THE ECOLOGY OF BLACKBIRD (AGELAIUS) SOCIAL SYSTEMS GORDON H. ORIANS Mluseumof VertebrateZoology and Departmentof Zoology, Untiversityof California,Berkeley, California* TABLE OF CONTENTS PAGE PAGE INTRODUCTION. ...... ...... .... 285 Initiation of Breeding ..... .. 295 Nesting . STUDY AREAS .. ........... .. 286 Habitat ................... 297 Time of Breeding . 297 METHODS. .... .. 287 Territory ... ..... 298 GENERAL BIOLOGY OF THE SPECIES 2.... 87 Mating System . ... 299 THE SOCIAL SYSTEMS 289 Clutch Size . .. .... 300 1. The Red-wingedBlackbird Nesting Success ... 300 Non-breedingPeriod .289 Feeding Behavior of Adults 301 Initiationof Breeding. ... 290 Colony-size Limitation ... .. 301 NestingHabitat .290 TIME AND ENERGY BUDGETS . .. 302 Time of Breeding .... .. ......... 290 Territory ... ...................... 292 THE EVOLUTION OF 'MATING SYSTEMS 306 Mating System ..... ........ 294 ClutchSize and Nesting Success .. 294 THE EVOLUTION OF BLACKBIRD SOCIAL: SYSTEMS 308 Feeding Behavior of Adults . 294 SUMMARY .310 2. The TricoloredBlackbird Non-breedingPeriod ........ ........ 295 LITERATURE CITED .. 311 INTRODUCTION systems from the modern ecological viewpoint has The conspicuousness of adaptive radiation in lagged behind other approaches because few observers morphology tends to conceal the fact that often the have made use of the background of a century of slight differencesbetween closely related species give Darwinian thinking in evaluating their observations. no clues to their widely differing ecologies, because In this study I have considered all features of many of the important differences between species social systems to be the products of natural selection are the result of behavioral and not morphological just as are any physiological or morphological adapta- adaptations. This study analyses the role of social tions. To the question whether or not differences organization of the Red-winged Blackbird (Agelaius between social systems are adaptive, three types of phoeniceus) and the Tricolored Blackbird (A. tri- answers are possible. Firstly, it may be assumed color) in the differentways in which these two species that the particular features of a social system are exploit their environment. surely adaptive. Secondly, it may be assumed that Knowledge of avian social systems began with the traits are purely fortuitous, without selective natural history studies, but certain phases, such as significance. Thirdly, it may be assumed that the territoriality, early attracted special consideration. particular traits are not adaptive but that they are In the 1930s, social systems began to be studied from associated with other, as yet unrecognized, differences the viewpoint of the comparative ethologist, who is which are adaptive (Maynard Smnith1958). In this primarily interested in the motivational and evolu- paper I shall attempt to interpret as far as possible tionary aspects of behavior patterns, but whose pub- the characteristics of social systems in the light of lications contain a wealth of information about many the first of these three assumptions. The second is ecological features of avian social systems. The rejected because it is sterile as a basis for research mathematical approach to population parameters has and because the widespread and consistent differences provided a basis for considering the consequences of to be discussed cannot be without selective signif- changes in social system characteristics upon basic icance. The third can never be easily accepted, for population parameters, but biologists have in general unless this statement of faith is followed by attempts been suspicious of this approach, which seemed to to discover the traits of adaptive significance and rest upon assumptions of doubtful biological validity. their connection with the supposedly unadaptive trait, The result is rather widespread failure to realize nothing is really explained. Furthermore, no such the significance of certain features of social systems case involving polygenic traits has been shown to be in quantitative terms, and failure to record and pub- true, and separation of desirable from undesirable traits will almost occur with time. lish relevant information. Finally, the study of social certainly Because the closely related and morphologically * Present address: Dept. of Zoology, Univ. of Washington, Seattle 5, Washington. similar Red-winged and Tricolored blackbirds differ This content downloaded from 168.150.38.178 on Wed, 10 Feb 2016 18:05:39 UTC All use subject to JSTOR Terms and Conditions 286 GORDON H. ORIANS Ecological Monographs GORDON H. ORIANS ~~~~~~~~~Vol.31, No.3 strikingly in their social organization, they are ex- cellently suited to studies developed from the premises just given. Furthermore, these and other species of the fainily Icteridae are common, easily observed, and well-known. Moreover, their social systems range fromnroutine territorialityto extreme coloniality and fronm monogamy to promiscuity and parasitism. My studies of these two blackbird species were carried out in north-central California from 1957 through the spring of 