Grassland Birds Orient Nests Relative to Nearby Vegetation

7 Pages·2002·5.1 MB·English

by  Steven T Hoekman

Wilson Bull., 1 14(4), 2002, pp. 450^56
GRASSLAND BIRDS ORIENT NESTS RELATIVE
TO NEARBY VEGETATION
STEVEN T HOEKMAN,! 24 i j BALL,* AND THOMAS E FONDELL* ^
—
ABSTRACT. We studied orientation ofnest sitesrelativeto nearby vegetationfordabblingducks(Cinnamon
Teal, Anas cyanoptera\ Blue-winged Teal, A. discors; Gadwall, A. strepera\ Mallard, A. platyrhynchos; and
Northern Shoveler, A. clypeata) and Short-eared Owls (Asioflammeus) in ungrazed grassland habitat during
1995-1997 in westcentral Montana. We estimated an index of vegetation height and density in intercardinal
directions (NE, SE, SW, NW) immediately around nests. All species oriented nests with the least vegetation to
the southeast and the most vegetation to either the southwest or northwest. Furthermore, maximum vegetation
around nests shifted from the southwest to the northwest with increasing nest initiation date, apparently as a
response ofindividuals tracking seasonal change in the afternoon solarpath. Thus, nests were relativelyexposed
to solarinsolation duringcool morninghoursbutwere shadedfromintenseinsolationintheafternoonthroughout
the breeding season. We suggestthat nest microhabitat was selected in partto moderatethethermalenvironment.
Received 15 May 2002, accepted 24 August 2002.
Natural selection should favor nest sites Webb 1993, Nelson and Martin 1999). Grass-
that maximize reproduction and survival of lands birds are exposed to wide daily and sea-
adults, and selection of nest microhabitat ap- sonal fluctuations in temperature, wind, and
pears to have evolved in response to predation moisture, making nest microhabitat especially
and microclimate (Walsberg 1981, Gloutney important. We studied nest microhabitat selec-
and Clark 1997). Nest microclimate can influ- tion by dabbling ducks (Cinnamon Teal, Anas
ence the energetic costs of incubation and the cyanoptera\ Blue-winged Teal, A. discors',
development and survival of eggs and young Gadwall, A. strepera'. Mallard, A. platyrhyn-
(Haftorn and Reinertsen 1985, Webb 1987). chos', and Northern Shoveler, A. clypeata) and
Many birds select nest microhabitats that ame- Short-eared Owls {Asioflammeus). Specifical-
liorate climatic factors and reduce physiolog- ly, we asked if orientation of nests relative to
ical stress on adults, eggs, and young. Nest nearby vegetation differed from random.
sites often are oriented nonrandomly relative Based on patterns observed in our data, we
to nearby objects or vegetation, presumably to also asked if a relative shift in orientation
accrue thermal benefits and protection from from southwest to northwest oecurred with in-
severe weather (Walsberg 1981, With and creasing nest initiation date.
Webb 1993, Norment 1993). Birds that con-
METHODS
struct nests in cavities or with tunnel entrances
—
frequently orient entrances relative to the solar Study area. We conducted research on 227 ha of
path and prevailing winds (Facemire et al. ungrazed grassland habitat in the Mission Valley (47°
1990, Hooge et al. 1999, Wiebe 2()01). Ori- 24' N, 1 14°24' W), 80 km north of Missoula, Mon-
entation of nests relative to surrounding veg- tana. during 1995-1997. Glacial topography character-
etation also has been observed for relatively izes the area, which exhibits low reliefand high den-
sities of wetlands (Lokemoen 1962). Vegetation was
small-bodied open cup nesters, but large-bod- typical of habitat managed for upland-nesting ducks
ied birds have received little attention (Wals- and Ring-necked Pheasants (Phasianus colchicus).
berg 1981, Petersen and Best 1985, With and Plant communities were structurally diverse, dominat-
ed by introduced cool season grasses, primarily inter-
mediate wheatgrass (Agropyron intermedium), quack-
' Montana Coop. Wildlife Research Unit, U.S. Geo- grass (A. repens), smooth brome {Bromus inermis),
logical Survey, Univ. of Montana. Missoula, MT Kentucky bluegrass (Poapratensis), andorchardgrass
59^812, USA. (Dactylis glomerat—a).
