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Winter Communal Roosting In the Pygmy Nuthatch 1
William J. Sydeman and Marcel Gilntert 2
Abstract.--We have studied tbe communal roosting behavior of Pygmy Nuthatches in one extremely important cavity
in a snag. Between 27 and 167 birds used the roost during
the winter of 1983. Some nuthatches moved almost 2 kilometers nightly to reach this cavity. The number of birds
and time of roosting were effected by weather conditions.
INTRODUCTION
Cavities in snags provide essential habitats
for birds during many phases of their life histories.
For migratory species, cavities provide the proper
thermodynamic and protective environments for rearing young. Resident species may use cavities for
predator avoidance and energy requirement during
the non-breeding season as well. In particular,
the survivorship of many over-wintering species
may depend solely on their use of snags. The
fmportance of snags may be underestimated with
regards to the survivorship of many forest dwelling
birds.
The Pygmy Nuthatch (Sitta pygmaea) relies
on cavities during the winter for survival. Pygmy
Nuthatches are one of the smallest (10-11 grams),
resident birds of the Ponderosa Pine (Pinus
ponderosa) forest near Flagstaff, Arizona. They
are highly social, and live in groups of 5-20 individuals throughout the non-breeding season. In
winter, nuthatches forage in flocks of 4-20 or
more that jointly defend a group territory from
conspecifics. In the late afternoon, birds gather
and travel to a snag within their territory to
spend the night. These communal roosts vary in
volume and number of birds using them. The snag,
where a communal roost is located, is the central
focus of wintering birds (Norris, 1958). The
largest reported roost of Pygmy Nuthatches is that
of Knorr (1957), who estimated 150 birds using
a snag in a montane area of Colorado.
factors that play a role in promoting large roosting
aggregations. Additionally, we will describe the
distances traveled by nuthatches to reach this
cavity, and the composition of this roosting association in terms of foraging flock membership.
METHODS
We have been studying Pygmy Nuthatch social
organization and breeding biology at Walnut Canyon
National Monument and in adjacent Coconino County
Forest Service land since October, 1980. The study
area is approximately 300 ha., 24.0 km. east of
Flagstaff, and is dominated by mature stands of
Ponderosa Pine with occasional patches of Pinyon
Pine (Pinus edibilus), Juniper (Juniperus spp.)
and Gambel's Oak (Quercus gambelii). Many snags
are available to the birds in the National Monument,
whereas few are present in the Forest Service land
due to firewood cutting practices.
A large percentage of birds within this area
have been color-banded to follow individual life
histories. Nuthatches were banded with a unique
color-combination. Group foraging territories
have been mapped as a result of afternoon observations on the number of birds and identity of individuals in each group.
At approximately 1500-1600 hours, observations
began at Walnut Canyon 50 (hereafter WCSO), the
communal roosting snag used extensively in 1983.
The arrival of nuthatches was recorded for as many
nights as possible from 14 January to 9 April,
·1983. Generally, two well-trained observers participated in gathering data on the number of birds,
and on the identity of individuals. Nuthatches
were viewed entering the cavity using either a
Bushnell Spacemaster o~ a Bausch and Lomb Explorer
spotting scope(s).
·
Here we report on the communal roosting behavior of a large population of Pygmy Nuthatches
in one particularly important snag during the winter
of 1983. We will examine the biotic and abiotic
1Paper presented at Snag Habitat Management
Symposium. Northern Arizona University, Flagstaff,
Arizona, June 7-9, 1983.
2william J. Sydeman is a graduate student
in the Department of Biological Sciences, Northern
Arizona University, Flagstaff, AZ, and Dr. Marcel
Guntert was a post-doctorate student at NAU.
Presently, Dr. Guntert is Professor of Anatomy,
Universitat Irchel, Zurich, Switzerland.
We restricted our analyses to evenings when
only a full complement of the data were obtained.
For example, only those evenings when a full count
was made on the number of birds roosting were used
for statistical analysis. Similarly, only those
nights when we were certain to have arrived before
121
any birds had entered the roost were used in the
analysis of time of roosting.
numbers. This is insufficient in describing the
observed roosting association. Either additional
data are.needed on the eff-ect of weather parameters
on nuthatch roosting behavior, or biological factors,
possibly social constraints, play a role in maintaining large numbers of roosting birds.
All regression models ..were developed- usingSPSS forwara or stepwise procedures.
RESULTS
Table I.--Analysis of Variance Table for the Regression of the No. of Birds Counted Nightly with
Weather Conditions (n=33).
Number of Birds and Time
of Roosting
A total of 33 accurate evening counts were
made on the number of birds roosting in WC50. The
count varied considerably with a high of 167 on
19 February and a low of 27 on 10 March (fig. 1).
Source
df
R2
F
Presence of Snow
Maximum Daily Temp.
BP @ 1700
Day
Residual
1
1
1
1
28
.26
.29
.33
.36
10.66
6.05
4.77
3.89
Signif.
p
p
p
p
<.005
< .01
< .01
<.025
Data presented with the addition of each
variable listed •
...
