from June l),HDishatchingdate Gu1iandays; esti- of bars in the most seriously affected feather, as
mated from the regression of primary length on well as the number of feathers with bars (scored as
age), ARE is chick age at rectrix eruption (days), ofor no feathers with bars, 1 for one feather only,
FL is feather length (mm),BD is bar distance (mm), 2 for two to four feathers and 3 for five or more
and GR is average daily growth rate (mm/day). feathers) was recorded. The alula could only be
To obtain an overall estimate of rectrix growth scored as 0 or 1. We adopted this method of plum-
and to check whether all rectrices grew at the same age scoring in order to be able to handle each chick
rate, the growth rate was estimated in three differ- quickly. If the nestling's distress was severe we im-
ent ways: (A) The average daily tail growth rate mediately returned it to the nest without com-
was detennined from tail length data collected on pleting all of the measurements. This happened
a weekly basis. (B) For feathers with bars only, the only rarely, but the sample sizes for the 12 plumage
average daily growth rate of individual rectrices groups therefore vary somewhat. Using a Spear-
was calculated. The growth rates of inner (inner man rank correlation matrix, we examined the as-
two rectrices on both sides), middle (middle two sociation between the number offeathers with fault
rectrices on both sides) and outer rectrices were bars in the different plumage groups on individual
compared in a one way ANOVA. (C) The rate of birds (excluding comparisons with the alula, as
movement of the most severe (intensity = 4) fault there were only two groups for this category). WeMachmer et al.: PLUMAGE IN OSPREY NESTLINGS 265
also correlated plumage scores to the total number
40
of rectrix fault bars in order to examine the validity
of using the latter measure as a general indicator.
RESULTS
A total of 1098 fault bars was observed in the
rectrices of III Osprey nestlings. Descriptive sta-
tistics for each of the three bar criteria are presented
in Table 2 and a frequency distribution ofthe num-
ber of fault bars measured per nestling is shown in
Fig. 2. The average nestling had 9.9 fault bars on
its rectrices (SD = 9.4), with an average intensity o 10 20 30 40 50 60
of 2.0 (SD = 0.5). There was large variation in the number of fault bars
expression of this phenomenon with numbers of
Fig. 2. Frequency distribution of the number of rectrix
bars per nestling ranging from 0-60. Most rectrix
fault bars measured per nestling.
fault bars formed when nestlings were young (Fig.
3) and the rate of formation declined as the nest-
lings aged. Central rectrices contained more fault sidered, because there are only two alula catego-
bars, and the number of rectrices diminished to- ries), all are positive, and 35 are significant at the
ward the outer rectrices in a symmetrical pattern 0.05 level. Also, a multiple rank correlation of
(Fig. 4). The distribution of fault bars and the mean plumage bar scores to the total number of rectrix
number of bars in other plumage groups is present- bars in an individual were positive and 9 were sig-
ed in Table 3. Certain plumage groups (e.g. alula, nificant.
primaries) consistently had few fault bars, while
others (e.g. back, head, median upperwing coverts)
had both high numbers of bars perfeather and more 140
feathers with bars. In general it seemed that plum-
age groups important for flight contained the few- "0
Q)
120 •
est fault bars. A multiple rank correlation of the bar E
scores between plumage groups within each indi- ,2100
vidual (Table 4) shows that, on average, the inci- rn'"
.0 80
dence of fault bars is consistent in all plumage ~
:::>
groups. Of the 55 plumage group comparisons in ~ 60
Table 4 (comparisons with the alula are not con- '0
CD 40 •• • •
•••• •
.0
E
Table 2. Mean values per nestling and standard devia- :::>
c:: 20
tions (SD) for fault bar criteria in Osprey nestling rec-
trices (n = 111 nestlings). 0
15 20 25 30 35
• 45
nestling age (d)
Bar criterion Mean SD
. _--- Fig. 3. Pattern of fault bar formation with nestling age.
Number of bars 9.9 9.4 The figure shows the total number of fault bars measured
Intensity of bars 2.0 0.5 in the rectrices of all Osprey nestlings. Regression equa-
Adjusted number of bars 0.08 0.Q7 tion is y = 151.4 - 3.93 x, n = 28 days, r2 = .82, n = 1098
bars, P < 0.001.266 ARDEA 80 (2), 1992
rectrices (right 3-4, left 3-4) growing most rapidly,
150 whereas method (C) suggested that inner and mid-
dle rectrices grow more rapidly than the outer rect-
rices, but the differences are not significant. We
~ 130
,Q calculated an overall rectrix growth rate, incorpo-
==::J rating a weighted mean of all three methods, and
co
-110
'0 used this estimate to calculate the formation date
~ of all bars.
