Table Of ContentThe Wilson Journal ofOrnithology 118(4):452—460, 2006
INVESTMENT IN NEST DEFENSE BY NORTHERN1 FLICKERS:
EFFECTS OF AGE AND SEX
RYAN J. FISHER123 AND KAREN L. WIEBE
—
ABSTRACT. At early breeding stages, male woodpeckers invest heavily in nest construction and defense,
but parental contributions to brood defense among Picidae are not well known. We studied the Northern Flicker
(Colaptes auratus) to determine whether sex, age, brood size, body size, or body condition influenced defense
behavior. When presented with a model predator (red squirrel, Tamiasciurus hudsonicus) during the brood-
rearing period, parents exhibited a range ofbehaviors, such as blocking the nest hole, diving at the model, and
striking the model; however, defense scores did not differ between males and females aged 1, 2, or 3+ years
old. Although we predicted that defense level would be positively correlated with brood size, we found no such
relationship. Adultbody size andcondition also were not relatedtodefense intensity. Weconcludethatthe sexes
may exhibit similar levels of defense because they have similar apparent annual survival rates and males are
only slightly larger than females. Ifflickers optimize clutch size according to the numberofoffspring they can
rear, then there may be no relationship between defense and brood size. Received20September2005, accepted
6 July 2006.
Although nest defense may deter predators, experience also may be willing to defend their
it may place the parent bird at considerable nests more aggressively (Veen et al. 2000).
risk while requiring significant energy expen- Levels of defense also may vary between
diture (Blancher and Roberstson 1982, Nealen the sexes (e.g., Breitwisch 1988, Sproat and
and Breitwisch 1997, Olendorf and Robinson Ritchison 1993, Tryjanowski and Golawski
2000). For many birds, the intensity of nest 2004) because ofintersexual differences in fu-
defense may increase (1) as the breeding sea- ture survival and body size (Montgomerie and
son and reproductive value of the brood in- Weatherhead 1988). The sex with the lower
creases (see Montgomerie and Weatherhead survival rate and, consequently, the lower
1988 for a review), (2) as the potential for probability of future breeding, should defend
renesting declines (Andersson et al. 1980), broods more vigorously than its partner
and (3) with clutch or brood size (Olendorf (Montgomerie and Weatherhead 1988). Mor-
and Robinson 2000). Moreover, the intensity tality is usually female biased in many bird
of defense may depend on the sex of the par- species, likely as a result ofhigh reproductive
ent defending the nest (Breitwisch 1988, costs (Promislow et al. 1992). Generally, the
Sproat and Ritchison 1993, Nealen and Breit- larger sex defends the nest more aggressively,
wisch 1997). perhaps because the risk of injury is lower or
Age may be correlated with the level of because larger birds are able to mount strong
nest defense for several reasons, but this has attacks (Tryjanowski and Golawski 2004). Be-
rarely been tested (Veen et al. 2000). Older cause healthy birds may have relatively great-
birds have a lower probability offuture repro- er energy reserves, they may take more risks
duction; thus, they should invest more in when defending their nests than birds in poor-
broods than younger individuals (Hatch
er condition (Martin and Horn 1993). For ex-
1997). In addition, it is often difficult to sep- ample, females may be in poorer condition af-
arate the effects of age from experience with
ter incubation and defend the nest less ag-
predators because they are often directly cor-
gressively than the male (Sproat and Ritchison
related. Similar to olderbirds, birds with more
1993).
Cavity nesters may rely more on the inac-
1 Dept, ofBiology, Univ. ofSaskatchewan, 112 Sci- cessible or cryptic nature oftheir nest than on
ence Place, Saskatoon, SK S7N 5E2, Canada. active nest defense (Weidinger 2002); how-
2Current address: Dept, of Biology, Univ. ofRegi- ever, there have been few studies of wood-
na, 3737 Wascana Pkwy.. Regina. SK S4S 0A2, Can-
pecker behavioral responses to predators at
ada.
