The Condor98~259-27 I 0 The CooperOrnithological Society 1996 DIFFERENTIAL REPRODUCTIVE SUCCESS OF BROWN-HEADED COWBIRDS WITH NORTHERN CARDINALS AND THREE OTHER HOSTS DAVID M. SCOTT AND ROBERT E. LEMONY Department of Zoology, Universityof WesternOntario London, Ontario, N6A 5B7, Canada Abstract. To understandlow productionof Brown-headedCowbirds (Molothrus ater ater) by a largehost, Northern Cardinal (Cardinalis cardinalis), we comparedthat production with thatof threesmall hosts: Song Sparrow (Melospiza melodia), Chipping Sparrow (Spizella passerina),and Yellow Warbler (Dendroicapetechia). Cowbirdswere present in only 11 of 63 broodsfledged by cardinalsbut in 62 of 93 broods fledged by other hosts (P < 0.001). Notably, neither the frequency of parasitized nests nor the number of cowbird eggsper parasitized nest varied significantlyamong the hosts. Unusual features of cardinals as hosts accountedfor the low production of cowbirds by cardinals.First, cardinal eggswere about 50% larger than cowbird eggs.Many cowbird eggs, as well as cardinal eggs,disappeared from nests, which remained active. Second, the incu- bation period of cardinals was short, only about 10 h longer than for cowbird eggs.Third, cardinal clutcheswere small (mode = 3 eggs).Many cowbird eggswere laid after incubation had begun. These features combined to produce great differencesin body massesof young cardinals and cowbirds, even when cowbirds hatched first. Differences were accentuated when cowbirds hatched after cardinals. Competition in mixed broods often resulted in underweight cowbirds, which usually died before or soon after fledging. Cowbirds reared without cardinal nestmatesgrew well and usually fledgedand survived well. Finally, inter- brood intervals were much longer for cardinals than for Song and Chipping Sparrows. Cardinalshad the lowest number of successfulbroods per host-pair. Cowbirds thrived when reared by cardinals in broods with only one or no cardinal nestmates.We suggestthat host brood-reduction enables cowbirds to exploit large hosts, particularly when the host-incu- bation period is short. Key words: Cowbird reproductivesuccess; cowbird/host laying synchrony;nesting mor- tality; Brown-headedCowbird Molothrus ater ater; Northern kardinhl Cardinaliscaminalis; Chipping Sparrow Spizella passerina;Song Sparrow Melodia melospiza;Yellow Warbler Dendroicapetechia; Gray Catbird Dumetella carolinensis. INTRODUCTION (Southern 1958) may rear many cowbirds (unless Much has been written about the effect of the otherwise stated, cowbird refers to M. ater ater). Brown-headed Cowbird (Molothrus ater), a brood Most hosts are smaller and lay smaller eggs parasite, upon the successof its hosts, but rather than cowbirds. In nests of tolerant hosts, young less about the details of its reproductive success cowbirds usually thrive, often to the detriment with different hosts.Passerine species vary great- of their smaller nestmates (Mayfield 1965, Gra- ly in their tolerance of parasitism (Rothstein ham 1989). There are, however, tolerant large 1975). For example, Gray Catbirds (Dumetefla hostswhose eggs are appreciably larger than cow- carolinensis) are intolerant, although often par- bird eggs,including Eastern Meadowlarks (Stur- asitized (Scott 1977), and rarely rear cowbirds nella magna), Red-winged Blackbirds (Agelaius (Friedmann and Kiff 1985). In contrast, tolerant phoeniceus), and Northern Cardinals (Cardinalis speciessuch as Red-eyed Vireos (Vireo olivaceus) cardinalis) (hereafter, cardinal). Nestling cow- birds might be expected to do poorly in com- petition with larger foster-siblings, but, appar- ently, this does not always occur. Nestling cow- birds grow well togetherwith nestling Red-winged LReceived 25 August 1995. Accepted 26 January Blackbirds (Weatherhead 1989, Ortega and Cruz 1996. 2 PresentAddress: Department of Biology,McGill 199 1). On the contrary, we have noted that cow- University, 1205 Dr. Penfield Avenue, Montreal, Que- birds often did not survive well, either as nest- bec, H3A 1Bl , Canada. lings or fledglings, when reared with young car- PW 260 DAVID M. SCOTT AND ROBERT E. LEMON TABLE 1. Number of successfulbroods including cowbirds from clutchesbegun by three hosts” during the cowbird laying season.Percentages in brackets. Number of broods Total broods NOIE Otte TWO Per pair Northern Cardinal 71 21 (30) 37 (52) 13 (18) 0.89 Song Sparrow 42 7 (17) 25 (60) 10 (24) 1.07 Chipping Sparrow 27 1 (4) 18 (17) 8 (30) 1.26 G = 10.4; df = 4; P < 0.05 * Yellow Warbler omitted becauseof small sample;no warblerwas known to be successfullydouble-brooded. dinals. At London, Ontario, between 1955 and not made daily but only periodically to deter- 1961, about 80% of Northern Cardinal nests mine hatching times, appropriate times for