414
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The Condor9R414-417
0 The CooperOrnithologicalSociety1996
LACK OF SPERM STORAGE
SIGNIFICANCE
BY FEMALE
MIGRANTS
OF COPULATIONS
AND
THE
EN ROUTE’
JAMESV. BRISKIE~
Department of Biology, Queen’s University,Kingston,Ontario K7L 3N6, Canada
Key words: Sperm storage;sperm storagetubules; route copulationsmay have an adaptive function, such
as minimizing the time and costs of mate choice or
migrant warblers;copulationbehavior;Manitoba.
ensuring females have an adequate supply of sperm
In all speciesstudiedthus far, female birds storesperm should they not find a mate on the breeding grounds.
Unfortunately, the role that en route inseminations
in specializedsperm storagetubules (SSTs) located at
the junction of the uterusand vagina (Fujii 1963, Shu- play in the eventual fertilization of eggsremains unaart 1988. Birkhead et al. 1990. Briskie and Montaomerie 1993). SSTs appear necessaryfor the long-term
survival of spermatozoawithin the female reproductive tract, becausesperm not stored within SSTs soon
die or are displacedby the passageof subsequenteggs
(Lorenz 1966). The length of time that sperm can remain viable in storagevaries from speciesto species,
rangingfrom about six days in the Ring Dove (Streptopeliaroseogrisea;
Zenone et al. 1979) to over 72 days
in the domestic Turkey (Meleagris gallopavo;Lorenz
1950). Why sperm storageduration varies among speciesis not clear, but it may be related to the proximity
of males and females during pre-laying (Birkhead and
Moller 1992a). For example, sperm storagedurations
are particularly long in seabirds,in which females are
separatedfrom their mates for prolongedperiods prior
to laying. In contrast,males of many passerinespecies
typically remain closeto femalesthroughoutpre-laying
and these specieshave correspondinglyshorter durations of sperm storage(Birkhead and Meller 1992a).
In most birds, copulationsare generallyrestrictedto
a brief prelaying period, with the greatest frequency
occurringjust prior to the laying of the first egg. For
migratory species,pairing and mating are thought to
take placelargelyon the breedinggroundsafter the end
of springmigration. However, by usinglavagesto census sperm within the cloacaeof male and female migrant passerines,Quay (1985a, 1989) found that many
TennesseeWarbler ( Vermivoraperegrina)femalesand
possibly some Blackpoll Warbler (Dendroica striata)
females had been inseminated during migration and
south of their nearest breeding grounds. In the TennesseeWarbler, about 25% of femalescontainedsperm
in their cloacae, indicating that copulatory behavior
during migration may be widespread(Quay 1989). Recently, Moore and McDonald (1993) suggestedthat en
knOWll.
If copulations during migration play a role in the
fertilization of eggs,then females must be able to store
sperm acquireden route and to arrive on the breeding
groundswith this spermstill viable (Ouav 1989. Moore
and McDonald 1993). Since sperm canremain viable
for extended periods only if stored within the SSTs,
migratory birds must have active and developed SSTs
if en route copulationsare to have any functional significance. In this study, I examined the reproductive
tractsand SSTsof four speciesof migratory passerines,
including Tennesseeand Blackpoll Warblers, to determine if females can store sperm prior to their arrival
on the breedinggrounds.My resultssuggestthat sperm
are unlikely to remain viable from inseminations occurringen route, and that suchbehavior may be simply
an epiphenomenonof the recrudescenceof the reproductive organsduring migration.
METHODS
Six female TennesseeWarblers and three female Blackpoll Warblers were collected during the last week of
May in 1992 and 1994 at Delta Marsh, Manitoba,
Canada.Both speciesare common migrantsin this area
and were moving throughin largenumberson the days
they were collected. Neither species nests at Delta
Marsh, the southernmostbreeding location for TennesseeWarblers is about 100 km north of the collecting
site, while the nearest record for Blackpoll Warblers
lies about 450 km farther north (Godfrey 1986). I also
salvagedone female Black and White Warbler (Mniotilta-varia)on 14 May 1992 and one female Ovenbird
(Seiurusaurocapillus)on 22 May 1992 from window
kills in the southend of the city of Winnipeg, Manitoba.