-Current address: Inst, of Arctic Biology, Univ. of Field methods. We commenced nest searching in
Alaska-Fairbanks, Fairbanks, AK 99775, USA. late April and searched twice more at 21- to 25-day
^Current address: Alaska Science Center. 1011 E. intervals using a cable chain device (Higgins et al.
Tudor, Anchorage. AK 99503. USA. 1969). We monitored nests and estimated nest initia-
^Corresponding author: E-mail: tion dates following Klett et al. (1986). Ob.servers
[email protected] could not always distinguish between female Cinna-
450
Hoeknmn et cil. • GRASSLAND BIRDS ORIENT NESTS 451
TABLE I. Candidate models examining support for orientation of nests and random locations relative to
nearby vegetation and examining support for a seasonal shift in orientation for six species of ground-nesting
grassland birds in westcentral Montana, 1995-1997.
Modelset Model“ Interpretation
Orientation Constant No support for orientation.
Direction Orientation by intercardinal direction, consistent across
years.
Direction X Year Orientation by intercardinal direction, variable among
years.
Shift Constant No support for shift.
Initiation Date Shift explained by individual response to nest initiation
date.
Median Initiation Date Shift explained by interspecific differences in nesting
phenologies.
Initiation Date Species Shift explained by individual response to nest initiation
date, but species differ in orientation for a given date.
Initiation Date X Species Shift varied among species.
Modelsdesignatedbysourcesofvariationindata.
mon and Blue-winged teals, and we assumed that our (SPSS, Inc. 2000). We used Akaike’s Information Cri-
sample of nests reflected the local predominance of terion corrected for sample size (AIC^) to select par-
breeding Cinnamon Teal (U.S. Fish and Wildlife Ser- simonious models for parameter estimation and infer-
vice, National Bison Range unpubl. data). These teal ence (Burnham and Anderson 1998, Anderson et al.
arecloselyrelated,physically similar(JohnsonandSo- 2000). For each analysis, we created an apriori set of
renson 1999), and likely selected similar nest sites. candidate models that mathematically represented dif-
Therefore, we pooled these species (hereafter “teal”) ferent biological hypotheses concerning orientation;
in analyses. We sampled vegetation when nests were Akaike weights allowed us to assess relative support
no longer active, because we suspected that intensive among these models. To assess the importance of in-
sampling could increase nest abandonment and pro- dividual variables, we summed Akaike weights for all
vide visual and olfactory cues that may increase nest models containing a variable.
predation. We assumed that rates of change in vege- To examine orientation, we modeled variation in the
tation around each nest were similar and that relative deviation of estimates of vegetation at each sampling
differences at the time of sampling reflected patterns point from the mean for that location. We considered
at the time ofnest site selection. We used a 3.5 X 3.5- the categorical predictive variables intercardinal direc-
cm pole alternately marked black and white in 2-cm tion (Direction) and year (Year). For each species and
intervals (modified from Robel et al. 1970) toestimate forrandom points, we created three apriori models to
a“nvegientdaetxioonf”)v.egFertoamtioanhehiegihgthtofan1dmdaennsditaydi(shtearnecaeftoefr eaxpaemricneeivseudppsoeratsofnoarlorshiiefnttaitniornel(aTtaibveleve1g).etTaotieoxnamfirnoem
4 m during 1995 and 2 m during 1996-1997, we re- southwest to northwest, we modeled variation in the
corded (in cm) the lowest visible interval. We short- difference between estimates in these directions. We
ened the observation distance because we thought that hypothesized that orientation shifted in response to
estimates taken from 2 m were more likely to be in- seasonal northward change in the solar path and that
fluenced only by vegetation near the pole. We made a shift could result eitherfrom response ofindividuals
estimates while facing nests in the four intercardinal or interspecific differences. A shift could occur if in-
directions (NE, SE, SW, and NW) with the pole cen- dividuals, regardlessofspecies,alterednestorientation
tered in the bowl; estimates were calibrated to ground inresponse to nest initiation date. Thus, we wouldpre-
level. We conducted identical sampling at random lo- dict that the individual covariate nest initiation date
cations. We identified random locations from a grid (Initiation Date) would best explain seasonal change
superimposed on an aerial photo and then chose each in orientation. Alternatively, if individuals within a
sampling point by tossing a stick in a randomly se- species did not alter nest orientation in response to
lecteddirection. Nestorientationtypicallyhasbeende- initiation date, a shift could occur if each species se-
fined relative to a salient object (e.g., tunnel entrance lected orientation appropriate to its nesting phenology.