CD
,Q
90
E
::J
C
Relationship to brood size and rank
(ij
70 Average values and significance levels for bar
§
criteria in nestlings from different brood sizes are
presented in Table 6. There were no significant dif-
ferences among nestlings from broods of one, two
tail feather
or three chicks for any of the three bar criteria.
However, the number of bars showed an (insignif-
Fig. 4. Pattern of fault bar distribution on rectrices of icant) tendency to increase with brood size. The
nestling Ospreys, Note the symmetrical pattern proceed- average bar scores of nestlings of different rank are
ing from outer to inner rectrices,
presented in Table 7. Again, there were no signif-
icant differences between A, B or C chicks for any
A summary of tail growth rate calculations is of the criteria. However, for all three severity mea-
presented in Table 5. Method (B) produced signifi- sures, a trend of increasing bar scores for A, B and
cantly different rates of rectrix growth with middle C chicks, in that order, was apparent.
Table 3. Fault bar scores in the body plumage of nestling Ospreys.
No. nestlings in plumage
score category* Mean no. fault
bars per affected
Plumage group 0 2 3 feather n
Primaries 41 24 25 5 1.7 95
Secondaries 27 18 43 8 1.9 96
Alula** 86 9 1.6 95
Greater primary coverts 47 21 23 6 2.3 97
Greater secondary coverts 10 16 45 28 4.4 99
Median upperwing coverts 6 3 32 57 7.4 98
Lesser upperwing coverts 13 3 31 49 3.5 96
Scapulars 19 13 38 25 3.5 95
Upper tail coverts 23 14 41 21 2.5 99
Under tail coverts 33 6 43 14 2.0 96
Back 0 3 26 70 3.6 99
Head 2 5 18 72 4.5 97
o = no feathers in group have fault bars ** Alula is only feather in group
I = one feather has fault bar and hence can score only 0 or 1.
2 = 2 - 4 feathers have fault bars
3 = > 4 feathers have fault barsMachmer et al.: PLUMAGE IN OSPREY NESTLINGS 267
Table 4. Spearman rank correlations between fault bar plumage scores for each Osprey nestling. n ~ 95 for all
comparisons. The critical value of r = 0.205.
PRI SEC GPC GSC MUC LUC SCA UPTC UNTC BACK HEAD TOT
PRI 1.00
SEC 0.31 1.00
GPC 0.25 0.33 1.00
GSC 0.12 0.20 0.26 1.00
MUC 0.20 0.33 0.21 0.60 1.00
LUC 0.27 0.24 0.16 0.34 0.58 1.00
SCA -0.01 0.09 0.18 0.43 0.33 0.28 1.00
UPTC 0.25 0.29 0.18 0.12 0.22 0.14 0.16 1.00
UNTC 0.16 0.26 0.18 0.40 0.46 0.37 0.18 0.32 1.00
BACK -0.00 0.23 0.14 0.11 0.30 0.35 0.24 0.34 0.26 1.00
HEAD 0.17 0.20 0.23 0.20 0.33 0.38 0.24 0.22 0.35 0.49 1.00
TOT 0.32 0.30 0.21 0.34 0.38 0.36 0.36 0.20 0.25 0.19 0.23 1.00
PRI = primary, SEC = secondary, GPC =greater primary coverts, GSC = greater secondary coverts, MUC = median
upperwing coverts, LUC = lower upperwing coverts, SCA = scapulars, UPTC = upper tail coverts, UNTC = under
tail coverts, TOT = total numher of rectrix fault bars.
Relationship to weather distribution of the nestlings to calculate the expect-
The most striking temporal pattern in the for- ed number offault bars formed on each day during
mation of fault bars is that related to age (Fig. 3). the nestling period. To examine if weather had any
We used the regression equation of bar formation effect on bar formation rate, we compared this ex-
rate on nestling age in Fig. 3 and the known age pected distribution with that observed, reasoning
Table S. Summary of tail growth rate calculations.
