s 542 .18 S74 no.;43 1965 Role of Habitat in the Distribution and Abundance of Marsh Birds by Milton W. Weller and Cecil S. Spatcher Department of Zoology and Entomology Special Report No. 43 Agricultural and Home Economics Experiment Station Iowa State University of Science and Technology Ames, Iowa- April 1965 IOWA STATE TRA YEUNG LIBRARY DES MOlNESt 'IOWA CONTENTS Summary ---------------------- -- --------------------------------------- --- ------------------------------ --- ----------- 4 Introduction ------------------- ---- ------ --- -------- ----- ------------------------------ ---------------------- --- ---- 5 Study areas --------- -- --- --- --- -------------------------------- ---------------------- ----------------------- --------- 5 Methods ----------- --- ----------- --------- ------------------------------------------------------- --- -------------------- 6 Vegetation ---------------------------- ------------ --- -------------------------- --- ------------------ -- -------- 6 Bird populations ---------------------------------------------------------------- -------------------------- 6 Results ______ _ __ ____ __ _ __ ___ __ __ __ ______ __ ___ ___ __ _ _ _ ____ __ __ ___ __ ______ __ __ _____ ______ ____ ___ __ _ _ ____ ___ _____ __ __ ___ ___ _ 6 Species composition and chronology of nesting -------------------------------------- 6 Habitat changes at Little Wall and Goose lakes ------ -------------------------------- 8 Bird populations in relation to habitat ----------- ---- ----------- -------------------------- 11 Distribution of nests in relation to vegetative changes ____________________ __ __ 14 Redwinged and yello·w-headed blackbirds ___ ____________________________________ _15 Coot and common gallinule ---------------- ----- ------------- ---------- -------- ------------ 16 Black tern -- -------------- --- ----------- -- -------------------- ------------- ------------- -------------- 16 Pied-billed grebe ------------------------------------------------------------ ------ ----------------16 Changes in bird populations and vegetation on Goose Lake transecL17 Habitat selection and adaptability __ ____________________________ ___________________ ___________ 17 Redwinged and yellow-headed blackbirds _________________ ___ _______________ ____ 18 Coot and common gallinule ------- ---------------- -------------------- --------- ------------22 Black and Forster's terns __ ______ ____ ____ ___ ________ ___ ______________________________________ 23 Pied-billed grebe ------------ ---- -------------------------- -- --------------------------------------23 Least bittern -- --- --- -------------- -- ----- ----------------------------- ---------------------- --- -----23 Discussion --------------------------------- ---- ------------------------------- -- --------- ------------------- ----- ---- 24 Evolution of habitat niches ___________________ ______ ______________________________________________ 24 Dynamics of habitat and animal populations in marshes _____ __ ___ ____________ 25 Causes and length of the "cycle" -------------------- ------- -- ------------------------- 26 Muskrat populations ------------- -------------- --- ----------------------------------------- -- -27 Bird populations ----- -------------------- --- ---------- -- --- -- -------------------------------------27 Productivity of marshes ________________ _________ _______ __ __ ___ ______ ____ __ ________________________ _.28 Marshes and concepts of succession --------------------------------------------------------29 Literature cited ----------------------- ---------- ------------- ----------------------------------------------------30 SUMMARY Severe drouth during the 1950's produced dra­ ant of open-marsh tages, and nest losses in coots matic changes in the vegetation of midwestern at that time often were due to wind damage. glacial marshes and in the abundance and dis­ Black terns selected low, natural nest sites or tribution of marsh birds. Changes in marsh hab­ built nests low to the water in sparse emergent itat quality and quantity were studied in relation vegetation where they were protected from wave to bird populations in two small central Iowa action. Forster's terns nested on higher sites, marshes, Little Wall and Goose lakes near Jewell. such as active muskrat houses, often in open­ General observations also were made on several water areas, or built nests higher abo.ve the water larger marshes in northwest Iowa near Ruthven. than those of black terns. These marshes were nearly dry in 1956 and be­ The only competition noted was among shore­ came densely vegetated. With gradually rising ward nesting redwings and over-water nesting water levels, plants flourished, and bird popula­ yellowheads. Some interspecific chases were ob­ tions