Behavioral Neuroscience
2008, Vol. 122, No. 3, 527–534

Copyright 2008 by the American Psychological Association
0735-7044/08/$12.00 DOI: 10.1037/0735-7044.122.3.527

Sex Differences in the Effects of Captivity on Hippocampus Size in
Brown-Headed Cowbirds (Molothrus ater obscurus)
Lainy B. Day

Marjorie Guerra and Barney A. Schlinger

University of Mississippi, University of California, Los Angeles,
and University of California, Santa Barbara

University of California, Los Angeles

Stephen I. Rothstein
University of California, Santa Barbara
In brood parasitic cowbirds, hippocampus (Hp) size is correlated with environmental spatial memory
demands. Searching for host nests is the presumed causal factor influencing cowbird Hp size, because Hp
volumes vary across species, sexes, and seasons according to nest-searching participation. Brown-headed
cowbirds have female-only nest searching and, at least in the eastern subspecies, a larger Hp in females
than in males, suggesting that nest searching influences cowbird Hp size. We predicted that female
brown-headed cowbirds housed in aviaries lacking host nests would have a smaller Hp than wild-caught
females whereas males would be unaffected. We found that the Hp was smaller in captive females, but
not males, compared to their wild-caught counterparts. This did not appear to be due to general effects
of an impoverished environment on all brain regions. Our results imply that interruption of nest searchingin cowbirds prevents seasonal increase in Hp size in females. Future studies should isolate which
behavioral differences between wild and captive birds contributed to captivity-induced changes in Hp
volume in females while not affecting males.
Keywords: cowbird, hippocampus, sex differences, brain plasticity, brood parasitism

foraging-related rate of movement in lacertid lizards (Day, Crews,& Wilczynski, 1999), home-range size in voles (Gaulin, 1992;
Jacobs, Gaulin, Sherry, & Hoffman, 1990; Sherry, Jacobs, &Gaulin, 1992), and breeding-habitat complexity in petrels (Abbott,
Walsh, Storey, Stenhause, & Harley, 1999) are all associated withenlarged Hp size, presumably due to the demands of spatial memory. In addition, seasonal variation in food-storing demands is
associated with seasonal changes in Hp volume (Smulders, Sasson,& DeVoogd, 1995) in black-capped chickadees and rodents (Jacobs, 1996). Males have a larger Hp than females in a vole species
in which males have larger home ranges but not in vole species
that do not have sex differences in home range (Gaulin, 1992).
These results provide evidence that environmental or experiential
seasonal changes can alter Hp volume and that natural and sexual
selection on a variety of spatial-related skills has produced a larger
Hp in some species or sexes than in others.
Although correlations of Hp volume with ecological demands
for spatial ability are numerous, the specific mechanisms that
contribute to such adult variation in the volume of the Hp are notwell understood. We do know that species-specific reactions tospatial experience (food storing or other spatial learning tasks)
during development contribute to differences between some species of food-storing versus non-food-storing birds. In adults, foodstoring experience alone does not appear to alter the volume of the
Hp in some species (Cristol, 1996); although combined with otherseasonal effects, it may increase Hp volume, cell number, or new
cell recruitment in other species such as the black-capped chickadee (Barnea & Nottebohm, 1994; Smulders et al., 1995; Smulders, Shiflett, Sperling, & DeVoogd, 2000). Changes in Hp neuroanatomy across seasons that are correlated with seasonal demand

Variation in environmental spatial demands is related to variation in hippocampus (Hp) size among closely related species,
between sexes, and across seasons in mammalian, avian, andreptilian species. For example, food-storing behaviors in birds(Clayton, 1995a; Hampton, Sherry, Shettleworth, Kurgel, & Ivy,
1995; Hampton & Shettleworth, 1996; Healy & Krebs, 1992;
Krebs, Sherry, Healy, Perry, & Vaccarion, 1989; Sherry, Vaccarino, Buckenham, & Herz, 1989; Volman, Grubb, & Schuett,1997) and rodents (Jacobs, 1992; Jacobs & Spencer, 1994),

