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Host Choice of a Generaüst Brood Parasite, the Brown-Headed Cowbird (Mocothnrs Der)

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Host Choice of a Generaüst Brood Parasite, the Brown-Headed Cowbird (Mocothnrs Der) Host choice of a generaüst brood parasite, the brown-headed cowbird (MoCothnrs der) by Naomi Dawn Grant A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Biology School of Graduate Studies and Research Laurentian University Sudbury, Ontario O Naomi Dawn Grant 1998 National Library Bibliïthèque nationale du Canada Acquisitions and Acquisitions et Bibliographic Senrices services bibliographiques The author has granted a non- L'auteur a accorde une licence non exclusive licence ailowing the exclusive permettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distrr-bute or sell reproduire, prêter, distriibuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/nlm, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thése. thesis nor substantial extracts fkom it Ni la thése ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. The brood parasitic brown-headed cowbird (Molothrus ater) lays its eggs in the nests of many host species. Little is known about how cowbirds locate and select host nests. They may do so nondiscriminatorily (the shotgun parasitism hypothesis) or according to a cost-benefit trade-off (the host quality hypothesis). Evidence from the iiterature supporting the host quality hypothesis versus the shotgun hypothesis was reviewed. Selection of highquality hosts would enhance cowbird success. However, shotgun parasitism may be the best strategy if host quality is difncult to assess, or if the production of many eggs decreases the value of each egg to the cowbird's reproductive success. Recenr estimates of the number of eggs a cowbird lays are lower than previously believed, more consistent with the hoa quality hypothesis than with the shotgun parasitism hypothesis. Indeed, it is clear that cowbirds do not lay entirely nondiscriminatorily. Most cowbird eggs are laid in active nests at an appropriate nesting stage, with host species capable of raising cowbird offspring. Preference for hosts that do not eject cowbird eggs also appears to be real, although more data are required. However, in this study no solid evidence was found that cowbirds discriminate beyond this level. Frequency of parasitism was not higher for host species that were lest iikely to desert after parasitism, that fledged a higher percentage of cowbird eggs, andfor that produced cowbird nestlings that grew faster. Frequency of parasitism is not necessarily a measure of preference, and host quality is infiuenced not only by the quality of foster care but also by the costs of locating, assessing, and parasitizing host nests. For these reasons, it is difficult to assess preference for one host species over another. Examining host choice within a single host species may be more informative. In the second part of this study, selection of red-winged blackbird (Agelaius phoeniceus) hosts by the brown-headed cowbird was investigated at Delta Marsh, Manitoba. The host quality hypothesis predicts that cowbirds will select those redwings with the highest iv nest and nestling success. Naturdy parasitized nests were no more likely to succeed than nonparasitized nests. although statistical power to detect such a difference was low. However, parasitized redwings had traits that have been associated with a greater probability of nest success in other populations: primary female status and more aggressive male nest defense. Cowbird and redwing nestlings appeared to be more successful in naturally parasitized nests, although small sample sizes weaken these conclusions. As predicted by the host quaiity hypothesis, parasitism was associated with host traits related to greater nestling success. Primary female red-winged blackbirds were parasitized more fiequently. Rimary females are expected to be better parents because they are more experienced breeders and their nestlings are more likely to receive male provisioning. Indeed, cowb~dsin the care of primary versus secondary females grew less variably. As well, nestlings of primary females grew more rapidly. Naturally parasitized nests had a larger proportion of female redwing nestiings that may offer less cornpetition than the larger male redwing nestlings. Cowbirds also appeared to parasitize less aggressive females. Lower aggression in females was associated with greater nestling success, perhaps because of a trade-off between nest defense and nestling care. DEDICATION This thesis is dedicated to Gustavo Arteca, who knows much more about cowbirds and redwings than he ever expected to, and is still interested in their many secrets. ACKNOWEDGEMENTS 1 would like to sincerely th& my CO-supervisor,Dr. S.G. Sealy (University of Manitoba). His enthusiasm and support in the field made this project possible despite the distance, and he provided constructive, insighdul, and prompt comments on ail the matenal with which 1 fded his mailbox. 