The salvagesite for these specieslies within their respectivebreedingranges,but it is likely that both were
migrantsbecauseof the early date and the unsuitability
of the salvage site (parking lot and apartment complexes)for breeding. For comparison with migrants, I
I Received 28 August 1996. Accepted 26 January collectedone additional BlackpollWarbler in breeding
1996.
condition at Churchill, Manitoba on 10 June 1994.
* Current address: Department of Zoology, South This female had an egg in the oviduct and several
Parks Road, University of Oxford, Oxford OX1 3PS, enlargedfollicles in the ovary. Although sample sizes
United Kingdom, e-mail:[email protected]. were small, it is likely the results presented here are
ac.uk
representative,as previous work has shown little vari-
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ation in SST morphology betweenfemalesof the same
speciesrelative to that observedamong species(Birkheadand Hunter 1990, Briskieand Montgomerie 1993).
Within an hour of obtaining the specimens,the oviduct wasremoved, pinned straightto a cork board, and
then immersed in 10% formalin. After fixing for 24 h,
the tissue was transferred to a labeled vial and stored
in 10% formalin for a further four to six months before
processing.The method used for preparing oviduct
tissuefor the examination of SSTs has been described
in detail by Briskie and Birkhead (1993). Briefly, I first
exposedthe oviductal lumen by making a longitudinal
incision throughthe vagina and uterusand pinning the
tissueflat onto a cork dissectingtray. The folds at the
uterovaginal junction were counted, and a random
sampleof three folds was removed. Each fold wasthen
cut alongthe mid-ridge, and eachhalf waslaid flat onto
a glassslide with the lumenal surfacefacing down. A
drop of phosphate-bufferedsaline was added, and the
tissuewas covered with a glasscover slip.
Sperm storagetissue was examined at high magnification (x 400) with a light microscope.For each fold,
I counted the SSTs; the mean number of SSTs from
the three folds wasthen multiplied by the total number
of folds to estimate the total number of SSTs for that
individual. Next, I measured the outside length of 30
SSTs per individual (10 SSTs/fold) with a calibrated
ocular micrometer. SSTs for measuringwere selected
by running a transectacrossthe fold from the uterine
to vaginal end. This ensuredthat about equal proportions of SSTsfrom all regionsof the uterovaginaljunction were sampled. Finally, I examined each SST for
the presenceor absenceof sperm. Passerinespermatozoa have a characteristicspiral-shapedhead and thus
are readily recognizable within the lumen of a SST
(Briskie and Montgomerie 1993).
RESULTS AND DISCUSSION
SSTs were observedin all individuals of each species
examined (Fig. 1). The estimatednumber of SSTsvaried from 327 + 39 (n = 6 females) in the Tennessee
Warbler to 492 SSTs (n = 1 female) in the Black and
White Warbler (Table 1). The number of SSTs in each
415
FIGURE 1. Photographs of sperm storage tubules
(SSTs) in a (A) migrant and a (C) breeding female
BlackpollWarbler and line tracings(B, D) to illustrate
structuralfeatures.SSTsappear as small, round structures embedded in the mucosa of the oviduct in migrating birds (A, B); the lumens of SSTs at this time
are indistinct (dotted lines in B) and no sperm were
observed. By breeding (C, D), SSTs have elongated
about seven-fold and contain many sperm. The line
tracingswere made directly from the photographsbut
eachwasenhancedto show featuresnot readily visible
in a singleplane. Bar is 50 Mm. Symbols:W = wall of
SST, L = lumen of SST, SH = head of sperm, ST =
tail of sperm, OV = lumen of oviduct.
of the four speciesof warblers studied here was less
than half that expectedfor their body mass (Birkhead
and Moller 1992b, Briskie and Montgomerie 1992,
1993). The reasonfor this difference is not known but
could be either a characteristicof parulid warblers in
generalor the resultof a seasonalchangein the number
of SSTs as egg laying approaches,as all birds in the
other studieswere in breedingcondition. Since all the
SSTsI observedin migratingbirds were typically small
(see below), it is also possible that I missed some of
the very small SSTs in my census.However, seasonal
TABLE 1. Number and length of sperm storage tubules (SSTs) in individuals of four species of migrant
passerinescollected in southern Manitoba, Canada. SST length calculatedfrom 30 SSTs per bird.