Lainy B. Day, Department of Biology, University of Mississippi, Department of Physiological Sciences, University of California, Los Angeles,and Department of Ecology, Evolution and Marine Biology, and Department of Psychology, University of California, Santa Barbara; Marjorie
Guerra and Barney A. Schlinger, Department of Physiological Science,
University of California, Los Angeles; Stephen I. Rothstein, Department of
Ecology, Evolution and Marine Biology, University of California, Santa
Barbara.
This work was supported by 5 F32 MH12326 and NSF, IBN 9728091.
We thank Thomas Pilsbury for assistance with brain sectioning, Zoe
Lieberman and Rachel Pauley for help with staining and coverslipping, andDr. Deborah Olster for the use of her laboratory and for helpful commentson the manuscript. We appreciate the efforts of individuals involved in the
cowbird control program who brought us cowbirds for this work. Day was
involved in all aspects of this work. Guerra measured brain regions anddiscussed results. Rothstein provided resources and advice on all aspects ofthe project and manuscript. Schlinger provided resources and manuscript
comments, as well as participating in interpreting results.
Correspondence concerning this article should be addressed to Lainy B.
Day, Department of Biology, University of Mississippi, 219 ShoemakerHall, University, MS 38677. E-mail: lainyday@olemiss.edu
527

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DAY, GUERRA, SCHLINGER, AND ROTHSTEIN

for spatial memory appear to be triggered directly by changes inphotoperiod in some mammals (Prendergast, Nelson, & Zucker,
2002). In eastern gray squirrels, however, seasonal changes inphotoperiod and food storing are not accompanied by changes in
Hp volume (Lavenex, Steele, & Jacobs, 2000). In black-cappedchickadees, a food-storing bird with seasonal changes in both Hpvolume and food-storing behavior, photoperiod does not appear toinduce changes in Hp volume or cell proliferation even thoughphotoperiod can induce changes in food-storing behavior(Macdougall-Shackleton, Sherry, Clark, Pinkus, & Hernandez,2003). Furthermore, causal links between behavioral traits that are
related to species and sex differences in Hp size, for example, foodstoring and mate searching, and actual Hp growth or attrition havenot been established in most cases. Thus, more studies are neededthat examine mechanisms for changes in Hp volume and that
demonstrate that changes in the behavior proposed to alter Hpvolume actually do so.
Cowbirds provide an interesting group in which to examine
the link between behavior and Hp volume. These brood parasites show species, sex, and seasonal variation in Hp volume inconcordance with demands for locating host nests to parasitize
when compared to other species in the same family, the Icteridae, that are not parasitic (Clayton & Reboreda, 1997; Reboreda, Clayton, & Kacelnik, 1996; Sherry, Forbes, Khurgel, &Ivy, 1993). In addition, males and females search for host nests
together in some cowbird species whereas only the female
searches for hosts in others. The Hp is larger in females than inmales only in those species studied in which the females search
for nests alone (Reboreda et al., 1996; Sherry et al., 1993). Thisis a particularly interesting result as it counters the more common male bias in Hp volume in rodents (Roof & Havens, 1992).
Furthermore, Hp size in two species of parasitic cowbirds
appears to be seasonally plastic as Hp volume is larger in thesummer when breeding and thus nest searching is taking placethan in the winter nonbreeding season. Additionally, sex differences in Hp size seen in the breeding season are not found in
the nonbreeding season in a species with female-only nest
searching, the shiny cowbird (Molothrus bonariensis; Clayton& Reboreda, 1997).
Together, these results suggest that searching for nests is related
to Hp volume. Thus, we hypothesized that preventing cowbirds
from searching for nests should result in a decrease in Hp volume.
To prevent nest searching while reducing the general effects ofdeprivation (Gage, 2002; McEwen, 1999; Rosenzweig & Bennett,
1996), we held brown-headed cowbirds in large mixed-sex outdoor
aviaries where they could have normal social interactions but
without the provision of host nests for egg laying. Additionally, we
created a specifically impoverished group by transferring somefemales from the outdoor aviaries to comparatively small isolationchambers. Because we used brown-headed cowbirds for this study,
we expected females, but not males, to be sensitive to prevention
from nest searching as only females normally search for nests.
Thus, we predicted that preventing birds from nest searching byholding them in captivity would selectively decrease the volume of
the female Hp but not the male Hp compared to wild-caught
counterparts. If general deprivation is the main difference between
captive and wild-caught birds, then both sexes should show decreased Hp volume as a result of captive housing and females

housed in isolation chambers should show a greater decrease in Hp
volume than aviary-housed captive males and females.