3Corresponding author; e-mail: the nest site. Wiebe (2004) examined respons-
[email protected] es of the Northern Flicker (Colaptes auratus)
452
—
Fisherand Wiebe • NORTHERN FLICKER NEST DEFENSE 453
ato ktlhee—pEtuopraorpaesainteStaorflicngav(iSttyurnneusstsvul(gKaraipsp)es (=me2a6n Acplurticlh--2iniJtuilayt;ioMnodoartee =199135,MaKyL,Wranugne-
1997) but found no sex- or age-related dif- publ. data). Each year since 1998, the area has
ferences in cavity defense. Ingold (1994) also been surveyed in spring (22 April-15 May,
described aggressive interactions between 1998-2005) for finding newly excavated cav-
starlings and flickers, but did not examine sex ities and to check old cavities for new breed-
or age differences in these behaviors. Law- ing pairs (flickers tend to reuse old cavities
rence (1967) described woodpeckers defend- more often than other woodpeckers; Moore
ing their nests from inside their cavities, en- 1995, Aitken et al. 2002, Wiebe et al. 2006).
gaging in alarm vocalizations and diving at- Tape-recorded territorial playback calls also
tacks; she also reported a male Northern were used to locate flicker territories and nest
Flicker that delivered a blow with its beak to sites. Average clutch size in this area is eight
a squirrel entering a nest hole, effectively de- eggs and mean number of young fledged per
terring the squirrel from entering. successful nest is six (Wiebe 2003). Once a
In this study, we presented a model predator clutch was complete, we cut a small door into
(red squirrel, Tamiasciurus hudsonicus) at the side of the nest tree for access to adults,
nest sites of Northern Flickers to examine eggs, and nestlings (see Wiebe 2001). Flickers
adult nest-defense behavior in relation to age, seem to tolerate the doors and readily re-use
sex, brood size, body size, and body condi- such cavities (Fisher and Wiebe 2006a). Ap-
tion. Because flickers are relatively short-lived proximately 18% of monitored nests are dep-
and their probability of survival is indepen- redated annually by mammalian predators,
dent ofage (Fisher and Wiebe 2006a), we pre- mainly red squirrels (Fisher and Wiebe
dicted that there would be no differences in 2006b).
defense between young and older birds. Sim- We captured flickers by flushing individuals
ilarly, mark-recapture models suggest only a from the nest cavity into a small net placed
2% difference in annual survival rate between over the cavity entrance (Fisher and Wiebe
the sexes (Fisher and Wiebe 2006a), and the 2006b). Three colored plastic and one alumi-
sexes invest about equally in nestling provi- num band were attached to each individual to
sioning (Moore 1995, Wiebe and Elchuk aid in individual identification (>95% of the
2003). Thus, we predicted that male and fe- known annual breeding population is color
male flickers would defend their broods with banded and individually identifiable). During
similar intensity. We also predicted that indi- banding, we used molt criteria to determine
viduals in better condition and with larger the birds’ ages (up to 4 years old; Pyle et al.
broods would defend their nests more aggres- 1997). We developed an index of flicker body
sively. size (i.e., score on the first axis of a principle
components analysis based on six measures:
METHODS
— bill depth, and lengths of the wing, bill, tail,
Study site and study species. Our study tarsus, and ninth primary) and body condition
site was near Riske Creek, British Columbia (i.e., residuals of a regression of body mass
(51° 52' N, 122°21' W), and encompassed ap- on body size); because of sexual size dimor-
proximately 100 km2 90-120 pairs offlickers phism, we made separate calculations for
;
nest there each year (Fisher and Wiebe males and females (see Wiebe and Swift
2006a). Habitats on the site are patchy and 2001). A year-specific estimate of body con-
variable. Flickers prefer grasslands for forag- dition was made only for individuals that were
ing (Elchuck and Wiebe 2003) and patches of trapped and weighed in 2003 and 2004; thus,
quaking aspen Populus tremuloides and only individuals captured during 2003 or 2004
( )
lodgepole pine (Pinus contorta) for nesting were included in analyses with body condition
(Martin and Eadie 1999). Continuous forests as a covariate (see below). We assumed that
of Douglas-fir (Pseudotsuga menziesii) and body size (i.e., the structural size of an indi-
hybrid spruce (Picea engelmannii X P. glau- vidual and not body mass) did not change
ca) also occur. from year to year. —
Flickers migrate to the area in mid-April Model presentations. Birds with altricial