SST length
Species
TennesseeWarbler
Blackpoll Warbler
Black and White Warbler
Ovenbird
Number of SSTs
Mean k SE
256
476
380
360
260
232
415
292
225
492
377
118.3 + 6.9
96.9 ? 5.3
143.0 2 6.0
123.0 Z! 5.9
148.8 f 8.1
96.6 f 3.6
140.5 + 7.6
75.8 + 3.4
74.4 + 4.1
57.1 + 4.3
58.1 + 3.2
(pm)
RFUlge
47.5-180.0
42.5-167.5
82.5-217.5
57.5-187.5
52.5-275.0
52.5-132.5
67.5-210.0
42.5-l 10.0
40.0-137.5
22.5-102.5
27.5-102.5
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increasesor small size are unlikely to explain the low
number of SSTs in Blackpoll Warblers, as the number
of SSTs in the single female collected on the breeding
grounds(368 SSTs)was very similar to that observed
in migrants of this species (331 ? 56 SSTs, n = 3
females; Table 1).
Despite the presenceof SSTs, no spermatozoawere
observed in any migrating female of the four species
examined (Fig. 1). SST length ranged from 121.2 t9.0 pm (n = 6 females) in the TennesseeWarbler to
only 57.1 rrn (n = 1 female) in the Black and White
Warbler (Table 1). SST length for migrating Blackpoll
Warblers (96.9 * 21.8 pm, n = 3 females) was less
than one-seventh that observed during the breeding
season(725.8 f 45.8 pm, n = 1 female). Although I
was unable to measurethe length of SSTs in breeding
TennesseeWarblers, Black and White Warblers. and
Ovenbirds, SSTs in’these specieson migration ‘were
generallymuch smallerthan thosemeasuredby Briskie
and Montgomerie (1993) in other passerinespeciescollected during the breeding season.This suggeststhat
SSTsin all the migrants I sampled were not developed
nor potentially capableof storingspermif inseminated.
This lack of developmentis particularlyevident when
SST length is compared against sperm length. In the
TennesseeWarbler, sperm length averages 187.8 pm
(n = 2 males; unpubl.) or about 55% greater than the
averagelengthof the SSTsduring migration ( 121.2 pm;
Table 1). Of the 180 SSTs measured in the six TennesseeWarbler females, only seven (3.9%) SSTs were
sufficiently large enough to accommodate the entire
length of a spermatozoa.Since sperm not completely
protectedwithin a SST are unlikely to survive (Lorenz
1966), it appears that most SSTs are too short and
undevelopedto store sperm during spring migration.
The same pattern was evident in the three Blackpoll
Warblersexamined.In this species,spermlength(269.5
Nrn, n = 1 male; unpubl.) was almost three times the
averagelengthof the SSTs(96.9 urn:Table 1) and none
of the90 SSTsexamined in the three migratingfemales
were large enoughto accommodateeven a singlespermatozoa. No information is available on the length of
sperm in either Black and White Warblers or Ovenbirds; however, both speciesalso had very short SSTs,
suggestingthat they too would not be able to store
sperm if inseminated at this stage.
The lack of sperm storageby female migrants is not
a product of the lack of sperm production in males.