MethodSubjects and Housing Conditions
Thirty-two brown-headed cowbirds (M. ater obscurus) were
captured in walk-in traps in southern California in 1999 and 2000
along the Santa Clara River or in Simi Valley (both in VenturaCounty) as part of a federal- and state-run cowbird population
control program designed to aid the endangered least Bell’s vireo(Vireo bellii pusillus; Griffith & Griffith, 2000). Because of possible population differences in ecology and neuroanatomy (Uyehara & Nairns, 1992) in cowbirds and because differences inneuroanatomy are known to occur as a result of differential experience and/or age (Clayton, 1995b; Healy, Gwinner, & Krebs,
1996), we attempted to select only adult sexually mature birds thatwere from the local Ventura population using wing length toidentify subspecies and plumage characteristics to identify age
classes (Fleischer & Rothstein, 1988; Lowther, 1993). Among
adult males, second year (SY) individuals and after second year
(ASY) males show different patterns of daily movements (Rothstein, Yokel, & Fleischer, 1986), so we selected only ASY males.
All females were adults, minimum SY, as determined by wing
cord, but they could not be further aged into SY and ASY. Because
all females mate and parasitize nests as yearlings (Darley, 1983;
Lowther, 1993; Payne, 1973; Rothstein et al., 1986), all were inbreeding condition and were presumed to be engaged in nest
searching when caught. It is possible that females were, on average, younger than males. However, all birds were breeding adults
and thus brains were unlikely to be more plastic in females becauseof age. All birds were killed between May and June, a period thatcoincides with the breeding season in California cowbirds (Payne,1973).
We had three conditions: wild-caught birds, birds held captive inaviaries, and birds housed in isolation chambers (females only).
Captive birds were six males (CM) and five females (CF) captured
between late April and early July of 1999 and housed in a large
outdoor aviary (1.2 ⫻ 2.7 ⫻ 6.0 m) in mixed-sex groups until
killed between May 12 and June 30 of 2000, approximately 1 year
after their capture. An additional five females (ISO), similarly heldin the outdoor aviaries for 1 year, were placed in visual andacoustic isolation chambers (internal diameter 53 ⫻ 28 ⫻ 30 cm)
on natural photoperiods on April 1, 2000, about when breeding
normally begins in southern California. These birds were killed 5
weeks later on May 8, 2000. Wild-caught breeding birds, 10 males(WCM) and 6 females (WCF) were captured between May 8 andJune 20 of 2000 and were killed within 10 days of capture (anonsignificant negative correlation between Hp size and capture
time for WCM and WCF suggest this brief captivity was insufficient to alter Hp size). As wild-caught birds came in from the field,
we killed them in matched pairs with captive birds. All birds inboth years were caught at one of the two sites in Ventura County.
All birds were given food (Roudybush Maintenance Crumble;
Roudybush, Woodland, California) and water ad libitum. All conditions were approved by the University of California Institutional
Animal Care and Use Committee (Protocol 185). Captive males
and females used in this study were all in apparent breeding
condition based on courting and mating behaviors.

SEX-SPECIFIC EFFECTS OF CAPTIVITY IN COWBIRDS

Histology and Morphometrics
Birds were injected intramuscularly with a ketamine (50 mg/kg)/xylazine (20 mg/kg) mixture followed by 8 mg/kg sodium
pentobarbital and perfused transcardially with 0.8% saline andneutral buffered formalin (NBF). Brains were postfixed in NBF for
11 or 12 days then transferred to 30% sucrose in 0.1 M Phosphate
Buffer until sinking. Brains were cut in the coronal plane on acryostat at 40 ␮m; every fifth section was mounted on a slide andNissl stained with cresyl violet. Slides were coded so that individuals doing measurements were unaware of the sex and treatment of
individuals. We computed the volume of the Hp, telencephalon(Tel), and nucleus rotundus (NRot), a visual, thalamic nucleus.
The Tel and NRot were measured as control regions to determine
whether captivity affected brain regions outside the Hp and tocontrol for shrinkage of tissue during processing by adjusting Hp
volume for Tel volume. Volume was computed by capturing
images of slices with MRGrab (Carl Zeiss, Inc., Thornwood, NY)
software via a light-microscope equipped with an AxioCam MRc
digital camera (Zeiss) and measuring surface areas. Our sampling
procedures and volume calculations followed standard stereological methods (Gundersen et al., 1988). Standard neuroanatomical
landmarks were used to delineate the boundaries of the brain
regions. In birds, the Hp is bounded by the midline, the lateral
ventricle, and the brain surface. Laterally it can be distinguishedfrom the apical part of the hyperpallium (HA) by an obvious
increase in cell density at the start of the HA (Figure 1A; Day,
Westcott, & Olster, 2005; Sherry et al., 1989). Following previous
work with brown-headed cowbirds, we measured the Tel from thefirst slice in which Hp appeared to the last slice in which wemeasured Hp (Sherry et al., 1993). On sections in which thetelencephalic hemispheres were joined with the diencephalon,
prominent markers such as the occipito-mesencephalic tract andthe anterior commissure were used to determine telencephalicboundaries (Brenowitz, Lent, & Kroodsma, 1995). The NRot
boundaries are particularly clear in Nissl-stained sections (Figure
1B). Hp and NRot were measured on every mounted slice (distance between slices 200 ␮m), and Tel was measured on every
other mounted slice (distance between slices 400 ␮m). We measured both the left and right Hp and Tel on four wild-caught malebrains and found no significant laterality (paired t test, p ⬎ .05). In
addition, no laterality in Hp volume was found in a previous study
that measured six male and five female brown-headed cowbirds(Uyehara & Nairns, 1992). However, the left eye–right hemispheresystem in birds has been shown to perform more spatial informa-

