42 Lyon. 4 COWBIRD PARASITISM OF THE NORTHERN YELLOW-THROAT. s¾ r. s. nofst, UND

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1 [ Auk 42 Lyon. 4 COWBIRD PARASITISM OF THE NORTHERN YELLOW-THROAT s¾ r. s. nofst, UND DURXNG the summers of 948 through 95 I made an ntensve study of the lfe hstory of the Yellow-throat (Geothlyps trchas). The summers of 948 and 949 were spent n studyng a populaton near Ann Arbor, Mchgan, and the study was contnued n 950 and 95 at Duluth, Mnnesota. An mportant part of the study was the relatonshp between the Yellow-throat and the Cowbrd ( Molothrus ater). Amount and Type of Injury to the Host.--Twenty nests, or about 38 per cent of the Yellow-throat nests found n the four-year study, were parastzed by the Cowbrd, and only three of these nests produced any Yellow-throat young. Almost 58 per cent of the total egg loss of the Yellow-throats studed could be attrbuted to the Cowbrd. Thrty-three per cent of ths egg loss was due to the removal of host eggs, 20 per cent to punctured eggs, 3 per cent to nest desertons caused by Cowbrd actvty, and 33 per cent to the loss of heat to the larger Cowbrd eggs. Ths nformaton s summarzed for each year n Table. TABLE INJURY TO THE YELLOW-THROAT BY COWBIRD PARASITISM Total number of Yellow-throat nests found Number of Yellow-throat nests parastzed 7 Yellow-throat eggs known lad Egg loss Egg removal attrbuted to Cowbrd Punctured eggs attrbuted to Cowbrd Eggs lost through nest deserton caused by Cowbrd actvty Eggs abortve, loss attrbuted to loss of heat to Cowbrd eggs Total egg loss attrbuted to Cowbrd 6 2 Percentage of egg loss attrbuted to Cowbrd I was never fortunate enough to see the removal of an egg by a Cowbrd. I have assumed, therefore, on the bass of the layng of Cowbrd eggs after or before the host eggs dsappeared and the wellauthentcated proof that Cowbrds do remove host eggs establshed by Hann (937: 204, 94: 2-22), that the ten host eggs and three

2 9s l HOFSLUN ), Cowbrd and Yellow-throat 43 Cowbrd eggs that dsappeared from nne nests were removed by the paraste. There were other nstances where, undoubtedly, host eggs were removed whch do not enter nto these fgures. For nstance, one nest, when found, contaned two Cowbrd eggs and two Cowbrd young, but no host eggs or young. Undoubtedly, Yellowthroat eggs were removed, but just how many s questonable. Consderng both host and paraste eggs, the number of eggs removed from the nest was equal to 8 per cent of the total Cowbrd eggs lad. Ths compares to Hann's (937: 204) fndngs of 85 per cent n hs Oven-brd study. Some of the host eggs left n the nest showed tny punctures n the shell of a type and shape apparently caused by the claws of the Cowbrd. There has been some dsagreement as to whether punctures of ths type are accdental or delberate, and whether they are caused by the nals or the mandbles (Norrs, 947: 88). The sze and the shape of the punctures I found n Yellow-throat eggs ftted the claws better than they dd the mandbles, and I feel, as dd Fredmann (929: 86), that such punctures are caused by the claws of the Cowbrd when the clumsy female tres to place herself on a nest too small for her. I found seven of these punctured eggs durng the nvestgaton, and none of them hatched. Four nest desertons were attrbuted to Cowbrd actvty at the nest. Two nests were undoubtedly deserted because the contents of an egg leaked out on the other eggs through the punctures caused by the Cowbrd. At another nest the Cowbrd lad eggs before the Yellow-throat eggs were lad, and at a fourth nest t seemed lkely that the removal of several eggs may have caused deserton. There was a drect relatonshp between the hatchng success and the number of Cowbrd eggs lad n the nest. No more than two Cowbrd eggs, or one Cowbrd egg and two Yellow-throat eggs hatched n any nest; and f more than one Cowbrd egg was present, no Yellowthroat eggs hatched. Ths relatonshp held true for all parastzed Yellow-throat nests under observaton, and apparently, as ndcated by the number and knds of young, t was true also n broods out of the nest for whch nests were never found. Roughly, the heat of ncubaton each Cowbrd egg receved was enough to prevent the hatchng of two Yellow-throat eggs. Harm (947: 74) estmated that the probable lmt of egg volume that an Oven-brd can hatch successfully was between.3 and.8 tmes the volume of the normal fve-egg clutch, and that t was probably nearer.3 than.8. The normal Yellow-throat clutch s four (Hofslund, 953: 69), and although fve-egg clutches are not un-

