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Seasonal Selection ·of Tree Cavities by Pygmy
Nuthatches Based on Cavity Characterlstics 1
Douglas B. Hay2 and Marcel GUntert3 • 4
Abstract.--Eighteen characteristics of 46 cavities used
seasonally by Pygmy Nuthatches were compared to determine
factors influencing cavity choice. Seasonally used cavities
differed significantly in from 1-5 characteristics. Snag
management should provide for seasonality in snag/cavity
quality and bird choice.
~
INTRODUCTION
The importance of snags and dead wood within
live trees to populations of secondary cavity
nesting birds is well documented (eg. Allen and
Nice 1952, Balda 1975, Cunningham et al. 1980).
Further support for their value has been obtained
where cavity density was increased by the supply of
nest boxes (eg. von Haartman 1957). Several
investigators have offered management proposals of
a minimum number of snags necessary to maintain
populations of secondary cavity nesters (SCNs)
(eg. Balda 1975). Silviculture practices have
begun to incorporate these recommendations.
A basic assumption in most present management
plans is that a sufficient quantity of snags of a
minimum size will provide cavity nesters (both
primary and secondary) with suitable nest sites.
Yet to the birds a snag may not be a snag; a
cavity may not be a cavity. Variation in the
macro- and/or microhabitat of the snag or cavity
may affect the suitability of the site for a nest.
Furthermore many SCNs are resident species and may
select different cavities as nocturnal roost sites
(Cunningham et al. 1980). These species, more
correctly classified as secondary cavity users
(SCUs) may take advantage of a cavity's qualities
seasonally at which time the site is preferred.
....
,
unpub.). This pattern is observed throughout the
year, with the exception of the nesting period.
Pygmy Nuthatches are further known to select
different cavities for nesting and winter roost
sites (Cunningham et al. 1980). Guntert (unpub.)
has identified summer and fall/spring roosts as
well.
The objectives of this study were: 1) to
determine characteristics which distinguish those
cavities preferred td thin a season by Pygmy
Nuthatches, 2) to identify the possible benefits
of those cavity characteristics selected within a
season, and 3) to outline important considerations
for the future management of habitat for Pygmy
Nuthatches and other SCUs.
STUDY AREA
.,
The most common permanent resident SCU in
coniferous forests of western North America is the
Pygmy Nuthatch (Sitta pYgmaea). The species is
unique in that it roosts communally in tree
cavities normally in groups of 10-14 birds (Guntert
~aper presented at the Snag Habitat Management
Symposium (Northern Arizona Unitersity, Flagstaff,
June 7-9, 1983).
2Douglas B. Hay is Instructor of Biological
Sciences, Northern Arizona University, Flagstaff.
3Marcel GUntert is Research Associate,
Northern Arizona University, Flagstaff.
4Present address is Museum of Zoo.logy,
University of Zurich, Switzerland.
The study area was located within 3.4 km of
the canyon rim at Walnut Canyon National Monument,
Coconino County, Arizona. Ponderosa pine was the
predominate vegetation type in the area. Mature
pine stands with high snag densities characterize
the community within the monument. North of the
monument boundary in Coconino Nat:l. onal Forest land
logging and fuel wood collecting have removed
virtually all snags and have greatly reduced foliage
volume. Forest Service road 303 separates the two
jurisdictions.
METHODS AND MATERIALS
Characteristics of 46 cavities used seasonally
by banded flocks of Pygmy Nuthatches were recorded.
Cavities were assigned to general seasonal categories
(eg. nests, summer roosts, ~linter roosts, and fall/
spring roosts) according to observed use by the
birds.
Fourteen characteristics were used to discriminate between seasonal cavity usage groups. The
total tree height, DBH, and general condition were
recorded for the tree containing the cavity. The
height to the cavity and diameter of the trunk or
branch surrounding the cavity were recorded.
117
Measurements were taken of cavity depth, width,
vertical length, entrance hole area, and accessory
holes. The hole's direction of orientation and
placement ,.,ere noted. Cavity placement (branch,
trunk) and angle (of the branch) were also noted.