Figure 1. Photomicrographs of hippocampus (A) and nucleus rotundus(B). In A, arrows indicate hippocampus boundaries. The left section is froma captive female (CF) and right section is from a wild-caught female(WCF). We attempted to take photos at the same rostral-caudal position for
CF and WCF.

529

tion processing than the other hemisphere (Clayton & Krebs,
1993), and sex differences in Hp volume favoring males occur
only in the right dentate gyrus of rats (Roof, 1995; Roof & Havens,
1992; Tabibnia, Cooke, & Breedlove, 1999). Thus, we continued
measurements only on the right, more “spatial,” side of all brains,acknowledging the possibility that we could miss laterality differences if they exist in this species. After all brain regions were
measured, we randomly selected four slices per brain region fromeach bird and remeasured these areas. All regions showed a meanvariation of less than 3.5% between first and second measurements.

Statistical Analyses
Whether comparisons were significant or not was not affectedby log transformation, thus we used raw values. We subtracted thevolume of the Hp from Tel volumes (TelH) so that these two
measures were independent. To compare volumes of each brain
region across groups, we performed one-way analyses of variance(ANOVAs) followed by Bonferroni/Dunn post hoc tests using analpha level corrected for all possible pairwise comparisons inSuperAnova (Version 1.11; Abacus Concepts, Berkeley, CA).
Variation in the size of the Tel can affect Hp volume; therefore, weused simple regressions to examine the relationship between Hp
volume and TelH volume using Statview (Version 5.0; SAS Institute, Cary, NC). Note that simple regression analyses showed thatweight did not covary with Hp size, and thus weight was notconsidered further in analyses. To examine changes in brain region
volume adjusted for changes in the covariate, we used a generallinear model (GLM; SPSS 14; SPSS Inc., Chicago, IL). To determine whether we met the assumptions for performing GLM, we
first ran the model with a covariate–treatment group interaction
term to verify that slopes were not significantly heterogeneous
across groups. We then ran the model without the covariate interaction term and used Levene’s test to check for significant difference in error variance across groups before interpreting results.
GLM was followed by comparison of covariate-adjusted means
using Sidak test adjusting for multiple comparisons. For all comparisons, the alpha or multiple-comparison corrected alpha was set
at p ⬍ .05. Error bars on graphs and error values reported in text
are standard errors of the mean.

Results
Hp and Tel
The absolute volume of the Hp was smaller in the two captive
female groups (CF ⫽ 8.6 ⫾ 0.3, ISO ⫽ 9.1 ⫾ 0.4) than inwild-caught females (WCF ⫽ 11 ⫾ 0.3) and the two male groups(CM ⫽ 10.8 ⫾ 0.3, WCM ⫽ 10.4 ⫾ 0.3; see Figure 2): main groupeffect, F(4, 27) ⫽ 9.93, p ⬍ .0001, pairwise values all ps ⬍ .0001.
Hp volume was not significantly different in CM and WCM. Thus,
captivity does not appear to alter the size of the male Hp, but bothgroups of captive females (CF and ISO) had a smaller Hp thanwild females. The TelH was larger in the two male groups (CM ⫽250.3 ⫾ 7.5, WCM ⫽ 249.4 ⫾ 6.1) compared to the two captive
female groups (CF ⫽ 209.2 ⫾ 10.3, ISO ⫽ 207.5 ⫾ 4.5; see
Figure 3), F(4, 27) ⫽ 8.98, p ⬍ .0001, pairwise values all ps ⬍.0002. The TelH of WCF (234.8 ⫾ 4.5) was not significantlyDAY, GUERRA, SCHLINGER, AND ROTHSTEIN