or stem of nesting shrub) that describes aspect ofex- Thus, we would predict that the group covariate of
posure. Lacking a clearly defined reference object, we median nest initiation date for each species (Median
defined orientation as nest placement relative to veg- Initiation Date) would best explain seasonal change in
etation immediately around nests. orientation. We developed five models to assess sup-
—
Statistical analyses. We modeled orientation at portfora shift and, ifpresent, toattempttodistinguish
nests and random points using General Linear Models if it was better explained by differences in orientation
452 THE WILSON BULLETIN • Vol. 114, No. 4, December2002
TABLE 2. Selection results for models examining orientation of nests relative to nearby vegetation for six
species ofground-nesting grassland birds and random locations during 1995-1997 in westcentral Montana. We
present results for models with AIQ weights >0.025.
AICc
Group n Model^ log(Lf AAICc‘' weight^
Teak 168 Direction 5 -272.5 0.00 0.93
Constant 2 -278.2 5.23 0.07
Gadwall 152 Direction 5 -262.9 0.00 0.67
Constant 2 -266.8 1.42 0.33
Mallard 276 Direction 5 -513.2 0.00 0.54
Constant 2 -516.5 0.34 0.46
Northern Shoveler 196 Direction 5 -245.1 0.00 0.99
Short-eared Owl 100 Direction 5 -167.3 0.00 0.98
Random 1244 Constant 2 -1,676.7 0.00 0.63
Direction 5 -1,674.2 1.05 0.37
“Modelsdesignatedbysource(s)ofvariationin vegetationimmediatelyaroundnests.
^Numberofestimatedparameters.
Maximizedlog-likelihood.
DifferenceinAICj.relativetomodel withlowestvalueforeachgroup.
®Weightofevidenceforbeingthebestapproximatingmodel foreachgroup,
fCinnamonandBlue-wingedtealspooled.
among species or responses of individuals regardless points, the Constant model received moder-
of species (Table 1). We also considered the categori- ately more support (AIC^ weight 1.7X) than
cfaelrepnrceedsicitnororsipeenctiaetsio(nSpaemcioensg)tsopeecsiteismaatfeteprocteonnttiraollldiinfg- the Direction model (Table 2). Although the
for a seasonal shift. To assess support for a seasonal Direction model received some support, esti-
shift in orientation at random locations, we considered mated differences in vegetation among direc-
a constant model versus a model with the continuous tions from this model were small, and 95%
covariate date (Date). Cl’s for all estimates overlapped zero (Fig. 1).
RESULTS Direction X Year models received virtually no
— support (AICc weights <0.025).