0
Weather conditions notionly affected the number
of birds using this snag, t~ey were important determinants of the initiation of roosting. Temperature at 1700 hours and the presence of snow account
for 54% of the variation in: roosting times (Table
2).- Temperature is positiv~ly correlated (r = .71,
p < .0005) with the time of roosting. On days
when the temperature at 1700 hours was higher,
the birds roosted later. Snow was negatively correlated (r = -.61, p < .0005). When snow covered
the ground, birds roosted at an earlier time. A
confounding effect is seen with day in the winter
cycle which contributes an additional 16% to the
model. As would be expected, daylight hours
lengthened following the winter solstice, and the
birds entered the roost at later times. The full
model in this case predicts accurately when birds
go to roost. From these data, it does appear that
temperature and snow change Pygmy Nuthatch roosting
behavior.
'TI 12
IZI
ii
0
"'0
c
...zm
()
0
2 MAR
FIGURE
1.
I 8 MAR
liO MAR
9 APRIL
NUMBER OF BIRDS AND GROUPS COUNTED NIGHTLY
Figure I.--Number of birds (left y-axis) and number
of banded group (right y-axis) observed each
night coming to WC50.
A steady increase in the number of roosting birds
and the number of banded groups coming to roost
is apparent from the beginning of the study through
the first three weeks of February. Subsequently,
the number of roosting birds dropped and peaked
until the middle of April. These data suggest
that roosting is a group phenomenon, and that the
decision of where to roost is carried out as a
group function, and not by individual birds. In
addition, the consistent use throughout early
February followed by a sudden decrease in numbers
within a matter of days suggests that nuthatches
are behaviorally tracking changes in environmental
conditions. A threshold temperature or weather
condition may trigger use of this snag as a roost.
Table 2.--Analysis of Variance Table for the
Regression of the Time of Roosting with
Weather Conditions (n=33).
Multiple regression analysis reveals that
weather conditions do indeed affect the number
of roosting birds. The presence of snow is the
best predictor of cavity use and accounts for 26%
of the variability in roosting numbers (Table 1).
Temperature, barometric pressure and day in the
winter cycle contribute an additional 10% in explaining the number of roosting individuals. Other
predictor variables that were entered and found to
be insignificant are minimum nighttime temperature,
relative humidity, wind speed, cloud cover, and
changes in barometric pressure. The full model
explains only 36% of the variation in roosting
Source
df
R2
F
Signi£.
Temp. @ 1700 hrs.
Day
Presence of Snow
Residual
1
1
1
29
.50
.66
• 70
31.319
29.159
22.985
p < .0005
p < .0005
p < .0005
Data presented with the addition of each variable listed. Regression Equation: Y= 1405.43 +
5.35 (Temp. @ 1700) + 1.01 (Day) - 37.28 (SnowP).
Roosting Composition
The roost contained not only banded, but also
many unhanded individuals. The number of banded
and unhanded birds in each group, and the distance
traveled by each flock to the communal roosting
snag are summarized in Table 3. Most of the groups
122
I
I
contributed more to the roost than others. Generally,
these were groups closest to the roosting snag.
1
Light Blue-east and west\and Orange compose the
core groups of the WC50 roosting association. White
and Red, whose territories are 1.7 and 1.5 km.,
respectively, from the roost, exhibited similar
contributions to the cavity as did these closer
groups. The Light Green and Dark Blue groups contribute least, although their presence at the roost
is consistent. Yellow-east and west and Mauve
contribute significantly to the roost and their
distances range from .9 km. to almost 1.5 km.
Table 3.--Characteristics of Foraging Flocks of
Pygmy Nuthatches.
Group
No.
Banded
No.
Unhanded
9
5
6
2
2
6
5
6
3
3
9
0
2
2
5
3
2
5
12
3
13
LB-east
Or
LB-west
DB-8
Y-east
Y-west
LG-west
DB-east
LG-east
Mauve
Red
White
Park-west
11
9
8
9
1
Total
No.
12
8
15
2
4
8
10
9
13
14
20
12
14
Distance
to
WC50(km.)
0.664
o. 715
0.859
0.952
0.974
1.046
1.176
1.248
1.256
1.429
1.509
1.682
0.332
Cavity Characteristics
WC50 is located in a dying Ponderosa Pine 78
feet above ground. The cavity has two entrances at
equal heights, and located on the underside of
broken-off branches where they intersect the trunk.
One entrance faces to the northeast, while the
other has a southwest exposure.
have at least 50% of their birds banded. All banded
groups were located east of WC50. The distance
traveled to the roost varied from .3 km. to 1.7
km. A total of 141 birds were monitored by taking
observations on these banded foraging flocks. At
least 79 of these birds were banded and 62 were
unhanded. On 19 February, when the highest count
of 167 birds was obtained, 96 unhanded birds were
observed. Thus, in addition to our banded flocks,
other groups of nuthatches from outside the study
area were using WC50. The number of these groups
is unknown, but the distance traveled to this snag
was probably similar to those groups located to
the east.
All of the banded flocks have been observed
roosting at WC50. Banded birds whose complete
color-band combination was determined comprised
approximately 50% of all banded birds observed.