530

Hp Volume mm3*
12

Hp Vol mm3

11
10

13

CF

12

CM
ISO

11

WCF
WCM

10
9
8

9

7
180

8

200

260

280

Tel volume mm3 (minus HP vol)

7
6
ISO

CF

WCF

CM

WCM

Figure 2. Mean hippocampus (Hp) volume for captive males (CM) andfemales (CF), wild-caught males (WCM) and females (WCF), and isolated
females (ISO). Asterisk above brackets indicates that the hippocampus was
significantly smaller in both captive female groups than in WCF as well as
CM and WCM. The two male groups did not differ from each other.

different from any other group (post hoc p value was not significant). Thus, TelH size is not altered by captivity in males or
females. However, both CF and ISO have a smaller TelH volume
compared to both CM and WCM.
We found a significant positive relationship between Hp and
TelH (see Figure 4; R2 ⫽ 0.45, p ⬍ .0001). We thus examinedchanges in Hp volume controlling for changes in TelH, the relative
Hp volume (RHp). There was no significant interaction between
TelH and treatment, F(4, 22) ⫽ 1.01, p ⫽ .42. Thus, the interaction
term was removed. Variance in RHp was not significantly heterogeneous among groups, F(4, 27) ⫽ 0.22, p ⫽ .93. There was asignificant treatment effect, F(4, 26) ⫽ 4.47, p ⫽ .007 (see Figure5). Post hoc test showed that WCF had a larger RHp volume than
CF. No other groups differed from each other (WCF vs. WCM,
p ⫽ .12). Thus, aviary housing affected females but not males.

Figure 4. Mean hippocampus (Hp) volume plotted against telencephalon(Tel) volume (minus Hp volume) for groups studied (R2 ⫽ 0.45, p ⬍.0001). CF ⫽ captive females; CM ⫽ captive males; ISO ⫽ isolated
females; WCF ⫽ wild-caught females; WCM ⫽ wild-caught males.

Nucleus Rotundus
There were no significant differences in the size of the NRot,
our control region, across groups (CF ⫽ 1.24 ⫾ 0.36, CM ⫽1.56 ⫾ 0.45, WCF ⫽ 1.29 ⫾ 0.32, WCM ⫽ 1.80 ⫾ 0.21, ISO ⫽1.58 ⫾ 0.26), F(4, 27) ⫽ 0.62, ns. The size of NRot was notcorrelated with Tel volume (R2 ⫽ 0.001, p ⫽ .88), therefore we did
not need to perform analyses adjusting for Tel size. However, wenote that correcting NRot volume for Tel volume as we did for Hp
did not result in significant differences among groups.

Discussion
Our results demonstrate sex-specific changes in the size of brain
regions in birds and agree with the hypothesis that experience withnest searching is necessary for the seasonal increase of Hp volume
seen in brood-parasitic female cowbirds (Clayton & Reboreda,1997). Female brown-headed cowbirds held for 1 year in large
outdoor aviaries and killed during the breeding season had asignificantly smaller Hp and Hp volume controlling for Tel size
than WCF. Hp volume was similar in CM and WCM brownheaded cowbirds. In addition to these sex-specific effects, theeffects of captivity also appeared to be specific to the Hp. Al-

*

280
260
240
220
200
180
ISO

CF

WCF

CM

WCM

Hp Vol (Adj for Tele Vol)

Tel Vol mm3 (minus Hp)

240

220

*

12
11
10
9
8
ISO

Figure 3. Mean telencephalon volume (Tel Vol) minus hippocampus(Hp) volume (TelH) for captive males (CM) and females (CF), wild-caught
males (WCM) and females (WCF), and isolated females (ISO). Asterisk
above brackets indicates that TelH size is smaller in both captive female
groups compared to both male groups. TelH size did not differ amongfemale groups.

CF

WCF

CM

WCM

Figure 5. Hippocampal volume (Hp Vol) controlling for the effects oftelencephalon volume (Tele Vol; minus Hp Vol). Hp Vol is significantly
smaller in captive females than in wild- caught females. ISO ⫽ isolated
females; CF ⫽ captive females; WCF ⫽ wild-caught females; CM ⫽captive males; WCM ⫽ wild-caught males.