3 [ Auk 44 HOl*SLUND, Cowbrd and Yellow-throat [Vol. 74 common, frequently only four of the fve eggs wll hatch. Usng Sch6nwetter's formula (Nce, 937: 3) and average measurements for the Yellow-throat and Cowbrd eggs examned durng the study, I calculated the average volume of a Yellow-throat egg to be near.7 cc. and that of a Cowbrd egg to be near 3. cc. The volume of a normal four-egg clutch would thus be approxmately 6.8 cc.; of two Cowbrd eggs 6.2 cc., or almost the same volume as the fouregg Yellow-throat clutch. The replacng of two Yellow-throat eggs by two Cowbrd eggs would brng the total volume to 9.6 c½., or.4 tmes the normal egg volume. Whle these fgures are calculated and have not been tested n the feld, the number of Yellow-throat-Cowbrd broods I have observed whch had the relatonshp prevously mentoned seems to ndcate that.3 tmes the normal clutch volumes closely represents the lmt of egg-volume that a Yellow-throat wll normally hatch, and ordnarly one can say that a nest wth more than one Cowbrd egg n t s doomed to falure as far as the Yellowthroat eggs are concerned. Shaver (98: 0) blamed the falure of a Yellow-throat egg to hatch on a Cowbrd egg-shell that had slpped over t and adhered there. Hann (937: 204) found that ths type of accdent had no effect on the hatchng of the Oven-brd egg. My fndngs bear out Hann's concluson. I observed three nstances of the half-shell of a hatched Cowbrd egg slppng over the smaller Yellow-throat egg. In one case the shell was removed by the adult Yellow-throat and the egg hatched. In the second nstance I removed the shell, but despte what I consdered as help, the egg faled to hatch. In the thrd nstance the shell remaned and the egg faled to hatch. The evdence seemed to ndcate that the Cowbrd egg-shell dd prevent the hatchng of the latter two eggs, but when the contents of these two eggs were examned they were found to contan embryos that had not progressed beyond the 4 to 5 mm. stage, ndcatng death of the embryo long before the adherence of the Cowbrd halfshell. Shaver made no menton of the Yellow-throat egg beng ppped, and as the contents of the egg were not examned, the embryo may have been dead before the shell adhered to t. In the Ann Arbor regon, Yellow-throats normally rased two broods a season. If an attempt was unsuccessful they tred agan. These attempts contnued untl at least one was successful, or presumably, physologcal changes prevented further effort. One par of Yellow-throats n the study made at least three and perhaps four attempts before they were fnally successful n rasng a brood, whch n ths case conssted of two Cowbrds. Theoretcally, a par of

4 0s7 I-IoFs JND, Cowbrd and Yellow-throat 45 Yellow-throats should rase from seven to ten young a season. The rasng of Cowbrds only, however, satsfes the physologcal urge to rase young. Thus f the frst attempt at rasng a brood produces nothng but Cowbrds, one Yellow-throat brood s lost, and only one more attempt wll be made. If ths also should produce only Cowbrds, t s stll a successful attempt as far as the parents are concerned, and no further effort wll be made. Therefore, Cowbrd nterference under these condton has caused the loss of from seven to ten Yellow-throats. In areas such as Duluth where only one brood s normally rased, t s concevable that a heavy Cowbrd year could cause serous nroads on the populaton. The above nstances are all concerned wth egg loss. I found no loss of nestlngs that could be attrbuted to the Cowbrd wth the possble excepton that Cowbrd nestlngs seem to be more nosy than Yellow-throat nestlngs, and there s the possblty that predators mght be attracted more to nests contanng Cowbrds than to those that have Yellow-throat nestlngs alone. The Cowbrd may hatch from one-half to one day earler than the Yellow-throat, and usually they reman n the nest from onehalf to one day after the Yellow-throats leave, thus exposng the parent brds over a longer perod to the attendant dangers of the nestng cycle. There were no losses of young Yellow-throats durng the four years of study from suffcaton or starvaton, and Yellow-throats n parastzed nests developed at about the same rate as dd those n unparastzed nests. The Cowbrd s an mportant check on the Yellow-throat populaton, but t does not seem to be a crtcal factor, as wtness the fact that although the ncdence of parastsm was relatvely hgh, the reproductve success of the Yellow-throats durng the four-year study was slghtly better than 5.9 per cent. One mportant factor here was that only rarely were cases of parastsm found that occurred after the frst part of July. Second and thrd attempts were generally free of Cowbrd parastsm. In Duluth, where only one brood was rased, the breedng season of the Yellow-throat normally s enough later than the Cowbrd's that the relatve frequency of parastsm usually s qute low. Success of the Cowbrd n Yellow-throat Nests.--At least 75 per cent of the parastzed nests contaned more than one Cowbrd egg (Table 2), the most common number n a sngle nest beng two, and wth an average of 2.0 per parastzed nest, a fgure comparable to the.8 average found by Stewart (953:3) n hs Yellow-throat study.