Two male TennesseeWarblers collected on migration
were estimatedto have approximately 1.3 and 5.8 million sperm, respectively,in their seminal glomera (the
site of sperm storagein male birds). The seminal glomera of a singleBlackpoll Warbler salvagedfrom a window kili also contained numeroussperm, althoughthe
exact number was not estimated. In a more extensive
survey of sperm production by migratory birds, Quay
(1985a. 1985b. 1986a. 1986b) found that males in a
wide variety of passe&es initiated sperm production
while on migration and far southof their nearestbreeding grounds.Thus, if copulationsdo occur en route in
some migratory passerines,it is likely that males are
capable of inseminating females with viable sperm,
even though theseinseminationsdo not appearto lead
to subsequentsperm storage.
In a study of seasonalchangesin SST morphology
in the Yellow-headed Blackbird (Xunthoce~halusxunthocephalus),I found that SST; were present in all
females and in roughly equal number throughout the
season(Briskie 1994). As with the migratory warblers
examined in this study, SSTs in blackbirds collected
on the day they arrived on the breeding grounds(and
thus at the end of their spring migration) were small
and regressedand did not contain any sperm. After
females arrived on the breeding grounds, their SSTs
rapidly doubled in size and reached maximum length
in about eight to fourteen days, although sperm were
not observedwithin SSTsuntil three to five daysbefore
clutch initiation (Briskie 1994). However, it is interesting to note that SSTs in newly arrived birds were
almost double the length of thosefound in post-breedine birds (Briskie 1994). This sueeeststhat some de’ of the SSTsmust occurwhile on migration,
velopment
althoughnot nearly enoughto allow for sperm storage.
Although I examined SSTsin only a few speciesand
a small number of individuals, it was clear that none
of the femalescollectedon migrationeitherstoredsperm
or appearedcapable of storing sperm if inseminated.
Why then do some speciescopulate en route to their
breedinggroundsif suchbehavior doesnot lead to the
subsequentfertilization of eggs?Quay (1989) suggested
that en route copulations may provide some physiological or behavioral advantageto the individual in a
kind of “inseminatory practice run” before the breeding territory is reached. However, why this should be
necessaryin some speciesand not others is not clear.
It is also possiblethat en route copulationsmay function in pair formation (Moore and McDonald 1993).
Such an function should be most advantageousin
northern specieswith a limited breeding season,and
indeed,the two speciesknown to copulateon migration
(Tennesseeand BlackpollWarblers) are two ofthe most
northerly breeding speciesof warblers. Nonetheless,it
seemsunlikely that extensivepairing occursduring migration in most speciesof passerinesbecauseof the
differential timing of migration between the sexes
(Francis and Cooke 1986).
If femalesparticipate in en route copulationsonly if
their reproductive tracts are physiologicallyready to
receive and storesperm, then the lack of sperm storage
I observedmay have been due to inadequatesampling.
This hypothesissuggeststhat females have two strategies, with some individuals pairing, copulating and
storing sperm on migration (SSTs developed), while
other individuals remain sexuallyinactive and do not
mate until they arrive on the breeding grounds(SSTs
undeveloped).If females usingthe former strategyare
uncommon, then it is possiblethey were missedas my
samplesizewassmall (n = 11 femalesin total). Instead,
only females of the latter strategywere collected and
they would therefore not be expected to store sperm.
Determining whether such a dicotomy exists among
femaleswould be relatively easyto test but will require
a more extensive survey of birds during migration.
Perhapsthe most parsimoniousexplanation for why
some birds copulate en route is that such behavior
resultsfrom migration occurringduring the recrudescenceof reproductive tissues.In other words, the hormonal systemnecessaryto stimulate the regrowth of
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417
BRISKIE,J. V., AND R. MONXOMEIUE. 1992. Sperm
sizeand spermcompetition in birds. Proc. R. Sot.
Lond. B i47:89-95:
BRISKIE.J. V.. ANDR. MONTGOMERIE.1993. Patterns
of sperm storagein relation to sperm competition
in passerinebirds. Condor 95~442-454.
FRANCIS,C. M., AND F. COOKE. 1986. Differential
timine of snrina m&ration in wood warblers (Parulin&). Auk l?l3:548-556.