and begin egg-laying in early- to mid-May young generally defend their nests most
454 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 4, December 2006
strongly during the nestling stage and as nest- that the flicker was responding to the model
lings age (Montgomerie and Weatherhead at distances ^10 m from the nest once we
1988). We measured nest defense when nest- judged that it could see the model. If parents
lings were 10-15 days old to control for ef- did not return to within 10 m and in sight of
fects ofnest stage and nestling age on defense the model in 1 hr, then these trials were re-
behavior. At each nest, we tested nest defense moved from all analyses. After an adult(s) re-
once with a predator (taxidermic model of a turned within <10 m, we recorded its behav-
red squirrel) and once with a control (taxider- ior for 5 min (if both parents returned simul-
mic model of a Yellow-headed Blackbird, taneously, we treated them as individual re-
Xanthocephalus xanthocephalus, or a Cedar sponses). Flickers respond to models with
Waxwing, Bombycilla cedrorum). The same slow, deliberate movements (Wiebe 2004), so
individuals were tested only once with each the 5-min period should have provided a rep-
model during the 2-year study to avoid poten- resentative sample of behavior. We quantified
tial habituation of parents to the models defense levels based on four behaviors re-
(Knight and Temple 1986a, 1986c). Blackbird corded during the 5-min period: (1) number
and waxwing models were used as controls of alarm calls {peak and wicka calls; Moore
because they are both common in the study 1995); (2) the closest distance that the re-
area and neither poses a threat to flicker sponding parent approached the model (m; a
broods (Wiebe 2004). In 2004, during 60% of visual estimate); (3) whether or not the parent
control trials we used the waxwing because dived at or hit the model (dichotomous vari-
the blackbird model was irreparably damaged able); and (4) time (sec) an individual spent
from transportation to and from trials. inside the cavity during each trial (flickers en-
Predator and control trials were conducted tered cavities and then peered back out, usu-
randomly at a given nest, with 1-5 days be- ally with their beaks protruding from the cav-
tween trials (i.e., one trial = one model pre- ity entrances). Time spent in the cavity should
sentation). Because the perceived threat from reflect investment in nest defense because
a predator could vary with distance between blocking the entrance prevents predation of
the predator and the nest (Ratti 2000), we fas- the nest (Cordero and Senar 1990). Assessing
m
tened the models at a fixed distance (1 be- the risk a parent incurs by blocking the cavity
low the cavity entrance) with a bungee cord entrance is difficult. This defensive strategy
tied to the tree trunk. The model squirrel was may be safer than others because most of the
attached to a small, flat board base that was parent’s body is inside the cavity (Cordero and
then attached to the tree trunk. Control models Senar 1990); conversely, there are no avenues
were mounted in an upright, perched position of escape for the parent.
—
on a natural branch, which was then attached Statistical analyses. Response time was
to the tree trunk. During a given trial, terri- square-root transformed to meet assumptions
torial “chatter” calls of squirrels or songs of of normality, and we analyzed it separately
Yellow-headed Blackbirds or Cedar Wax- from other defense variables because it was
wings were played at the base of the nest tree unlikely to have been influenced by model
to increase model detectability (Ghalambor type (parents presumably had not had time to
and Martin 2002). After models were placed see the model before returning). We used an
at the nest, we retreated to a concealed posi- ANCOVA to test whether age, sex, brood
tion >15 m away to record responses of the size, and/or body condition affected response
returning parents. time to the predator model (we assumed that
The first variable we recorded was response the structural size of an individual would not
time of the adult (i.e., sec between when we influence response time). Because data trans-
had set up the model and were hidden, to formations ofthe other four defense variables
when the parent returned and wejudged it was did not result in normality, we used non-para-
m
within 10 of the nest and in sight of the metric tests for subsequent analyses. Statisti-
model). Ten meters from the nest was usually cal significance was set at P < 0.05.
