CAUSES AND CONSEQUENCES OF SIBLING AGGRESSION IN NESTLING OSPREYS (WDIONHALIAETUS) by Marlene Machmer B .Sc., Simon Fraser University, 1988 THESIS SUBMIlTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Biological Sciences O Marlene Machmer 1992 SIMON FRASER UNIVERSITY October 1992 All rights reserved. This work may not be reproduced in whole or in part, by photocopy or other means, without the permission of the author. APPROVAL Name: MARLENE MACHMER Degree: Master of Science Title of Thesis: CAUSES AND CONSEQUENCES OF SIBLING AGGRESSION IN NESTLING OSPREYS (PANDION HALIAETUS). Examining Committee: Chair: Dr. R. Mathewes, Professor 7 --. br. R.C -iate Professor, Senior Supervisw Department of Biological Sciences, SFU Department of Biological Sciences, SFU / - a .J.N.M. Smith, Department of Zoology, dverSig of ~riishColumbia of ~iolo~icalSciefi'ces, SFU I hereby grant to Simon. Fraser Unfvarsl$y the tlght to lend my ?thesis, proJsct or extended essay*(the ?It la of xhlch is shown below) to users of tha ShhFraser Unlverslty ~l br$ry, and to make partfnl or sfngla copies ant y for such users or f n response to a request f tam the l ibrary of any oihsr untverslty, or other $ducat tona l i nst f tut ion, on its own behalf or far one of its users. 1 further agrea that permission for-mulf.lp#s copylng of this work for scholariy purposes my be granted by me or tho Daan of Graduate Studios. it Is undersTood that,copyfng or pubflcatlon of ihls work for flnanclat galn, shall not be af towed without my wrltten permission. fftje of Thssfs/ProJect/Extended Essay Author: , .,- . a lslgnaturet ( name I Abstract Sibling aggression and brood reduction were studied in a population of Ospreys (Pandion haliaetus) breeding in the Kootenay region of British Columbia in 1989 and 1990. Almost all of the aggression observed occurred early in the nestling period when broods were less than 20 days of age. Senior siblings were more likely to use aggression and during the first 20 days of the nestling period, they received ~i~cantlygreater shares of food relative to their junior nestmates. After 20 days of age, aggression was rare and all brood members received equal shares of food. In cases of partial brood loss, over 90 percent of mortality involved junior chicks and all known mortality occurred when victims were less than 20 days old. Based on the pattern of food allocation and aggression within broods, and a general decline in aggression with nestling age, brood reduction appeared to be an outcome of aggression-mediated starvation. Brood reduction at successful nests occurred at rates of 12 and 31 percent in 1989 and 1990, respectively. Nestlings grew more slowly in the latter year, suggesting that mortality occurred in response to food limitation. There was tremendous variation in the level of sibling aggression observed between broods. Observations of broods under natural conditions provided little evidence that food amount, within-brood competitive asymmetry or brood size influenced this variation. Results of a controlled feeding experiment in 1989 and 1990 supported the food amount hypothesis for sibling aggression in nestling Ospreys. Broods were slightly but significantly more aggressive when they were hungry indicating that sibling aggression in this species is proximately influenced- - - by food shw-geta. Senior siblings took a greater share of food when they were hungry however this additional food did not come from the shares of younger nestmates. Instead, females compensated for the hunger of their broods by taking smaller shares for themselves and increasing the meal sizes fed to their broods. Experimental results also indicated that some of the inter-brood variation in aggression may be explained by differences in competitive asymmetries between nestmates. Aggression tended to decrease with an increase in brood age asynchrony and with an increase in brood weight asymmetry. The latter decline was significant. This effect is consistent with studies that have . .. manipulated brood hatching asynchrony or weight asymmetry and suggests that wuma&qp adjust their aggression levels based on the perceived level of competition within their broods. Acknowledgements I would like to express my thanks to the members of my committee for their help in improving this thesis. Ron Ydenberg, my senior supervisor, provided guidance and always found time for me during my impromptu visits. I thank him for his patience and understanding given the extended nature of my degree. Bernie Roitberg and Jamie Smith provided helpful criticism on earlier versions of this work. Thanks also to my collegues Scott Forbes and David Green both of whom worked through previous drafts of this thesis. Scott had useful suggestions during the planning of this project and lazy afternoon discussions with David helped me clarify my thinking. I owe special thanks to a number of enthusiastic assistants who I coaxed and who coaxed me up to Osprey nests. John Black and Ann Bussell persevered with me and the mosquitos through an extended field season in 1989. Their sympathetic pregnancies gave us all a good chuckle. My thanks also go to Kyla Super who volunteered her help during the fledgling study. David Green was tremendous help in 1990. His enthusiasm kept me going through a virtually sleepless field season. Over time, I discovered and came to share the secret to David's energy: yogwt icecream and chocolate waffle cones. Thanks to the staff at the Creston Valley Wildlife Management Area for their cooperation during this project. On behalf of the Ospreys, compliments to the Fish & Wildlife Branch in Nelson for the many frozen fish they supplied and thanks to Guy Woods in particular for his continued support of this project. Roger and Charlotte Perquy gave us a home in 1989 and made us feel part of their family. Roger's mirror and Charlotte's cinnamon buns were valuable additions to the study. Travis who made his presence felt at the beginning of the first field season, provided me with lucid insight on the theory and practice of parent-offspring conflict. I thank him for being a good-natured adventuresome baby and for teaching me the meaning of compromise. I hope he suffers no permanent damage as a resdt of the wrath inflicted on him by the senior sibling at nest N78. Last of all, I want to save special thanks for Chris Steeger. Not only was he "super dad" for a summer, but he also found time to collect data, struggle through numerous drafts of this thesis and put up with my endless thesis babble. I could not have done it without him. My research was supported by an NSERC Postgraduate Scholarship and a Graduate Fellowship Award from SN. The World Wildlife Fund and an NSERC operating grant to Ron Ydenberg provided additional financial support. TABLE OF CONTENTS Approval ............................................................................................ ii ... Abstract ............................................................................................. u1 Acknowledgements ............................................................................... v List of Tables ...................................................................................... ix List of Figures ..................................................................................... x I . GENERAL INTRODUCTION ......................................................... 1 I1. SIBLING AGGRESSION AND BROOD REDUCTION IN THE KOOTENAY OSPREY POPULATION Introduction ............................................................................... 4 Methods ................................................................................... 7 Results .................................................................................... 14 Extent. Timing and Variation in Sibling Aggression ................................ 14 Aggression and Food Allocation ...................................................... 19 Aggression and Nestling Growth ..................................................... 24 Nestling Growth and Mortality ........................................................ 24 Proximate Influences on Sibling Aggression ........................................ 29 Discussion ............................................................................... 34 Conclusions ............................................................................. 42 I11 . A TEST OF THE FOOD AMOUNT HYPOTHESIS FOR SIBLING AGGRESSION IN NESTLING OSPREYS Introduction ............................................................................... 43 Methods ................................................................................... 46 Results .................................................................................... 51 Hunger. Food Allocation and Aggression ....................: ....................... 51 vii Aggression and Relative Nutritional Condition ...................................... Aggression and Competitve Asymmetries Between Nestmates .................... Aggression and Brood Size ............................................................ Discussion ................................................................................ Conclusions .............................................................................. IV. GENERAL CONCLUSIONS ......................................................... LITERATURE CITED ................................................................. APPENDIX 1 ...........................................................................