SEX-SPECIFIC EFFECTS OF CAPTIVITY IN COWBIRDS

though the Tel (minus Hp) of CF and ISO was smaller than either
male group, there was no significant change in this Tel volume in
CF or ISO compared to WCF or between CM and WCM. In
addition, neither captive conditions nor isolating conditions had
any effect on the volume of the NRot.
Changes in Hp volume did not appear to be the result of generaleffects of an impoverished environment. CM did not have asmaller Hp than WCM. ISO had a Hp volume reduced in size
compared to WCF, but this treatment did not cause an additional
reduction in Hp volume compared to CF (although it should be
noted that longer isolation periods could possibly reduce Hp volume). Furthermore, when adjustments for changes in Tel size were
made, Hp volume did not differ between ISO and WCF. Thus, itappears that housing cowbirds in large outdoor aviaries without
host nests induces either a female-specific reduction in Hp volume
compared to wild-caught cowbirds or a failure of the Hp to growin captive birds compare to wild-caught birds. Although we cannot
differentiate between these two possibilities with these data, we
believe it is more likely that differences between WCF and CF are
due to a lack of seasonal increase in Hp volume in CF rather thanan induced decrease in Hp volume in CF. Although not yet
demonstrated for brown-headed cowbirds, the nest searchingsex(s) of other species of cowbirds have a larger Hp in thebreeding season than in the nonbreeding season (Clayton & Reboreda, 1997). We propose that CF did not undergo the normal
expansion of the Hp that occurs during the breeding season in
WCF. Further, experiments will be necessary to isolate the particular variable that led to differences between CF and WCF; possibilities include nest searching per se, behaviors otherwise related
to female breeding, physiological events altered by captivity infemales only, and sex-specific reactions to captive conditions.

Experience and the Hp
Any experience-driven change in Hp volume, whether as aresult of nest searching or some related behavior, is an example ofadult plasticity of the Hp. Experience-driven changes in neuroanatomical and neurochemical characteristics of the Hp occur as aresult of environmental enrichment, learning regimes, and exercise
in both young and adult laboratory rodents (Kempermann, Kuhn,& Gage, 1997; Kozorovitskiy et al., 2005; Rosenzweig & Bennett,
1996; van Praag, Kempermann, & Gage, 1999). In birds,
experience-driven changes in Hp volume have been demonstrated
in juvenile marsh tits, a food-storing bird (Clayton, 1995b, 1996).
Although adult Hp plasticity in food-storing birds is supported bystudies using black-capped chickadees (Smulders et al., 1995, but
see Cristol, 1996), it appears that captive conditions more restrictive than our outdoor aviaries (smaller aviary, all males) can resultin a decrease in Hp volume (Barnea & Nottebohm, 1994; Smulders, Casto, Nolan, Ketterson, & DeVoogd, 2000). It is not clear
what social or environmental factors excluded by captivity may
have played a role in changes in Hp volume, but these studies
suggest that experiences in addition to exclusion from nest searching can alter Hp volume. Thus, we must consider the possibility
that experience with nest searching is not the driving force behindthe changes we see in Hp size. Instead, the behavior thought todrive changes in Hp volume may coincide with other behavioral or
physiological variables that are the actual cause of the sex-specificneuroanatomical changes.

531

Behavioral Variables Altered by Captivity
In addition to prevention of nest searching, there are several
behaviors prevented in captivity that might affect females morethan males. These include changes in the ability to form dominance hierarchies and to defend breeding territories. In the wild,
both male and female cowbirds have dominance hierarchies. In
captivity, the dominant male in a cage (Dufty & Wingfield, 1986)
shows mate guarding and excludes subordinate males from courting females (Rothstein et al., 1986). Females show less clearly
defined dominance in captivity (Rothstein et al., 1986), possibly
because the resources that they normally defend, host nests, areabsent. The lack of spatial defense could affect both spatial memory requirements and hormone profiles that, in turn, could affectHp size (Galea, Spritzer, Barker, & Pawluski, 2006; Parducz et al.,2006). In fact, one could suggest that while females were prevented from nest searching and the tracking of other females,
males were still involved in keeping track of females and othermales—a task that is also likely dependent on the Hp (Day, 2003;
Volman et al., 1997). In addition, the establishment of dominanceitself can affect Hp cell proliferation or incorporation (Kozorovitskiy & Gould, 2004; Pravosudov & Omanska, 2005). Dominant
rats have more new cells in the Hp than subordinates (Kozorovitskiy & Gould, 2004; Pravosudov & Omanska, 2005). Similarly,
in mountain chickadees (Poecile gambeli), dominant birds show
higher levels of cell proliferation in the Hp (Pravosudov & Omanska, 2005). It will be important to determine if the effects seen in
mountain chickadees generalize to female cowbirds.
In addition to these social behaviors, captivity has a direct effecton a female cowbird’s reproductive condition. Although CM stillcourt and copulate in a manner similar to that seen in free-ranging
males, females show a dramatic decrease in egg laying (S. I.
Rothstein, personal communication, March 2007). Wild female
cowbirds normally lay 40 eggs or more over an 8-week breeding
season (Rothstein et al., 1986), which is far more than the average
passerine. This lengthy and robust egg-laying period is associatedwith longer elevation of estrogen (estradiol, E2) levels than in mostpasserines (Dufty & Wingfield, 1986). Although it is unclear
exactly how a reduction in egg laying could affect Hp volume, it
is likely that CF experience an altered hormonal profile comparedto WCF, which in turn could affect Hp volume as discussed in thefollowing section.