the maximum distance from which we could With respect to the four nest-defense vari-
observe a bird responding, because of dense ables, there was no difference between control
foliage around some nests. We were confident model types (blackbird versus waxwing;
Fisherand Wiehe • NORTHERN FLICKER NEST DEFENSE 455
PMa>nn0-.4W7h)i.tSniemyilaUratensdtsFailsshoesrhEoxwaecdtttheasttst:hearlel spoTnAdBinLgEto1.a mSoadmepllperesdiazteosro(freNdorstqhuierrrnel)Floirckceornstrroel-
were no significant differences between years (Yellow-headed Blackbird or Cedar Waxwing) placed
in terms of responses to control and predator at their nests during the brood-rearing stage at Riske
models (all P > 0.12). Therefore, we pooled Creek, British Columbia (2003 and 2004 data pooled).
all responses (for years and control models) Totals include instances in which both parents re-
in subsequent analyses. sponded to the models, plus those in which only one
We first analyzed each defense variable sin- tpoatraelntnurmesbpeorndoefd;tritahluss,cosnadmupclteedsizfeosraeraechlarmgoedretlhatnypteh.e
gly to determine which differed significantly
between control and squirrel models, without (toMtaoldenol.ttyrpiaels) Sex Age n
any other effects. This allowed us to eliminate
model type as a variable if it was non-signif- Control (91) Male 21 yyeeaarrs 1157
icant, thus simplifying subsequent models in- 3+ years 25
vaonldvibnogdaygecocnladsist,iosne.x,Wberooudsesdize,pabioreddy stiezset,s Female 21 yyeeaarrs 1196
(Wilcoxon’s signed-rank tests) to analyze min- 3+ years 15
imum distance to the model, time in the cav- Predator (94) Male 1 year 17
ity, and number of alarm calls to account for 23+yeyaerasrs 2149
both predator and control trials taking place at Female year 20
the same nest. This approach may have been 21 years 14
more stringent than necessary because it was 3+ years 13
not necessarily the same individual that re-
sponded to each trial; however, independent
test results were consistent with those of the of 3 indicates that the parent entered the cavity
paired tests. We used a Fisher’s exact test to and blocked it from the inside. Finally, a score
compare the frequency of diving at the squir- of4 indicates that birds dived at or hit the mod-
rel versus the control models. All means pre- el, indicating the riskiest and most energetically
sented are ± SD. expensive behavior to a defending adult.
After separate analysis of each defense be- For statistical analyses involving age, we
havior (see results), we constructed an overall categorized males or females as either 1, 2, or
defense score based on the three variables that 3+ years old, such that there was at least a
differed significantly between control and sample size of 13 in each age category (Table
predator models. This score was used in sub- 1). A further subdivision of age was not pos-
sequent analyses involving the relationship sible to analyze statistically, as it would have
between various parental attributes and resulted in some categories with a sample size
strength of response to the squirrel model. A <5. We used a Kruskal-Wallis test to examine
score of 1 indicated the bird returned to the whether the median defense scores ofbirds in
nest and was judged to be within sight of the the six different age-sex classes differed. To
model but did not dive at the model or enter analyze the effect of brood size on defense
the cavity, and always remained >2 m away score (a categorical variable), we used Spear-
from the model (there is a low probability that man’s rank correlations. Body size and con-
a squirrel could contact the parent at a dis- dition met assumptions of normality; there-
tance of 2 m). A score of 2 indicates that the fore, we could use parametric tests (two-factor
parent approached <1 m from the predator ANOVA) to assess the relationship between
model but otherwise performed no other nest- defense score and sex on body size and con-
defense behaviors. In developing score 2, we dition (dependent variables).
assumed that a squirrel might be able to phys-
ically contact a flicker <1 m away and that RESULTS
parents approaching within 1 m were placing We conducted 91 control trials and 94 pred-
themselves at a greaterrisk than those in score ator trials at 94 Northern Flicker nests in 2003
category 1. Responses in category 2 included and 2004. Control trials were not conducted
perching on the cavity lip from the outside or at three nests because nestlings were >15
on a branch within 1 m ofthe model. A score days old by the time the second model could
456 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 4, December2006
TABLE 2. Effects ofsex, age class (1, 2, and 3+
years old), brood size, and body condition of flicker
parents on theirresponse time (see description intext)
to a model nest predator presented at the nest during
the brood-rearing stage atRiskeCreek, BritishColum-
bia, 2003 and 2004. No predictor was significant ac-
cording to a 2-factor ANCOVA (n = 84 individuals)
using Type III sums ofsquares.