band- found during the cowbird laying seasonwere par- ing, and dates of fledging.A triple-beam balance, asitized (Scott 1963), yet few cowbirds fledged modified for field use, or Pesola scaleswere used and fewer survived for more than a few days to weigh eggsand nestlings. (Lemon 1957; Scott, unpubl.). Was this low production characteristicnot only HOST AND COWBIRD PRODUCTION of cardinals, but also of the local assemblageof species containing cardinals? To answer this We observed a total of 49 breeding pairs of car- question, we estimated the annual production of dinals from 1955 to 1961. For most pairs, the fledgling cowbirds by three other locally com- complete annual nesting history and cowbird mon accepter species:Song Sparrow (Melospiza production were determined. melodia), Chipping Sparrow (Spizella passeri- To examine cowbird production by other pas- na), and Yellow Warbler (Dendroica petechia). serines, in 1962 and 1963 we trapped and color- First, we present data on reproductive vari- banded at least one member of each of 22 pairs ables of hosts and cowbirds that affect variation of cardinals, 42 pairs of Song Sparrows, 27 pairs in parasitism. Second,annual production ofcow- of Chipping Sparrows, and 9 pairs of Yellow birds by Northern Cardinals and the three afore- Warblers. Also, we watched six pairs of Yellow mentioned species indicates that cardinals are Warblers in years after 1963. To determine the relatively poor hosts. Third, we analyze in detail number of broods and cowbirds produced by the successof cowbird eggsand nestlings in car- thesebirds we visited each pair of cardinals, Song dinal nests. Fourth, we suggestsome factors that Sparrows, and Chipping Sparrows weekly until contribute particularly to the failure of cowbirds we observed a brood (Table 1). Yellow warblers to succeedwith cardinals. Finally, we discussthe were visited less regularly. Once the presenceor conditions under which cowbirds are reared by absence of a fledgling cowbird in a brood had large hosts. been ascertained, we did not visit this host pair for another month, this being the minimum in- terval before another brood could be produced, METHODS barring overlapping nests. As our visits were not This study is based on data collected mostly be- daily, we could have missed broods or fledglings tween 1955 and 1968 from more than 700 car- that died soon after fledging. dinal nests,including about 230 parasitizednests, Our method of estimation in 1962 and 1963 that were observed on the campus of the Uni- was much less precise than in 1955 to 1961. In versity of Western Ontario, London, Ontario, the latter yearswe noted that some cowbirds died and some contiguous areas (maps in Darley et soon after fledging. Such events would have been al. 1971). Data on cardinals before 1962 were missed in 1962 and 1963. Thus, to make our basedon birds, usually color-marked, whosenests data from the two setsof years more comparable, were normally visited daily to mark cardinal and we consider only cowbirds that were seen two or cowbird eggsfor identification and to determine more days after fledging as a measure of pro- such variables as clutch size, incubation, nestling duction per host-pair. Thus, we could have un- periods, incidence of parasitism, hatching suc- derestimated, but not overestimated, the number cess,and so on, From 1963 onwards, visits were of cowbirds produced. COWBIRD REPRODUCTIVE SUCCESS 261 TABLE 2. Apparentinterval (days)between fledging groups: O-20 days and more than 24 days (Table and laying of the first eggof the next observednest or 2). brood. OTHER REPRODUCTIVE VARIABLES OF Days HOSTS AND COWBIRDS c-20 21-24 >24 Median Reproductive successof cowbirds among differ- ChippingSparrow 10 1 0 11 ent host speciesdepends upon the interplay of SongSparrow 9 0 2 14 Cardinal 6 1 15 28 many factors. First, the proportion of nests that are parasitized (incidence) and the number of cowbird eggs per parasitized nest (intensity of parasitism) set the basis of cowbird production. INTERBROOD INTERVALS AND At this point, several factors bearing on the NUMBER OF BROODS amount of interspecific competition within a nest Interspecific variation in interbrood intervals can come into play including the egg or hatchling influence the number of broods produced by dif- massesof cowbird and host, or the synchrony of ferent species during the period when they are hatching of the two species,which dependsupon vulnerable to parasitism. An interbrood interval the two incubation periods. These may overlap separatesthe date of fledging from the date of depending on the degree of synchrony of egg- laying of the first egg in the next nest. It should laying of host and cowbird, which in turn de- not be confused with the five or six days follow- pends in part