Hormones and Hp
Hp volume in female cowbirds may be influenced by changes in
E2 in a manner similar to the that in mammalian species where E2
is associated with increased neurogenesis, dendritic arborization,and synaptogenesis in the Hp (Galea et al., 2006; Parducz et al.,2006). In birds, we know that testosterone (T) and its metabolites,
E2 and 5␣-dihydrotestosterone (DHT), influence the anatomy ofthe vocal control circuit in adult songbird species (DeVoogd &Nottebohm, 1981; Schlinger, 1997; Tramontin & Brenowitz,
2000). In addition, E2 appears to regulate apoptosis after Hpdamage in the zebra finch (Saldanha, Rohmann, Coomaralingam,& Wynne, 2004). Last, the estrogen synthetic enzyme, aromatase,is abundant in brown-headed cowbird Hp (Saldanah & Schlinger,1997). Thus, it is possible that differences in E2 or T levels
between WCF and CF affected Hp size. It would not be surprising532

DAY, GUERRA, SCHLINGER, AND ROTHSTEIN

Table 1
Range Size and Morphology of Populations of Eastern and Western Cowbird Subspecies
Western male
Feeding range
Commute feeding to breeding

550 ha220 ha7 km or ⬎a,eEastern male

56 ha

a

Eastern female⬍ Western subspeciesa,ba

7 km or ⬎a,e⬍ Western subspeciesa,bFeeding may overlap breedinga,bMales ⬎ femalesa

Feeding sites visited
Breeding range

Western femaleaMales ⫽ femalesc
78 ha

a

b

6.6 haⱖ7.9 hac
20.4 had

4.5 hab
7.9 hac
20.4 had

Nest searching

No

Yes

No

Yes

Hippocampus

9–10.6 mm3
M ⫽ 10.4 mm3

10.3–12.4 mm3
M ⫽ 11.0 mm3

3.5–9.5 mm3
M ⬇ 5.5 mm3f

5–10 mm3
M ⬇ 6.8 mm3f

Telencephalon

220–276 mm3
M ⫽ 250 mm3

222–246 mm3
M ⫽ 235 mm3

110–290 mm3
M ⬇ 160 mm3f

125–240 mm3
M ⬇ 170 mm3f

41.3 g28.3 g43.9 g35.5 gWeightNote. For Western subspecies, behavioral data refer to two western subspecies M.a. obscurus and M.a. artemisiae, while neuroanatomical data are from
M.a. obscurus only. All Eastern subspecies data are from M.a. ater.
aRothstein et al., 1984, 1986. b Darley, 1968. c (Darley, 1983. d Dufty, 1982. e Curson, Goguen, & Mathews, 2000. f Measurements approximatedfrom single study presented in Figure 2 (Sherry et al., 1993).