Rectrix position Weighted mean
Method of growth growth rate
rate calculation Outer Middle Inner P* (mm/day)
--------- --~-_._------------~-----_._----------_._.-
(A) Average tail growth rate 5.37
(n= 45)
(B) Individual rectrix 5.20 5.67 5.21 0.029 5.34
growth rate (n = 63) (n = 17) (n = 18) (n = 28)
(C) Rates of movement 5.10 5.41 5.43 0.489 5.32
along individual (n = 16) (n = 10) (n = 21)
rectrices (n = 47)
Overall mean 5.15 5.58 5.30 5.34
(n = 33) (n = 28) (n = 45)
* test of the hypothesis that outer, middle and inner rectrix growth rates do not differ268 ARDEA 80 (2),1992
Table 6. The influence of brood size on rectrix fault Table 7. The influence of nestling rank on rectrix fault
bar severity of nestlings. bar severity of nestlings.
Brood size Nestling Rank
Bar criterion 2 3 p Bar criterion A B c p
Number of bars 5.5 8.7 11.9 0.2214* Number of bars 8.3 10.2 10.5 0.543
Intensity of bars 2.5 2.0 2.0 0.1502** Intensity of bars 2.0 2.0 2.3 0.447
Adjusted number of bars 0.07 0.Q7 0.09 0.9057* Adjusted number of bars 0.07 0.08 0.10 0.307
Number of nestlings 8 54 49 Number of nestlings 29 19 9
compared using Kruskal-Wallis test
** compared using one-way analysis of variance Relationship to handling intensity
The bar criteria measured in chicks experi-
that periods of bad weather (i.e. high winds, rain) encing different numbers of nest visits are com-
should increase the rate of bar formation relative pared in Table 8. Differences between groups are
to good weather periods. The results are presented significant for two of the three criteria. The data
in Fig. 5. Although bar formation is greater than indicates that the 1-3 visit group had the greatest
expected on days 35 and 38, rates are lower than bar scores, due in part to six nestlings with unusual-
expected on other bad weather days and sometimes ly high fault bar scores. These nestlings came from
elevated in the absence of bad weather. These re- two nests located side by side in the area of highest
sults indicate little or no effect of weather. Osprey nest density, at Creston. Three nestlings,
all from one nest, had by far the highest bar scores
of all nestlings that we measured, but eliminating
this nest from the analysis does not change the basic
result. Pooling'the 1-3 and 4-6 visit groups and
80 comparing this group to the no visit group produced
w: wind significant differences for two ofthe three bar crite-
R = rain
~ 60 • B: both
ria (K-W test; NOBARS: P =0.041,INT: P =0.082,
.0 ADfNOBARS: P = 0.006).
=:
::::l
9 km/hr), rain * compared using Kruskal-Wallis test
( > 5 mm/day), or both wind and rain are indicated. ** compared using one-way analysis of varianceMachmer et at.: PLUMAGE IN OSPREY NESTLINGS 269
currence of fault bars in plumage groups critical to
120
flight to be minimized.
-0
Cll
There was a marked decline in fault bar forma-
E 100
.2 tion as nestlings grew older. Perhaps they become
C1l
ro more robust and less sensitive to short term fluctua-
£J 80
::: tions in food supply. Also, as fledging approaches,
:::J
~ feathers take on a role of paramount importance
'0 60 and perhaps a greater proportion of total available
Q;
£J energy is shunted into feather growth to ensure ade-
E 40
:::J
C
quate plumage.
There was no significant difference in fault bar
20
overall mean rate severity amongst broods of one, two or three nest-
of formation
lings. While small sample sizes for broods of one
0 may have contributed to this result, other con-
321t123
BEFORE AFTER founding factors could have obscured patterns. For
nest visit day
example, there may have been differences in par-
Fig. 6. Frequency distribution of the number of rectrix ental quality which influenced the brood-sizes we
fault bars formed around each visit day for all regularly observed. Perhaps parents of single nestlings were
visited nestlings. 126 chick-visits were made, and 572 younger or less experienced, and therefore deliver-
fault bars measured. ed less food than parents of larger broods. This ef-
fect could, in part, offset the proportionately smal-
If the hypothesis that handling causes fault bars ler shares of food received by nestlings from large
is correct, then one would expect to see a peak in broods, assuming parental quality was constant.
bar formation on the visit day. A frequency distri- For two of the three criteria, there was a trend of
bution of the number of bars formed in the rectrices increasing bar severity with brood-size, suggest-
in relation to each chick-visit (n = 126) is presented ing that food stress may be related to fault bar oc-
in Fig. 6. The 572 fault bars measured in the currence. To make any conclusive statements, a
rectrices formed over a 42 day period, giving a larger number of one-chick broods would be re-
mean formation rate of 13.6 bars per day. Nest quired.