FUJII,S. 1963. Histologicaland histochemicalstudies
on the oviduct of the domestic fowl with special
I thank the University of Manitoba Field Station at
reference to the region of uterovaginaljuncture.
Delta Marsh for providing accommodationduring my
Arch. Histol. Japan 231447-459.
collectionsin the area. This work was supportedby a
GODFREY,W. E. 1986. The Birds of Canada. Natl.
grant from the Natural Sciencesand EngineeringReMus. Canada, Ottawa.
searchCouncil of Canada (NSERC). Collections were
L~RENZ,F. W. 1950. Onset and duration of fertility
made under a permit from the Canadian Wildlife Serin turkevs. Poul. Sci. 29:20-26.
vice. Bob Montgomerie, Stephen Yezerinac and two
L~RENZ,F. W. 1966. Behavior of spermatozoain the
anonymousreviewers made severaluseful suggestions
oviduct in relation to fertility. In C. Horton-Smith
in the preparation of this manuscript.
and E. C. Amorose leds.1.Phvsioloav of the domestic fowl. Oliver and Boyd,~Edinburgh.
MOORE,F. R., ANDM. V. MCDONALD. 1993. On the
LITERATURE CITED
possibilitythat intercontinentallandbird migrants
copulateen route. Auk 110:157-160.
BI~KH~AD,T. R., ANDF. M. HUNTER. 1990. Numbers
QUAY, W. B. 1985a. Spermreleasein migratingwoodof sperm-storagetubulesin the Zebra Finch (Poewarblers (Parulinae) nesting at higher latitudes.
phila guttutu) and Bengalese Finch (Lonchuru
Wilson Bull. 97:283-295.
striatu). Auk 107:193-197.
BIRKHEAD,T. R., ANDA. P. MOLLER. 1992a. Sperm QUAY, W. B. 1985b. Cloaca1sperm in spring migrants:occurrenceand interpretation. Condor 87:
competition in birds: evolutionarycausesand con273-280.
sequences.Academic Press,London.
QUAY, W. B. 1986a. Timing and location of spring
BI~KH~AD,T. R., AND A. P. MOLLER. 1992b. Numsperm releasein northern thrushes.Wilson Bull.
bers and size of sperm storage tubules and the
duration of sperm storagein birds: a comparative
981526-534.
QUAY, W. B. 198613.Cloaca1protuberanceand clostudy. Biol. J. Linn. Sot. 45~363-372.
acal sperm in passerinebirds:comparative study
BIRKHWU),T. R., J. E. PELLATT, AND F. M. HUNTER.
of quantitative relations. Condor 88: 160-168.
1990. Numbers and distribution of sperm in the
QUAY, W. B. 1989. Insemination of TennesseeWaruterovaginal sperm storagetubules of the Zebra
blersduringspringmigration. Condor 9 1:660-670.
Finch. Condor 92:508-5 16.
SHUGART,G. W. 1988. Uterovaginal sperm-storage
BRISKIE, J. V. 1994. Seasonalpatternsof sperm storglandsin sixteen specieswith comments on moraee in the Yellow-headed Blackbird Xunthocephologicaldifferences.Auk 105379-384.
pkzlus xanthocephulus.Ibis 136:323-330.
ZENONE, P. G., M. E. SIMS,ANDC. J. ERICKSON.1979.
BRISKIE, J. V., ANDT. R. BIRKHEAD. 1993. A review
Male Ring Dove behavior and defenseof genetic
of the methodsusedto studythe anatomy of avian
paternity. Am. Nat. 114:615-626.
sperm storage.Omis Stand. 24:323-329.
the genitals may also inadvertently trigger the corresponding sexual behaviors, such as singing or copulations. This may be especiallytrue for northern birds,
which face a relatively short window of favorable conditions in which to pair, mate, and raise offspringand
would be selectedto arrive on the breeding grounds
ready to breed. Thus, en route copulationsmay be an
epiphenomenonof the need for both malesand females
to arrive on the breedinggroundsin reproductivecondition and not as an adaptation to fertilize eggs.