if any housing-induced changes in hormones altered Hp volume infemale cowbirds only, given that effects of E2 on Hp neurogenesis,
spinogenesis, and synaptogenesis are low or lacking in male compared to female rats (Galea et al., 2006; Parducz et al., 2006).
Even if E2 affects Hp morphology similarly in male and female
cowbirds, unlike the case in rats (Galea et al., 2006; Parducz et al.,2006), captivity could differentially affect male and female hormone profiles. Although both CM and CF were in apparent breeding condition, it is possible that there were sex-specific effects ofhousing on hormonal profiles in our cowbirds. As mentioned, there
were likely greater differences in reproductive condition between
WCF and CF than between WCM and CM as a result of reduced
egg laying that could potentially affect female hormonal profiles.
In addition, females normally have elevated T levels related todefense of breeding areas (Dufty & Wingfield, 1986). Thus, a lackof this defense could have altered the secretion of T in females.
Last, it has previously been shown that peak T levels are similar
between WCM and CM during the time that we killed birds(Rothstein et al., 1986). Comparable studies have not been done infemale cowbirds. Because hormonal state can alter Hp neuroanatomy, future studies attempting to isolate how captivity alters
female Hp while not affecting male Hp should include measurement of levels of T, E2, and the estrogen synthetic enzyme,
aromatase, in blood and brain of males and females. Experiments
aimed at determining the direct effects of E2 on the Hp in cowbirds
and other species are also warranted.

Cowbird Population Differences
It is important to acknowledge that, unlike studies in the easternbrown-headed cowbird (M. a. ater; Sherry et al., 1993), our study
and two others (Uyehara, 1994; Uyehara & Nairns, 1992) thatexamined the western subspecies (M. a. obscurus) did not find that
WCF had a significantly larger Hp than WCM. These discrepancies could result from methodological differences. However, if we

perform our statistical comparison exactly as Sherry et al. (1993)
did, comparing only male to females and including weight as acovariate along with Tel, p ⫽ .13, and without weight as acovariate, p ⫽ .08. In Uyehara’s (1994) two studies that usedstatistics similar to Sherry et al. (1993), sex differences for western
cowbirds were p ⫽ .25 and p ⫽ .15. Thus, at the very least, there
appears to be a reduction in the extent of sex differences in thewestern subspecies compared to the eastern subspecies using identical methods.1 In addition to decreased sexual dimorphism, Hp
size and Tel size appear to be larger in M. a. obscurus than M. a.
ater despite the smaller body size of M. a. obscurus compared toM. a. ater (see Table 1). Pronounced differences in habitat use,such as the larger ranges in the western than eastern subspecies,
could account for the population differences observed in Hp sizeand extent of sexual dimorphism (see Table 1).
However, if M. a. obscurus has a less sexual dimorphic Hp thanM. a. ater because both males and females have large feeding
ranges, why would captivity in the present study not reduce thesize of the male Hp given the enormous reduction in home range?
Both male and female screaming cowbirds (M. rufoaxillaris) show
a reduction in Hp volume in the nonbreeding season compared tothe breeding season (Clayton & Reboreda, 1997), thus male Hp
plasticity is possible in cowbirds. Obviously, more research isneeded to isolate why we found sex-specific effects of captivity on
Hp volume in M. a. obscurus despite a lack of sex-specific Hp size
in wild birds of the population we studied. Future studies should
consider subspecies and within-subspecies population differences
as cowbird space use appears to depend on a number of habitat,
habitat use, and population size dimensions (Darley, 1982, 1983;
1

Using residual analysis or arcsine transformed Hp/Tel as per methods
in Reboreda et al. (1996) and Clayton and Reboreda (1997) for shiny
cowbirds yields results as per our GLM except WCF versus WCM issignificant.

SEX-SPECIFIC EFFECTS OF CAPTIVITY IN COWBIRDS

Dufty, 1982; Lowther, 1993; Rothstein, Verner, & Steven, 1984;
Rothstein et al., 1986). Precedents for such within-species population differences are found in black-capped chickadees (P. atricapilla). A larger Hp in Alaskan chickadees compared to those from
Colorado is thought to reflect adaptations to the limited and unpredictable food supply in Alaska (Pravosudov & Clayton, 2002).

Conclusion
As hypothesized, female but not male cowbirds (M. a. obscurus)
had a smaller absolute and relative Hp compared to wild-caught
counterparts when they were prevented from nest searching by
being held in large outdoor aviaries for 1 year. This effect did notappear to be general to the whole brain as no reduction in NRot
occurred. The effects of an impoverished environment cannot
adequately explain results. Males were not affected by captivity,and females placed in isolation chambers did not have a smaller
relative Hp volume than CF or WCF. The experiential or physiological mechanism underlying the differences we found remains to
be determined. Regardless of the exact mechanism, our results are
unique in demonstrating that experimental alteration in behavior
can be associated with a sex-specific effect on Hp volume in adultbirds.

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Received May 4, 2007
Revision received November 8, 2007
Accepted December 5, 2007 䡲