visits occurred at least a week apart and therefore There was a trend of increasing bar scores with
no bar is represented twice in Fig. 6. The figure a decrease inrank (Table 7) for all bar criteria. Since
indicates a general rise in the rate of bar formation lower ranking chicks, particularly C-chicks, re-
associated with nest visits. However the incidence ceive proportionately less food than their dominant
peaks the day before the nest visit itself. siblings (Poole 1979, 1982, 1984; Jamieson ct. al.
1983; Hagan 1986), these findings support a rela-
tionship between food shortage and fault bar for-
DISCUSSION mation. A more direct way to relate fault bars to
food shortage would involve monitoring the daily
Despite great variation in the incidence offault bars food intake of specific nestlings and correlating
among Osprey nestlings, some consistent patterns fault bar incidence to well-documented patterns of
were evident. Fault bars were relatively uncom- food intake.
mon in the flight feathers and relatively abundant There is no obvious relationship between fault
in the body coverts. Inner rectrices showed more bar formation and bad weather (Fig. 5). Although
fault bars than the outer rectrices. Considering that the rate of fault bar formation is elevated on days
there is a cost associated with a gap in the flight 34-40, a period during which two days of high
feathers (Prevost 1983), one would expect the oc- winds and rain occurred, such increases are not ob-270 ARDEA 80 (2), 1992
served consistently with bad weather. A compari- A second possibility is that an unforseen asso-
son of the observed seasonal pattern of fault bar ciation between periods of bad weather and nest
formation with that expected on the basis of the visits may have shifted the peak in bar formation.
age-dependent formation rate and the nestling age Forty-eight percent of the 126 nest visits occurred
structure indicates no strong difference. During the on the day after a bad weather day. On two occa-
period ofnestling feather growth, we observed only sions, we actually postponed all nest surveys due
two consecutive days of bad weather and it is pos- to high winds and rain, which made travel by boat
sible that this was not sufficiently prolonged to pro- on the lake unsafe. In two other instances, the
duce a weather-mediated decline in nestling food timing ofbad weather prior to nest visits was purely
intake. coincidental. We therefore re-examined the timing
The fact that nestlings visited repeatedly during of nest visits in relation to fault bar formation, this
the season had significantly more fault bars pro- time excluding from the analysis all visits occur-
vides some support for the handling stress hypo- ring after a bad weather day. A rise in fault bar for-
thesis. We feel that pooling the 1-3 and 4-6 visit mation associated with nest visits and a gradual
groups is valid because nestlings in the latter group peak centered on visit days resulted. These findings
were beyond the age of appreciable fault bar for- clearly support the role of handling in fault bar for-
mation when the additional nest visits occurred. mation. They also suggest a potential influence of
The rise in bar formation associated with visits to weather on the timing of fault bar formation.
the Osprey nests could also be interpreted as evi- There was no clear and compelling association
dence for the effects of handling. The marked peak between fault bar formation and food shortage in
in fault bar formation is somewhat puzzling in that this study. Our sample included only those chicks
it occurs one day before actual visit days. This sug- surviving to at least six weeks of age and presum-
gests that either the fault bars are caused by the ably to fledging, which may have contributed to
stress ofhandling during visits and that dating tech- this outcome. Our results do show that handling
niques are inaccurate enough to lead to a one-day may increase the likelihood that Osprey nestlings
error, or, that some other factor somehow associat- develop fault bars however this provides no ex-
ed with the timing of visit days has caused this pat- planation for the widespread and variable occur-
tern. rence of fault bars in unhandled birds. Presumably,
Error in our dating technique arises from mea- several factors may contribute to this phenomenon
surement ofthe feather length, the bar distance, and and very intensive monitoring of individuals may
from the calculation of growth rate. With practice, be required to find support for specific hypotheses.
feather length and bar distance measurements were If fault bars are representative of the sum total of
consistent to within one mm between observers, stresses to which a nestling is subject (i.e. food
and we attribute most of the dating error to our esti- shortage, thermoregulatory stress, sibling aggres-
mate of growth rate. We selected a representative sion), then they deserve further investigation and
fault bar dated at 36.4 julian days according to our may prove useful in future studies of offspring qua-
technique and then substituted the lowest and high- lity in relation to parental effort.