Cavity volume was directly measured in over 90%
of the cavities. A vacuum hand pump "ras used to
fill an elastic reservoir (condom) located within
the cavity. When the cavity space was filled the
pump was removed and the water within was measured
in a 500cc graduated cylinder. Cavity volumes
which could not be measured in this way were
estimated from inside dimensions.
fall/spring roosts, and winter roosts were tested
for significance using two-sample t-tests. A
discriminant analysis lfas employed to determine
those characteristics most important in distinguishing between seasonal usage groups. In this test
fall and spring roosts ~.,ere split into fall roosts
and those cavities used in both the fall and the
spring. Nests used through the summer and fall
were also assigned to a specific group. Each
cavity was assigned to only one of the usage groups.
RESULTS
Statistical comparisons between mean
characteristics of seasonal cavity groups indicated
Daily radiation absorbed by the cavities was
nest cavities were most distinctive (Table 1).
calculated for four representative days of the year.
J Nest cavities are significantly lower in the tree
A series of four clinometers l'Tere constructed, ~
'. than summer roosts (SR), fall and spring roosts
enabling the observer to trace the arc of the sun
> (F/S), and winter roosts (t=2.29, .010<pc.025;
on February 15, April 15, June 15, and August 15,
:.. t=3.88, pc.005; t=3.25, pc.005; respectively).
regardless of the date of data collection. The
SR, F/S, and winter roosts ~1ere located in signifi.. ....·,; degree of cover which lfould obstruct direct
cantly taller trees (t=2.57, .OlO•pc.025; t=5.34,
insolation were noted by the observer for each .'.date.
pc.005; t=3.62, pc.005; respectively). Nests
Data was called out to an individual recording ~the
averaged smaller entrance holes than SR and F/S
information on the ground. Qualitative cover
roosts (t= ~75, .025cpc.05).
categories of clear sky, light shade, medium sijade,
and heavy shade were used. Tabulated values of
Fewer characteristics distinguished between
direct radiation were corrected for shade periods
winter, SR, and F/S roosts. Winter roosts differed
by reducing the values by 20, 40, and 607. for light,
in having significantly smaller entrance holes
medium, and heavy shade, respectively. These
than SR and F/S cavities (t=44.8, pc.OOl; t= 42.8,
percent reductions \'lere determined from the measurep .001; respectively). F/S roosts were located
ment of qualitative shade classes on the penetration
higher in the tree than SR roosts (t=2.08,
of radiation recorded by a light meter. Indirect
.025cpc.05).
solar radiation was calculated as a time and date
specific percentage of the direct solar radiation
Cavities were usually located in the trunk of
penetrating the atmosphere on a clear day (Gates
snags or lightning strikes or dead branches of
1980). Calculations of daily absorbed radiation
live trees. The majority of nests and winter
were made using Morhardt's (1975) equation:
roosts were located in trunks (Table 2). SR and F/S
Q = (A•SA)DR + 0.5((A·IR) + A•R(DR + DI))
where A = absorbtivity, SA = radiated surface area
roosts were distributed somewhat evenly between
or ((cavity length/cavity diameter)/cosine of the
trunks and branches.
angle of the sun), DR= direct radiation, IRa
indirect radiation. Calculations were made using
Calculations of average daily absorbed radiation
a specifically designed computer program, which
indicated a wide degree of variation within a
corrected for shade.
seasonal usage group (Table 3). Comparisons of mean
absorbed radiation were only significantly different
between SR and F/S roosts (t=2.95, .OOScpc.OlO).
Differences between characteristics of seasonal
This difference occurred in the Spring (April 15)
cavities were evaluated by two methods. Mean values
during which time F/S roosts absorbed more radiation.
of recorded characteristics of nests, summer roosts,
Table 1.--Mean values of characteristics of seasonally used
cavities.