1also thank my CO-supervisorshere, Drs. J. da Silva and F-F. Mdiory, for taking on a student outside their own research interests. and for helpful comrnents on drafts of this thesis. Committee member Dr. J.-FI Robitaille aiso deserves thanks for his time and input Fhaiiy, many thanks go to the extemal examiner Dr. J. Picman (University of Ottawa) for his constructive critisism of this thesis, and hû interest, enthusiasm, and pertinent questions at the defense. 1 am grateful to many people for help in the field. 1 was lucky to work with the best field assistants one could wish for: Kim Caldwell, at whose feet all nests appeared, and Monika Tan, who was as curious as 1 about what those birds were up to, and was also an emergency babysitter and great friend. Claire Herbert and Karim Haddad also helped in the field. Jerry Alderson shared 'his' redwing nests, and Glen McMaster offered his experience, support, and some of his precious cowbird eggs. 1would aiso like to thank the staff of the University of Manitoba Field Station (Delta Manh) for their hospitality. A huge thanks goes to my family for aU their encouragement . Without protest, they bundled into a smaU car, over washed out roads, to spend their summer beside a marsh: for me, an incredible place fuil of birds, their nests, and their calls; to many others perhaps best described as a very successful breeding ground for voracious mosquitoes. Gustavo gave me the time 1 needed, much moral suppoa, and all sorts of other help too numerous to list. Despite leaming the word 'thesis' at an unconscionably young age, Ellen made the best of every situation, md kept wonder always in sight for all of us. Damiin gave me many toothless gnbs that couid make me laugh even at the most sleep deprived of times, and should be young enough to recover fiom the aversion to computers he has developed. Fially, 1 wouïd like to thank Catherine Danielles, Gina Schalk, and Carolina Caceres for their fnendship and insight. TABLE OF CONTENTS PAGE S.. Abstract.. ................................................................................ -111 Dedication.. ...................... .. ................................................... .v Acknowledgernents .................................................................... .xi Table of Contents............ .......-....-.........-..-......-...-...*.----.-.--..-...-... vil*. List of Figures ............................................................................ x List of Tables ............................................................................ xi 1. Literature review of evidence for and against selection of higher quality hosts by brown-headed cowbirds 1.1 Introduction........................................................................ 1 1.2 Indirect evidence for the host quality hypothesis versus the shotgun parasitism hypothesis................................................... 4 1.2.1 Behavioural observations of nest-searching behavio W.... ....4 1.2.2 Regional ciifferences in parasitism frequencies.. ............. ..4 1.2.3 The distribution of cowbird eggs among host nests ........... 5 . 1.2.4 Multiple parasitism ................................................. 6 1-2.5 Inappropriate laying.. ........... .... ........................ 13 1.2.6 Number and success of cowbird eggs ................. ..... 14 1.3 Support for predictions of the host quality hypothesis versus the shotgun parasitism hypothesis ........................................... -15 1.3 .1 Selection of high-quality hosts .................................. .15 1.3.1 .1 Appropnateness of the host ................... .15 1-3.1.2 Accepters versus rejecters of cowbird eggs.. 16 1.3.1 .3 Variation in the level of acceptance among accepter species.. ......... ... ................ -17 1.3.1.4 Nest success............... ,.. ................. 20 1.3.1.5 Nestling success: percentage of cowbird eggs fledging, and growth rates of cowbird nestlings. .................. .. ......-22 1.3.1 -6 Postfledging success. .......................... .29 1.3.1.7 Intraspecific host quality............. .. ..... ..30 1.3.2 Avoidance of high-cost hosts ..................................... 31 1.3.2.1 Nest-searching behaviour ....................... 31 1.3.2-2 The necessity of host activity for parasitism.. 32 1.3.2.3 The nesting
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