Effect ss df F P
Sex 231.67 1 1.18 0.29
Age 181.15 2 0.44 0.65
Sex X age 438.81 2 1.06 0.35
Brood size 16.50 1 0.56 0.46 Male Female
Body condition 589.02 1 2.84 0.10 Sexand age class
Sex X brood size 211.50 1 1.02 0.32
Age X brood size 92.43 2 0.22 0.80 FIG. 1. Nest-defense scores of parent flickers did
not differ by sex and age categories when responding
to a model predator (red squirrel) placed at their nest
during the brood-rearing stage in Riske Creek, British
Columbia, 2003 and 2004. Bold horizontal lines rep-
be presented. Parents occasionally returned to- resentmediandefensescores,boxesrepresent25thand
gether to defend the nest (16 out of91 control 75thpercentiles, anderrorbarsrepresent 10thand90th
and 13 out of94 predator trials) and responses percentiles. Because several birds within each age and
by these individuals were considered to be in- sex category received the same defense score, some
10th, 25th, 75th, and 90th percentiles overlap; thus,
dependent trials (i.e., two parents responding symbols foreach age and sex class are notnecessarily
increased sample size by two). Sample sizes apparent.
of responding parents of both age classes and
sexes varied according to model type (Table
1). — spectively; Wilcoxon’s signed-rank test: Z =
Response time anddefense behaviors. The -1.41, P = 0.16).
mwmoaedsaenlaowwvaeesraakl1l,t0rr9ee0nsdp±on(8sP7e6=tsie0mc.e1(0nt)o=tthh1ae0t7)pb.rierTddhsaetroiern dwiaasTnramdiaetrfsgeinonsfaeltlhsyecophraiergfehoenrrtmtaahnaldtestbhr^ao3nodoyf.e—aarTnshyoefotmahege-er
better condition responded to the predator age-sex category (Kruskal-Wallis test: x2 =
model more quickly, but there was no effect 6.63, df = 3, P = 0.085; Fig. 1). Brood sizes
ofage, sex, brood size, orbody condition, and of parents tested with the squirrel model
there were no interactions (Table 2). ranged from 2 to 9, but there were no signif-
Flickers dived significantly more at the icant correlations between brood size and
predator model (26% oftrials) than at the con- nest-defense score for the six age-sex classes
trol (2% of trials; Fisher’s exact test: P < when considered separately (Spearman’s rank
0.001). Parents also approached the predator correlations: all P > 0.28, but two-year old
model more closely (3 m ± 4) than the control males showed a marginally significant trend
model (5 m ± 4; Wilcoxon’s signed-rank test: of defending smaller broods more aggressive-
Z = —4.98, P < 0.001). During the 5-min ly, r = —0.45, P =0.060). Similarly, with all
trials, flickers spent significantly more time in ages and sexes combined, there was no effect
their cavities when responding to the predator of brood size on defense score (Spearman’s
model than to the control model (16% ± 33 rank correlation: r = 0.02, P = 0.83). In an-
versus 5% ± 20, respectively; Wilcoxon’s other analysis, we categorized brood sizes as
signed-rank test: Z = —2.35, P < 0.001). Par- small (<6 chicks, n = 45) versus large (>7
ents gave wicka and peah alarm calls in 36% chicks, n = 62). Approximately 30% of in-
of the trials, but there was no effect of model dividuals with large broods exhibited the most
type on the number ofalarm calls (mean num- intense defensive behavior (score = 4),
ber of alarm calls = 11 ±32 and 18 ± 37 in whereas 22% ofindividuals with small broods
response to predator and control models, re- had score 4; however, the overall frequency of
Fisherand Wiebe • NORTHERN FLICKER NEST DEFENSE 457
90- A dition and defense score (two-factor ANOVA:
F = 1.48, df =3, P = 0.84) for either sex (F
= 2.13, df = 1, P = 0.15; Fig. 2) or a sex X
88- f l i d=efense score interaction (F = 1.48, df =3, P
0.23; Fig. 2).