Nest orientation. We included 225 nests in Seasonal shift in orientation.—Among
our analyses: 42 teal, 40 Gadwall, 69 Mallard, models examining a seasonal shift in orien-
49 Northern Shoveler, and 25 Short-eared
tation, two models including Initiation Date
Owl. Sixty-eight nests were from 1995, 86
received similar support as the best approxi-
from 1996, and 71 from 1997. The Direction
mating model (Table 3). These models re-
model was selected as the best approximating ceived >3X more support than a model in-
model of orientation for each species (Table
cluding Median Initiation Date, and other
2). Strong support for the Direction model ex- models received little support. Summed Akai-
isted for three species (AIQ. weights were
>13X those for the next best model). Only vkaeriwaebilegshtpsrofvoirdemdod>e6lXs minocrleudsiunpgpoirntdifvoirdutahle
moderate support existed for Mallards and
predictive importance ofInitiation Date (0.85)
Gadwalls (AIC^. weights 1.2-2.0 those for the
next best model), but patterns in estimates and relative to Median Initiation Date (0.13), in-
effects were similar for all ducks. Vegetation dicating that a seasonal shift in orientation
to the southeast ofnests was relatively low for was best explained by individual response to
all species. Species nesting relatively early nest initiation date. The top model predicted
(Mallard, Northern Shoveler, and Short-eared that vegetation to the southwest relative to the
Owl) had relatively high vegetation to the northwest of nests decreased ([3, = —0.128;
southwest of nests, but species nesting rela- 95% Cl -0.047, -0.209; n = 225) as esti-
tively late (Gadwall and teal) had relatively mated nest initiation date increased (Fig. 2).
high vegetation to the northwest (Fig. 1). For The predicted shift in mean vegetation was
each species, the 95% Cl for the difference 10.6 cm across the range of nest initiation
between directions with highest and lowest es- dates, and the combined mean for both direc-
timates did not include zero. For random tions was 24.6 cm. Thus, nearly half(43%) of
Hoeknum et al. • GRASSLAND BIRDS ORIENT NESTS 453
ences in vegetation between southwest versus
northwest were predicted to be greater by
about 5 cm for Short-eared Owls relative to
[I^ ducks. In contrast to patterns at nests, support
for a seasonal shift at random locations was
Teal
(16May) equivocal (Table 3). The relatively small slope
from the Date model = -0.031; 95% Cl
((3,
=
0.009, —0.070; n 616) could not account
for patterns at nests.
g DISCUSSION
Gadwall
<D (28May) Microclimate is thought to be an important
c determinant of nest microhabitat selection for
.9 small birds (Walsberg 1985), because they are
ro
0 relatively susceptible to thermal stress. Many
O)
0> li, L small birds appear to orient nest sites to gain
o thermal benefits (Walsberg 1981, With and
OX0 (M2a9llAaprrdil) Webb 1993, Nelson and Martin 1999). Expla-
_c nations of nest orientation have focused pri-
c marily on optimization of thermal microcli-
0
0 mate relative to solar insolation and convec-
E L]
E tive cooling, the major sources of heat ex-
o Northern change at nests (Webb and King 1983). Birds
Shoveler nesting in cool or hot environments may in-
(3May)
crease or decrease exposure to the sun (Inouye
T
et al. 1981, Finch 1983, Petersen and Best
1985), and birds exposed to wide daily vari-
T ation in temperature may moderate thermal
variation by increasing heat gain in the morn-
SOhwolrt(1-eMaareyd) binegrgbu1t98d1e,crWeiatshinagnditWienbtbhe19a9f3t,erHnoooonge(Weatlasl-.
1999, Nelson and Martin 1999). Alternatively,
birds may orient nests relative to the prevail-
ing wind to facilitate convective cooling or to
.1, ^ reduce exposure to weather (Facemire et al.
Random 1990, Vihuela and Sunyer 1992, Norment
Locations
1993).