est growth rate estimates in Table 5 (5.10 and 5.67
mm/day, respectively) to calculate a range of 36.1
- 36.7 days. Assuming a one mm error in both the ACKNOWLEDGMENTS
feather length and bar distance and still using the We thank Brian Stushnoff and staff at the Creston Valley
lowest and highest growth rate estimates above, the Wildlife Management Area as well as Guy Woods at the
range becomes 35.7 - 37.1 days, respectively. The Nelson Fish & Wildlife office for their continued coop-
peak in fault bar formation one day before nest eration on this project. Peter Arcese provided helpful ref-
visits could therefore be attributable to error as- erences and was kind enough to share his unpublished
sociated with our dating technique. data. Thanks to John Krebs for his tree-climbing assis-Machmer et al.: PLUMAGE IN OSPREY NESTLINGS 271
tance. Funding was provided in part by a Natural Sci- Murphy, M.E., J.R. King & J. Lu 1988. Malnutrition dur-
ences and Engineering Research Council of Canada ing the postnuptial moult of White-crowned Spar-
(NSERC) summer student award to M.M., and by rows: feather growth and quality. Can. J. Zool. 66:
NSERC grant U0461 to R.c.Y. 1403-1413.
Newton, 1. 1986. The Sparrowhawk. T & A.D. Poyser
Ltd., Staffordshire, England.
Poole, A.F. 1979. Sibling aggression among nestling Os-
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SAMENVATTING het jong ontwikkelen (Fig. 3). Niettemin blijkt er tussen
jongen van dezelfde leeftijd een grote variatie te bestaan
In 1986 werden, tijdens het ringen en meten van nest- in het voorkomen van faultbars (Tabel 2).
jongen van Visarenden, op grote schaal veerafwijkingen Twee hypothesen werden geopperd ter verklaring
opgemerkt. Naar de oorzaak van deze afwijkingen werd van het ontstaan van faultbars, namelijk (a) voedselte-
door ons in het daarop volgende jaar een onderzoek kort en (b) stress opgelopen tijdens het hanteren (meten
opgezet in een populatie in de omgeving van Creston en ringen) van de jongen. De voedselhypothese bleek
(71 paar) en Nelson (47 paar) in Brits Columbia, Cana- geen bevredigende verklaring te bieden, omdat het op-
da. treden van faultbars niet significant samenhing met het
Faultbars zijn lichte smalle dwarsstrepen in een veer, aantaljongen op het nest (minder voedsel perjong: Tabel
veroorzaakt door een verstoorde ontwikkeling van de 6), met de dominantie tussen de jongen (Tabel 7), en ook
baardjes tijdens de groei van de veer (Fig. I), niet te ver- niet duidelijk samenhing met het weer (minder voedsel
warren met de regelmatig afwisselende donker-licht bij slecht weer) tijdens de groei van de veren (Fig. 5).
bandjes die ook tijdens de groei ontstaan doorverschillen De handling-hypothese bood een betere verklaring.
in metabolisme tussen dag en nacht. Faultbars kunnen Er was een positief significant verband tussen de mate
verschillen in intensiteit (Tabell). Ze komen in aIle veer- waarin het nest werd bezocht in de periode van veergroei
groepen voor, maar het minst in de duimvleugel (alula) en het optreden van faultbars (TabeI8). Bovendien kon
en de handpennen (Tabel 3). Waarschijnlijk beschikt de worden vastgesteld dat het merendeel van de faultbars
vogel over een mechanisme waarmee de vorming van gevormd moest zijn rond de tijdstippen waarop de nesten
faultbars in alula en handpennen zoveel mogelijk wordt bezocht werden (Fig. 6). Het wijd verspreide en variabele
tegengegaan, omdat er anders gemakkelijk breuken in voorkomen van faultbars in niet eerder gehanteerde jon-
deze veren zouden kunnen ontstaan. gen wordt hiermee echter niet verklaard. Waarschijnlijk
De mate waarin diverse veergroepen gevoelig zijn dragen verschillende factoren bij tot dit fenomeen. Om
voor het optreden van faultbars lijkt niet te verschillen meer specifieke hypothesen te toetsen zal het noodzake-
tussen individuen (Tabel 4, Fig. 4). De gevoeligheid lijk zijn om individueel bekende jongen intensief te vol-
neemt af naarmate de veren zich later in het leven van gen.You can also read