Figure 2 shows the percent contribution of each
banded group. It is clear that certain groups
.I 0
>
<
m .o 8
::u
>
Ci)
m
.o 6
0
.o 4
-
....r--
0
z
.....
~ .o 2c
.....
0z
A more realistic assumption may be that the
birds are not stacking, but are clinging to the
walls of the cavity. An estimate of minimum surface
area would then be appropriate in determining cavity
size. The area needed to accommodate the ventral
2
surface of a Pygmy Nuthatch is approximately 30 em •
Taking our high count of 167 birds~ this means that
a minimum surface area of 5,010 em is necessary.
A measurement of this magniture is equivalent to
a bit more than .5 m2.
,._
r--
r-1--
-
DISCUSSION
....-
The use of WC50 has increased dramatically
this year over previous winters. Traditionally,
only Light Blue-east and west, Yellow-east and west
and Orange have used WC50 as a winter roost. Why
have these additional flocks joined the roosting
association at WC50 this year? This is especially
puzzling considering that many winter cavities used
by these new groups in previous years are still
intact. We have already shown how weather ~banges
affect the number of roosting birds. -These data,
unfortunately, are insufficient in explaining the
total variation in roosting numbers. Since these
data are preliminary, we cannot explain fully the
increased use of this cavity.
....r--
-
Be OR 1.8111 DBB Ye YW 0111 DBe LOt
..
R
Exact cavity characteristics have not been
taken owing to the height of the entrances. However,
we can estimate the total volume and surface area
needed to accommodate the maximum number of birds
counted by extrapolating data collected on another
previously measured winter roosting cavity. One
particular cavity measured 1500 cc. and held, when
packed, a maximum of 42 birds (D. Hay, pers. comm.).
These nuthatches, in order to fit into this cavity,
must have been stacking on top of one another• ·
Stacking has been observed in aviary birds by groups
of 4-12 individuals (D. Hay, pers. comm.). If the
birds using WC50 stack, then a minimum of four times
1500 cc. must be available to fit the 167 birds
observed roosting on 19 February, 1983. Therefore,
the cavity must be at least six liters.
w
GROUP IDENTITY
FIGURE 2.PERCENT CONTRIBUTION OF BANDED GROUPS
Figure 2.--Percent contribution of each banded
flock to the roosting association. Contribution is computed using only banded birds.
Groups are listed in order from the closest to
the furthest from the roosting snag.
123
There are, however, a number of theoretical
hypotheses in reference to birds roosting in trees
or in open areas that may provide some insight
into communal roosting by the Pygmy Nuthatch. Lack
(1968) proposed that communal roosts function to
protect individuals from predators; certain birds
roosting in the center of the group may be buffered
by peripheral birds when a predator attacks. Ward
and Zahavi (1973) have been proponents of the information-center hypothesis which is based on a
differential in foraging success between individual
birds. Communal roosts then funcfion in the sharing
of food localities or foraging techniques between
less successful and successful forages. Weatherhead
(1983) has synthesized these two hypotheses and
suggests that successful foragers are simultaneously
dominant individuals in the roostfng aggregation,
and as such may secure the most predator proof
position within the roost. As of yet, we have
not tested any of the above hypotheses.
as these requirements change over time and are
different for each species. Finally, one might
argue that only one cavity is needed to provide
a large number of birds with a suitable roost.
However, the system we have studied is highly
plastic, with fluctuations in cavity usage between
years and within seasons. In previous years, other
winter roosting cavities were used with equal
frequencies of WC50 this year. The availability
of numerous snags with cavities having the proper
characteristics is critical to nuthatches and other
species that rely on snags for over-wintering
habitat.
Lastly, there are a number of important management implications concerning commooal roosting
in the Pygmy Nuthatch. First, if a large communal
roost can be identified, the birds using it may
be monitored to assess population density. As
the birds utilizing WC50 traveled
minimum of • 3
km. to almost 2 km. to reach this snag, a large
area may be effectively censused with little expenditure in time or manpower. The distance
traveled to WC50 also points out the importance
of a single snag to a large population of birds.
Identification of these primary roosting cavities
may be difficult, thus many snags within a particular
area are needed to provide suitable roosts. The
exceptional use of one cavity also points out the
need for more detailed work on the microhabitats
of cavities. Each cavity does not provide the
same essential habitat requirement~, especially
Knorr, O.A. 1957. Communal roosting of the
Pygmy Nuthatch. Condor, 59:398-399.
LITERATURE CITED
Hay, D. 1983. Personal conversation, Department
of Biological Sciences, Northern Arizona
University, Flagstaff, AZ.
a
124
Lack, D. 1968. Ecological adaptations for breeding in birds. Methuen, London.
Norris, R.A. 1958. Comparative biosystematics and
life history of the nuthatches, Sitta pygmaea
and Sitta pussilla. Univ. of Cal. Publ. in
Zool. 56:119-300.
Ward, P., and A. Zahav.i. 1973. The importance of
certain assemblages of birds as "information
centers" for food finding. Ibis 115:517-534.
Weatherhead, P.J. 1983. Two principal strategies
in avian communal roosts. American Naturalist
121:237-243.