!!:!.!
height
(m)
DBH
(em)
h.eight diameter
(m)
(em)
Cavity
depth width length volume accessory
hole area
(an2)
(mm) (mm) (mm)
(cc)
~
area azimuth
(cm2)
Nests i
(n=lO) SD
9.8
5.8
39.2
20.0
5.6
2.1
25.2
18.3
63.8
22.5
73.1
29.5
147.1
76.8
528.9
150.5
5.0
1.2
13.0
0.9
109.5
91.1
SR
i
(nell) SD
18.9
9.9
64.5
19.5
7.9
2.5
22.0
11.5
73.5
19.7
73.4
30.9
148.2
70.2
630.2
374.6
36.0
85.6
22.2
15.1
160.5
112.1
SD
24.5
6.1
63.3
17.3
10.8
3.5
34.9
26.1
75.6
24.0
68.1
13.4
172.6
24.9
616.2
226.8
1.5
0.4
21.7
14.0
161.8
130.7
Winter x
(n=8)
SD
23.2
9.1
73.1
20.3
9.3
2.6
56.6
22.6
73.2
21.6
68.7
18.9
137.2
44.5
683.1
393.5
3.6
0.5
11.2
6.0
188.2
58.6
F/SP
(n=9)
x
118
Table 2.--Loeation of seasonal cavities (%).
Branch
Trunk
Nests
(n=lO)
91.7
SR
{n=ll)
Table 4.--Significance between groups as defined by
discriminant analysis (n=46).
SR
F
p
Fall
F
p
N,SR,F
F
8.3
45.5
54.5
p
F/SP
(n=9)
66.6
33.3
Winter
Winter
(n=8)
87.5
12.5
F/SP
Table 3.--Mean daily absorbed radiation of
seasonally used cavities(Langleys/day).
Breeding
(Jun 15)
Nest
x
SD
SR
x
SD
F/SP
x
SD
Winter
x
SD
Summer
(Aug 15)
Spring
(Apr 15)
SR
Fall
1.68
.13
1.70
.13
2. 73
.01
1.23
.30
3.10
.01
2.21
.05
1.15
.35
2.00
.OS
2.15
.05
2.16
.05
1.31
.27
1.09
.39
Nests
F
p
F
p
3.99
.00
2.52
.02
N1 SR 1 F
Winter
0.91
.51
DISCUSSION
Several characteristics of the macrohabitat
distinguished beb~een cavities used seasonally.
Trees housing nest cavities t-rere shorter and consequently had a significantly smaller DBH. The
selection of these mmaller trees and lower cavities
within them probably represents a response to wind.
By utilizing cavities closer to the ground convective
heat loss may be greatly reduced. Greater cavity
height of Fall/SP roosts in comparison to SR was
identified in more absorbed spring/fall radiation
by the prior. In contrast nest cavities appear
to be positioned to obtain moderate insolation,
while being shielded from ,.,ind. Average radiation
absorbed by nests was not significantly different
from that calculated for any other cavity group
during the breeding season.
Winter
(Feb 15)
2625
1695
5470
2402
5939
2527
601
316
3392
694
6197
1980
6176
2909
925
339
7061
7179
10957
8983
14265
9134
1504
1229
4727
4680
7057
7114
6937
5654
989
957
A discriminant analysis was made to differentiate between six seasonal usage groups. These groups
were nests, summer roosts, fall roosts, nests used
during the summer and fall of the same year, winter
roosts (usually used during other seasons), and
fall and spring roosts. Eight variables were found
to be significant in explaining the variation
between groups: tree height (TH), DBH, height of
cavity in the tree (CH), diameter of the trunk or
branch surrounding the cavity (DC), cavity length
(CL), accessory hole area (AHA), compass orientation
of the hole {HAZ), and density of the wood (D).
On the basis of these eight characteristics
nests were found to be significant from all groups
except nests also used in the summer and fall
{Table 4). Summer roosts were significantly different from all other groups, except for those cavities
used only in the fall. All other seasonal usage
groups were insignificantly different from one
another.
Three discriminant functions were used in the
analysis. CH, AHA, DBH, and TH were included in
the first function. The second discriminant
function was based on DC. CH, RAZ, and D identified
the third function. Together these functions correctly classified 657. of the 46 cavities.
119
A variety of microhabitat variables were
useful in differentiating seasonal cavities.
Entrance hole area and the ares of other accessory
holes both are important factors affecting the
physical movement of .air through the cavity (Hay
unpub.). Larger hole areas and smaller cavity
lengths increase the relative air flux. In
addition the greater the relative difference in
outside ambient temperature vs. inside (roost)
temperature the more rapid the air movement.