increased. Gross cover maps demonstrated served; yellow heads dominated redwings in the the change in cover-water ratio and interspersion. ideal yellowhead habitat, but redwings occasion­ Population estimates showed the changes in dis­ ally nested in yellowhead territories in small tribution and density of various species of marsh patches of vegetation not used by yellowheads. birds. During dry periods, only adaptable species Evolution of nest-site selection seems to have such as redwinged blackbirds were present. As been influenced by general habitat of the ances­ water levels increased, densely vegetated areas tral stocks (terrestrial versus aquatic), by mode were opened up by muskrat cutting, and yellow­ of locomotion (perchers, walkers, swimmers and headed blackbirds, coots, pied-billed grebes and flyers) and by use of the major emergents (shore­ least bitterns became established and increased in ward or water's edge). The vertical height and numbers. Maximum bird numbers and diversity resulting "layers" of vegetation, their robustness were reached when a well-interspersed cover­ and their relationship to water, influence species water ratio of 50:50 occurred. By 1962, muskrats use and, thereby, species diversity. and high water had eliminated virtually all emer­ Short-term fluctuations in marsh habitat con­ gent vegetation with the result that all species ditions seem common in marshes as a result of except redwings were eliminated. A similar pat­ change in rainfall and subsequent water level tern occurred on marshes throughout Iowa, and changes. The dry and wet, open stages are the similar changes have been noted throughout the least productive of birds, while the hemi-marsh glacial marsh region during this and previous is ideal. Marsh birds have adapted to these con­ post-drouth periods. ditions, and marsh bird populations are charac­ Habitat changes permitted a measure of habitat terized by pioneering ability and mobility. A preference and adaptability in several species. variety of marsh types and sizes of marshes in a Populations shifted from area to area around the given area are essential to the preservation of marsh as conditions changed because of muskrat marsh bird diversity. cuttings. Redwings used shoreward vegetation Marshes are highly productive ecosystems char­ and were the most tolerant of changing condi­ acterized by dramatic short-term fluctuations. tions. They utilized a higher percentage of brush There are periodic invasions of terrestrial flora and tree nest sites over land as emergent vegeta­ and fauna during dry years, while wet years pro­ tion disappeared. Yellow-headed blackbirds were duce a pond or lake-type community. The view­ restricted to robust emergent vegetation standing point of marshes as transient seral stages is chal­ in water but used only those areas adjacent to lenged because of their duration of life and be­ open water. cause of the equally dramatic changes that may Coots and pied-billed grebes both nested over occur in surrounding terrestrial biomes. It is sug­ water in cover of medium density with sizable gested that a biome-type classification be applied adjacent water openings. Both were quite toler- to lakes, marshes, swamps and bogs. 4 Role of Habitat 1n the Distribution and Abundance of Marsh Birds t by Milton W. Weller2 and Cecil S. Spatcher3 The recent drouth of the late 1950's and early indicated that the phenomena viewed during more 1960's produced some dramatic effects on the intensive study at Little Wall and Goose lakes quality and quantity of emergent vegetation in were occurring there. Observational data from glacial marshes of the central United States and Iowa are integrated in the discussion with data Canada. These vegetative changes have had great from marshes in Utah and Texas and in Manitoba, impact on the distribution and size of marsh bird Canada, to present a synthesis of ideas concern­ populations. Similar plant responses must have ing evolution of habitat niches and ecosystem resulted from the dry years of the late 1800's and dynamics, productivity and succession in marshes. the 1930's, and intervening wet years produced The investigation was started by the senior opposite extremes on many marshes. Such hab­ author under a grant from the Iowa State Uni­ itat changes undoubtedly have occurred through­ versity Alumni Foundation and was completed out the racial history of many marsh animals, and under Project 1504 of the Iowa Agricultural and the maintenance of a species depended upon its Home Economics Experiment Station. The junior adaptability. author was financed during 1960 and 1961 by the Obviously, many factors other than habitat in­ National Science Foundation Teacher's Research fluence the size and species composition of a Participation