5 [ Auk 4-6 HOVSLUVm, Cowbrd and Yellow-throat tvol. 74 Ths s apparently a hgh percentage when compared to other passefne vctms. Fredmann (929: 78) found that only 33 per cent of over 9000 passefne nests had more than one Cowbrd egg; Nce (937: 56), 30 per cent of 98 Song Sparrow nests; Norrs (947: 89), 38 per cent of 73 passerne nests; Harm (937: 202), 55 per cent of 22 parastzed Oven-brd nests; and Berger (95: 33), 52 per cent of 2 passefne nests. TABLE 2 DISTRIBUTION OF COWBIRD AND YELLOW-THROAT EGGS IN Frst nestng PARASITIZED YELLOW-THROAT NESTS Subsequent nestngs Number of eggs Number of nests Number of nests Yellow- Cowthroat brd Total Total Average number of Cowbrd eggs per nest: Average number of Cowbrd eggs lad n the 20 parastzed nests: The Cowbrd receved as good care and solctude as the Yellowthroat nestlngs. The Cowbrd hatched after an average of.6 days of ncubaton, a fgure smlar to that found by Harm (937: 204) and Norrs (947: 95), and the young dd not leave the nest normally untl the nnth day after hatchng. They requred roughly a week less care before reachng ndependence than dd the Yellow-throats. Ther enemes must be consdered the same as the host, and that must nclude the adult Cowbrd, too. Paraste eggs as well as host eggs were removed, and sometmes the physologcal urge to lay eggs produced rather strange and, for the Cowbrd, unfortunate results. One Cowbrd female lad an egg n a depresson left after I removed a deserted Yellow-throat nest. Another lad n a nest that already had eggs ncubated for days, and stll another lad n a nest that had been deserted for two weeks.

6 957] HOFSLUND, Cowbrd and Yellow-throat 47 TABLE 3 SUCCESS AND MORTALITY OF COWBIRDS I ARASITIZING YELLOW-THROAT NESTS Yellow-throat nests parastzed 7 Per cent of total nests parastzed Cowbrds eggs lad n all nests Cowbrd eggs hatched Per cent of Cowbrd eggs lad that hatched Cowbrds fledged Per cent of eggs hatched producng fledglngs Loss of Cowbrd eggs Number of eggs lost Removed by Cowbrds Predaton Infertle or abortve 5 0 Deserton Loss of Cowbrd nestlngs Number of nestlngs lost Predaton Average number of Cowbrd eggs per parastzed nest Average number of fledglngs per parastzed nest When we compare the success of the Cowbrd eggs wth that of the host's, we fnd that the per cent of Cowbrd eggs hatched was less than that of the Yellow-throat, 42.5 per cent as compared to 65.8 per cent. Nce (937: 63) had a rato of 63.7 per cent Cowbrd eggs hatchng to 60.7 per cent Song Sparrow eggs. Only nne of the 20 parastzed nests produced Cowbrd fledglngs (Table 3) an average of.3 Cowbrds per successful nest, but only 0.6 per cent Cowbrds per parastzed nest. The Yellow-throat, therefore, can be consdered as only a farly favorable host. SUMMARY The Cowbrd, a frequent paraste of the Yellow-throat, s moderately successful wth ths host. It acts as an mportant check on the Yellow-throat populaton, wth most of the damage t does to the host comng durng the egg stage of the nestng cycle, when t may cause loss of host eggs through delberate removal, accdental punctures, causng of nest deserton, and loss of the heat of ncubaton, that normally would go to the host egg, to the paraste eggs. Ths paper s a part of a thess submtted to the Unversty of Mchgan n partal fulfllment of the requrements for the degree of Doctor

7 48 HOleSI,UND, Cowbrd and Yellow-throat [ Auk LVol. 74 of Phlosophy. The work was done under the drecton of Dr. H. W. Hann. LITERATURE CITED BEROER, A.J. 95. The cowbrd and certan host speces n Mchgan. Wlson Bull., 63: F IED tann, H The cowbrds. Charles C. Thomas, Sprngfeld, Ill. HANN, H.W Lfe hstory of the Oven-brd n southern Mchgan. Wlson Bull., 49: I-IANN, H.W. 94. The Cowbrd at the nest. Wlson Bull., 53: I-IANN, H.W An Oven-brd ncubatng a record number of eggs. Wlson Bull., 59: I-Iors,UND, P. B A lfe hstory study of the Yellow-throat, Geothlyps trchas. Ph.D. thess. Mcroflmed. NICE, M. M Studes n the lfe hstory of the Song Sparrow, I. Trans. Lnnaean Soc. V. Y., 4: NORRIS, R.T The Cowbrds of Preston Frth. Wlson Bull., 59: SHAVER, V. E. 98. A nest study of the Maryland Yellow-throat. Unv. Iowa Stud. n Nat. Hst., 8: -2. STEWART, R.E A lfe hstory study of the Yellow-throat. Wlson Bull., 65: Bology DePt., Unversty of Mnnesota, Duluth Branch, Duluth, Mnn., December 9, 954.