0
N DISCUSSION
'</)
o 86 Rela—tionship between sex and nest de-
o fense. Although a model predator may not
CO
o elicit the same intensity of nest defense as a
84- real predator, the fact that flickers responded
l i to it more intensely than to the control model
suggests that they did perceive danger. Con-
sistent with initial predictions, we found no
82-
differences between nest defense of male and
female flickers. Although many studies have
revealed sex-related differences in nest de-
fense among birds (Gill and Sealy 1996, Caw-
thom et al. 1998, Pavel and Bures 2001, Grig-
gio et al. 2003), others have not, including
studies on the American Goldfinch (Carduelis
tristis; Knight and Temple 1986b) and Red-
backed Shrike (Lanius collurio; Tryjanowski
and Golawski 2004). Adult male and female
American Goldfinches may exhibit equal de-
fense responses because they are monoga-
mous and both sexes are required to raise the
young (Knight and Temple 1986b). Tryja-
nowski and Golawski (2004) suggested that
net costs and benefits of nest defense by male
and female Red-backed Shrikes were equal
because males were larger than females, but
females had greater confidence ofparenthood.
FIG. 2. Mean and 95% Cl of (A) body size and
(B) body condition formale (filledcircles) and female For flickers, the sex-related differences in sur-
(opencircles)NorthernRickersperformingfourlevels vival (male survival is 2% lower than that of
of nest defense (1= least, 4 = greatest; see text for females; Fisher and Wiebe 2006b), body size
description ofdefense scores) in response to a model (males are —3% larger than females; Moore
predatorplaced at nests during the brood-rearing stage 1995, Wiebe 2000), and investment in the cur-
at Riske Creek, British Columbia, 2003 and 2004. rent brood (Moore 1995, Wiebe and Elchuk
Body size differed between the sexes, but defense
scores did not vary with body size or condition. 2003) are likely too small to alter the costs
and benefits of sex-related nest defense.
Among cavity nesters, male Eastern Screech-
defense scores was not associated with brood owls (Otus [currently Megascops asio de-
size (x2 = 2.48, df = 3, P = 0.48). fend nestlings more aggressively th]an fem)ales
As expected, adult body size was signifi- (Sproat and Ritchison 1993), as do male Great
cantly associated with sex (males were struc- Tits (Parus major, Currio and Onnebrink
turally larger than females; two-factor ANO- 1995) and male Tree Swallows (Tachycineta
wVaAs:nFo=re3la4t5i.o6n7s,hidpfb=et1w,ePen<b0o.d0y01s)i,zebuatndthdeer-e bicAogleon;aWnidnklneerst19d9e2f)e.nse.—In general, we
fense score (F = 0.33, df = 3, P = 0.80; Fig. found no significant association between age
2), nor was there a sex X defense score inter- and nest defense, although males ^3 years old
action (F = 0.41, df = 3, P = 0.75). Similarly, tended to engage in more risky defense be-
there was no relationship between body con- havior (attributed to their greater tendency to
458 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 4, December2006
block the cavity entrance) than the other head 1988). The few studies that have tested
groups. Blocking the cavity entrance may be for within-sex effects of body condition have
used by cavity nesters to prevent usurpation been equivocal at best, ranging from no effect
of cavities (Cordero and Senar 1990). With (Radford and Blakey 2000) to a sex-specific
the head and bill in striking position at the effect (Winkler 1992, Hamer and Furness
entrance hole, it also may be an effective strat- 1993). There is little direct evidence that body
egy for fending off an attack while minimiz- condition affects the intensity of active de-
ing risk to the rest of the parent’s body. The fense in any species, but good nutrient re-
lack of strong age or sex effects on any de- serves may allow a parent to reduce foraging
fense behavior suggests that individuals of time away from the nest and be more attentive
different ages perceive the overall costs and to the nest site during incubation and brooding
benefits of nest defense in a similar way. (Slagsvold and Lifjeld 1989, Wiebe and Mar-
According to economic models of nest de- tin 1997); in turn, these factors would result
fense (Montgomerie and Weatherhead 1988), in greater nesting success (Chastel et al.