1960. The behavioral aspects of this investigation are being treated separately, and a report on autumnal breeding in the Tricolored Blackbird has already been published (Orians 1960). This entire study was conducted under the super- vision an(1 guidance of F. A. Pitelka. The manu- script has also been read by P. Marler and G. L. Stebbins, both of whom have made valuable sugges- tions. Field work was made possible through the cooperation of F. Barnett of Lodoga, California, and C. Haskell of Marysville, who permitted access to their excellent marshes and also kindly provided lodging. A. S. Leopold generously made available his notes on blackbird activity at the Haskell Ranch since 1954. J. Parker of the East Bay Regional Park District permitted me to erect an observation FIG. 1. Study areas in central coastal California. tower on the small marsh at Jewel Lake during the Jewel Lake is marked by the large " X, " the East Park spring of 1958. G. M. Christman aided in the prep- Reservoirby the large circle, and the Haskell Ranch by aration of the illustrations and provided data on the large square. Other study areas are indicated by Californian Indians. At many times during the small dots. Areas of large breeding concentrationsof course of this study I was aided in the field by fellow Tricolored Blackbirds are shown by diagonal lines. graduate students, including J. L. Brown, R. B. Root, E. 0. Willis, M. Konishi, N. K. Johnson, D. R. Both species of blackbirds breed commonly at Medina and J. Frost. Exchange of ideas with G. the East Park Reservoir in the Coast Ranges of Collier, who is studying different aspects of the Colusa County, a body of water formed in 1910 by a same species at Los Angeles, has been a helpful dam across Stony Creek. Because its waters are stimulus. For the two academic years 1958-1960 I used to irrigate orchards in the Sacramento Valley was supported by a National Science Foundation near Orland, in the summer it may be reduced to a Graduate Fellowship which permitted me to spend inere remnant of its winter size and, as happened in extensive perio(ls in the field during the blackbird 1959, may completely dry up. Most of the shoreline breeding season. Travel expenses were in part de- of this large reservoir is devoid of emergent vegeta- frayed by a grant fromnthe Museum of Vertebrate tion but there are two large marches where two Zoology. Finally, my wife helped type the manu- major streams enter it. In addition, there are small script. To all I express my sincere appreciation. patches of cattails in some of the indentations along the shores (Fig. 2). The reservoir is surrounded STUDY AREAS chiefly by heavily grazed blue oak (Quercus doug- Field work was concentrated at four main areas, lasii) parkland, chaparral (chiefly chamise, Adeno- with supplementary observations at other localities stema fasciculatumr), and, at the south end, by cul- (Fig. 1). Studies were first begun at Jewel Lake, tivated fields. I firstvisited the area briefly in 1957 Tilden Regional Park, Contra Costa County, in 1957. and 1958, but at the end of March, 1959, I established The lake, formed by a damnconstructed across Wild- a camp on the shores of the reservoir which served as cat Creek in the Berkeley Hills about 1900, has a a base for field work through the breeding season. By maximum depth of 10 ft and is about 510 ft above early June the water had already dropped to such a sea level (Gullion 1953). About one-third of its 2.7 low level that no blackbirds remained there. Before acres is filled with emergent vegetation, chiefly cat- the reservoir completely refilled in March of 1960, tails (Typha angustifolia and T. latifolia), in which the cattails in the marsh at the southeast end were Redwings nest. No Tricolored Blackbirds breed completely burned. Blackbirds began to breed as there though occasional individuals roost in the soon as new growth permitted and regular observa- autumn and winter. During the non-breeding season tions were also made throughout the second spring. the marsh harbors a large roost of male Redwings, The Haskell Ranch, eight miles southeast of but females are normally present only during the Marysville, Yuba County, is located in the heart of a nesting season. This marsh was studied intensively ranching district. The marsh is surrounded by irri- during 1958 and less frequently in 1959 and 1960. gated pastures and owes its existence to the run-off This content downloaded from 168.150.38.178 on Wed, 10 Feb 2016 18:05:39 UTC All use subject to JSTOR Terms and Conditions Summer1961 THE ECOLOGYOF BLACKBIRD (Agelaius) SOCIALSYSTEMS 287 METHODS The basic methodin this study was simply to record in detail all features of social organization seen in the field,though advantage was takenof nat- ural experimentsoffered by habitat differences, variationsin weather,and human interference.In addition,I conductedelimination experiments in 1959 and 1960 to studyrepopulation. Each of the major displays and vocalizationsof both species was as- signed a symbolwhich enabled mneto record more V- fully the activitiesof an individualbird as long as it remainedin view.