Selection of cavities with large entrance areas
during periods of time when temperature differences
are small and the birds are communal (Summer and
Fall/Spring) encourages greater air flux. During
much of the nesting period fewer birds are roosting
together. Generally one to six nights after
fledging the family switches to a SR (Guntert,
unpub.). Winter temperature differences are high
offsetting the negative effect of a smaller hole
size on the mass transfer of air.
The location of more SR and Fall/SP roosts in
branches may further aid communal birds to acquire
adequate ventilation. Slanting branches allow
individuals to position themselves closer to the
cavity entrance, without the extra energy required
to cling to the walls of vertical cavities.
Other char acteri stics distinguishing the microclima te were identified as significant in the
di scriminant anal ysis. The diameter of the trunk or
branch at the caVity as Hel l as cavity densi t y and
compass orient a tion at the entrance (a zimuth) a ppear
rel a ted t o seasonal needs for convective a nd r adiant
heat gains or l osses by the cavity. Bigger cavities
with hol es oriented more t o the south may hea t up
more in the \-Tinter an d be on the l ee side of tr ees
when northerly \·linds bl o1·1. Nest cavities fa cing
east wi ll warm fas t er in the mornings and be more
protected by prevailing wes t erly wi nds . Less dense
wood in ne sts and \.,inter r oos t s buffers the cavities
from rapid t emperature shif t s .
On the basis of these and other si gnificant
fa ctors the discriminant a nalysis ~ras unable to
classify 35% of the 46 cavities correctly. However
of those incorrectly classif ied 95% are l oca t ed in
the section of t he study area ~rhi ch has been logged
and heavil y cut fo r fuel wood . Thi s strongly
suggests that a lter ation of the birds ' habitat,
resulting i n considerable reduction in snag
densities , forces Pygmy Nuthatche s to use a typica l
cavities. Cavi ty selection appears s trongly tied
to a variety of seasonal behavioral and physiological responses of the birds ( J~y , unpub.).
The management practices of snags ~rhi ch do not
provide a n adequate range in snag and/or cavity
quality may aff ect the over a ll biology of the
species, its survivorship, and r eproduction .
CONCLUSIONS
Seasonal cavities sel ect ed as nes t a nd/ or
roost sites by Pygmy Nuthatches were found to
differ in several characteris t ics . These fa ctor s
contribute to interseasonal variation in t he macroand/or microhabitat of the cavity. In the a bsence
120
of an array of cavi tie s from vrhich to choose, the
birds a re forced to u se a typica l cavities within a
season. As cavity selection i s interrelated to the
overall biology of the species, management of snag
and/ or cavity quality, r a ther than quantity, appear s
critica l. We recommend more baseline research of
this type conducted with the goal of determining
important cavity characteris tics for seasonal
usage by resident s econdary cavity users.
LITERATURE CITED
Allen, Robert R. and Hargaret 11. Nice . 1952.
A s tudy of the breeding biol ogy of the
purple martin ( Prognesubis). American
11idland Na tura lis t 47: 606-665.
Balda, Russell P. 19 75. The rela tionship of
secondary cavity nes ters snag densities in
western coniferous for ests. U. S.D.A. Forest
Service Wildlife l ~bi ta t Technica l Bulletin 1,
37 p . South\-Testern Region, Albuquerque, Ne\.,
Mexico .
Cunningham, James B., Russell P. Bal da, and William
S. Gaud. 1980. Selection and use of snags
by secondary cavity-nesting birds of the
ponderos a pine fores t. U.S.D.A . Forest
Service Res earch Paper RH-222, 15 p. Rocky
Mountain Forest a nd Range Experiment Station,
Fort Collins, Colora do.
Gate s , David 1'1 . 1980. Biophysical ecology. 611 p.
Springer-V erlag , Ne1'1 Yorl{.
Haartman, Lars von. 1957. Adaptation in hole
nes ting birds . Evolution 1 : 339- 347 .
Morhardt, Sylvia S. 1975. Use of climate diagrams
to des cribe microhabitats occupied by Belding
Ground Squi rr el s and to predict r ates of change
of body temperature. In Perspectives of BioPhys ica l Ecology (ed. D. H. Gates a nd R. B.
Schmerl) pp 303-324. Springer-Verlag, N.Y.