NE SE SW NW We believe that support for nest orientation
Intercardinal Direction in this study was novel for large-bodied grass-
FIG. I. Deviations (+I SE) in an index of vege- land birds, and we suggest that nest sites were
tation height and density in intercardinal directionsrel- selected in part to ameliorate nest microcli-
ative to the mean for that location for nests of six mate. Consistent patterns oforientation across
species ofground-nesting grassland birds (median ini- species suggested common thermal benefits to
tiation date) and random locations during 1995-1997 nest microclimate. The pattern of least vege-
in westcentral Montana. Reference lines indicate no tation to the southeast and most to the south-
difference from mean.
west or northwest ofnests suggested that birds
selected sites that were relatively exposed in
the total vegetation was predicted to shift di- the morning to maximize heat gain but shaded
rection, suggesting that the magnitude of the in the afternoon to avoid excessive insolation.
estimated effect was biologically important. Energetic costs of incubation increase as am-
The second best model predicted a more mod- bient temperature declines below a lower crit-
erate effect of estimated initiation date ([3, = ical temperature (T,^; Haftorn and Reinertsen
-0.083; 95% Cl 0.016, -0.181), and differ- 1985). Assuming body temperatures of 40° C
454 THE WILSON BULLETIN • Vol. 114, No. 4, December2002
TABLE 3. Selection results for models examining seasonal shift in relative vegetation immediately to the
southwestversus northwestofnestsofsix speciesofground-nesting grasslandbirdsandrandom locationsduring
1995-1997 in westcentral Montana. We present results for models with AIQ. weights >0.02.
AlCe
Group Model^ log(Lf AAICc‘‘ weight^
Nest Initiation Date 3 -458.8 0.00 0.45
Initiation Date + Species 6 -455.7 0.23 0.40
Median Initiation Date 3 -460.0 2.51 0.13
Random Date 3 -1,173.0 0.00 0.50
Constant 2 -1,174.0 <0.01 0.50
^Modelsdesignatedbysource(s)ofvariationindifference(southwest — northwest)ofvegetationimmediatelyaroundnests.
^Numberofestimatedparameters.
Maximizedlog-likelihood.
^DifferenceinAIC^relativetomodel withthelowestvalueforeachgroup.
^Weightofevidenceforbeingthebestapproximatingmodelforeachgroup.
aruj typical masses for breecding females, es- and thereby reduced energetic costs of incu-
timateti T,^. for our stu(jy species rangecd be- bation.
tween about 10°C for Mallarcds to about 16° In contrast, ambient temperatures in the af-
C for teal (Calder ancd King 1974: equation ternoon often reached 35°C, and ground tem-
20). Ambient temperatures on our stu(iy area peratures likely reached levels causing stress
early in the nesting season or at night typically to nesting females and reducing growth and
were below T,,. of females. Similarly, temper- survival of eggs and young (Webb 1987, Nel-
atures at nests of Mallards and Blue-winged son and Martin 1999, Conway and Martin
Teal have been demonstrated to be below their 2000). Nesting ducks typically take incubation
T,^. for most of the night (Gloutney and Clark recesses during early afternoon (Gloutney et
1997). Selecting nests with relatively high al. 1993). Unattended eggs can reach delete-
southeast exposure likely facilitated heat gain rious temperatures rapidly when exposed to
during the morning (Nelson and Martin 1999) insolation (White and Kinney 1974, Bennett
and Dawson 1979), suggesting that increased
shading from afternoon insolation may be im-
portant for moderating egg temperature.
Gloutney and Clark (1997) found that Mal-
lards and Blue-winged Teal selected nest sites
shaded from intense insolation during after-
noon hours, and patterns we observed were
consistent with that observation. Support for
a shift from southwest to northwest in vege-
tation near nests with increasing nest initiation
dates suggested that individual females
tracked seasonal changes in the solar path
when selecting nest sites. When nesting began
in early April, the sun was in the southern
hemisphere of the sky for most of the day.