an older bird should defend its current brood 1995). We found some evidence that birds in
more aggressively than a younger bird be- better body condition responded more quickly
cause it has a lower future reproductive po- to the predator model, which may provide
tential; however, we found no evidence for support for this hypothesis. Flicker condition
this in flickers. Winkler (1992) explained that was measured in the late stages of incubation
age-independent survival probabilities pre- or early stages of brooding when parents
cluded an effect of age on nest defense by could be captured; thus, they may not have
Tree Swallows. Similarly, the annual apparent been in exactly the same condition at the time
survival rates (42%) for flickers do not vary of our defense trials (about 10-15 days later).
with age, and the birds are relatively short- However, if relative rankings of body condi-
lived (Fisher and Wiebe 2006b), so it is prob- tion among individuals remain similar, we
ably not surprising that age has little influence should have been able to detect a pattern—.
on defense intensity. Effects ofbrood size on nest defense. We
Although future reproductive potential is predicted that male and female flickers with
one component that could lead to age-depen- larger broods should defend them more ag-
dent nest defense, experience also may be a gressively than flickers with smaller broods,
key factor if defense is learned and becomes but brood size was not correlated with any of
less risky for the adult over time (Montgom- the defense behaviors that we measured. Try-
erie and Weatherhead 1988). We could not janowski and Golawski (2004) suggested that
separate age from experience in our study and brood size manipulation experiments are
it is impossible to know the previous experi- needed to adequately test for effects of brood
ence that a wild bird may have had with a size on nest defense. However, even some ex-
predator. perimental studies have failed to reveal any
Effects ofbody size and condition on nest differences in nest defense as a result ofbrood
—
defense. It was surprising that neither body size (Tolonen and Korpimaki 1995). Ifparents
size nor condition were positively associated optimize their clutch size according to their
with our measures of flicker nest defense. Al- ability to raise all their young, then large and
though sexual-size dimorphism is often cited small broods may represent equal value to the
as contributing to differences in nest defense defending adults, in which case brood size
between the sexes (Tryjanowski and Golawski may not be expected to influence nest defense
2004), effects of body-size differences within (Tolonen and Korpimaki 1995, Dawson and
the sexes have rarely been tested (Hamer and Bortolotti 2003).
Furness 1993, Radford and Blakey 2000). If In summary, anecdotal data from the liter-
large and small birds are both—effective nest ature (Lawrence 1967) and video-tape evi-
defenders for different reasons for example, dence from our own study site (KLW unpubl.
if small individuals have greater maneuver- data) indicates that the defense behaviors we
abil—ity and large individuals are more power- observed may successfully protect cavity
ful then overall costs and benefits may be nests from live predators, such as red squir-
similar for each (Montgomerie and Weather- rels. Individual flickers varied in their re-
Fisherand Wiebe • NORTHHRN FLICKER NEST DEFENSE 459
sponses, but we were unable to find strong Fisher, R. J. and K. L. Wiebe. 2006a. Effects of sex
correlates of that variation associated with and age on survival of Northern Flickers: a six-
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Fisher, R. J. and K. L. Wiebe. 2006b. Nest site attri-
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butes and temporal patterns of Northern Flicker
ACKNOWLEDGMENTS nest loss: effects of predation and competition.
Oecologia 147:744-753.
We sincerely thank C. L. Galatiuk, H. J. Kalyn, J. Ghalambor, C. K. and T. E. Martin. 2002. Compar-
R M. Johnston, and K. M. Warner, who helped with ative manipulation ofpredation risk in incubating
the model presentations. We would also like to thank birds reveals variability in the plasticity of re-
K. Martin, who allowed us to conduct model presen- sponses. Behavioral Ecology 13:101-108.
tations on parts of her study area. D. J. Ingold, J. J. Gill, S. A. and S. G. Sealy. 1996. Nest defence by
Kappes, Jr., and one anonymous reviewer improved Yellow Warblers: recognition of a brood parasite
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