heiFgIhGt.a2n.d dDeinfsfietryenicmemebdeitawteeelnyatnoitnhdeexsoouftvhewgeesttatainodn However, the sun was near its northernmost
the northwest of nests of dabbling ducks (Cinnamon extent from late May to the end of July,
Teal, Anas cyanoptera: Blue-winged Teal, A. discors: spending nearly half of the afternoon in the
Gadwall, A. strepera: Mallard, A. platyrhynchos: and northern hemisphere of the sky and setting to
Northern Shoveler, A. clypeata) and Short-eared Owls the northwest. Others have observed relatively
(Asia flammeus) relative to estimated nest initiation abrupt seasonal switches from nest orienta-
date during 1995-1997 in westcentral Montana, with
95% Cl for predicted mean values. Positive values in- tions increasing to those decreasing heat gain
dicate more vegetation to the southwest relative to the as ambient temperatures increased (Austin
northwest; dotted line indicates no change over time. 1976, Finch 1983). However, we believe that
Hoeknum et al. • GRASSLAND BIRDS ORIENT NESTS 455
our interpretation ofa relatively subtle change typical orientation at a site often experience
in orientation to maintain shading in response decreased reproductive success (Austin 1976,
to seasonal change in the solar path is novel. Hogstedt 1978, Vifiuela and Sunyer 1992,
Alternatively, relatively greater vegetation Hooge et al. 1999, Wiebe 2001). Predation is
to the southwest and northwest may have pro- the primary cause of nest failure in most birds
vided protection and reduced convective heat (Martin 1993). Most studies of nest microhab-
loss at nests from prevailing winds from the itat selection by grassland birds have focused
west-northwest (National Oceanic and Atmo- on the relationship between concealment and
spheric Administration unpubl. data). How- predation, thereby tacitly assuming that mi-
ever, the shift in orientation through the breed- crohabitat selection responds primarily to pre-
ing season was more consistent with the hy- dation pressure (Clark and Nudds 1991,
pothesis that shading was primary to orienta- DeLong et al. 1995). Unsuitable nest micro-
tion. Gloutney and Clark (1997) reported only climates could increase energetic needs of
limited selection by Mallards and Blue- adults and hence decrease nest attentiveness,
winged Teal to ameliorate climatic factors at which could prolong exposure to predation
nests and suggested that microclimate may (White and Kinney 1974, Webb 1987, Yanes
have little influence on site selection in ducks. et al. 1996, Loos 1999) or increase vulnera-
However, effects of orientation may not have bility of nests to predators (Hogstedt 1978,
been detected because microclimate was mea- Martin et al. 2000a, 2000b). Further study of
sured >10 cm outside the center of nests (ori- selection of nest microhabitat in grasslands
entation unspecified). In our study, birds ori- will be needed to determine consequences for
ented nests relative to small bunches ofgrass, nest microclimate, parental energetics and be-
and microclimate benefits likely did not ex- havior, and reproductive success.
tend much beyond the nest bowl. ACKNOWLEDGMENTS
Selection of habitat type also may amelio-
rate microclimate. Ducks nesting in relatively We thank S. Garner, E. Hess, K. Jones, A. Knapp,
tall, uniform cover (e.g., the parkland habitat E. Leaf, B. Lowry, D. Miller, M. Myerowitz, G. Park-
in Gloutney and Clark 1997) may have little hurst, and S. Schmitz forassisting with field work. We
are grateful for funding from the U.S. Fish and Wild-
need or even opportunity to orient nests, and
life Service; Montana Fish, Wildlife, and Parks; U.S.
fine scale differences in microhabitat and mi- Geological Survey, Northern Prairie WildlifeResearch
croclimate may be small within suitable nest- CenterandCooperative Units Program; PrairiePothole
ing habitat. In relatively sparse and heteroge- Joint Venture; and the Univ. ofMontana. Logistic sup-
neous vegetation (e.g., sites dominated by port was provided by L. Clark, S. Cox, J. Grant, B.
bunchgrasses, grazed sites), fine scale varia- West, and D. Wiseman. Site access was provided by
tion in microhabitat and hence microclimate aMnodntWainladliFfiesh,SeWrivlidclei.feW,eantdhaPnakrksD.anPderktihensU.aSn.dFtiwsoh
may constrain suitable nest sites. anonymous referees for providing constructive com-
The orientation that we observed was un- ments.
likely to have been an artifact of vegetation
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