Dietary Relationships among Shrubsteppe Passerine Birds: Competition or Opportunism in a Variable Environment

Abstract

The suggestion that in less stable environments resource limitation and subsequent interspecific competition may be relatively unimportant in determining bird community structure is explored by examining the dietary relationships within a guild of three ground—foraging passerine birds (Horned Lark, Sage Sparrow, and Western Meadowlark) in the shrubsteppe of southeastern Washington, USA, an area of severe, arid, unstable climate. General dietary analyses indicated a strong temporal component to the organization of bird diets: different species collected at the same time ate much the same things while the same species collected at different times ate different things. This pattern is reinforced by cluster analysis and stepwise discriminant analysis. Similarities in diet extended to other components as well. Dietary diversities tended to be the same for contemporaneous collections of birds, as did average prey sizes, although the latter evidenced a few statistically significant exceptions. Theoretically predicted relationships between diet and trophic structure morphology emerged only at the most general level, and even then were not always observed. In general, differences in body size or bill length were insufficient to account for variations in prey sizes, although meadowlarks did on occasion take significantly larger items than the other, smaller species. Average prey size was significantly correlated with the proportion of seeds in the diet and varied seasonally as seed consumption varied. Several aspects of this study indicate that shrubsteppe passerines are largely opportunistic in their foraging and diet selection, and that the apparent absence of fine tuning to their competitive milieu is most likely a function of the variable environment in which they coexist.

Diet switching of seed-eating birds wintering in grazed habitats of the central Monte Desert, Argentina

“…Esta es, precisamente, la tendencia observada en la mayor parte de las especies del ensamble y está reflejada también en el incremento en importancia de los gremios asociados en sentido amplio a la vegetación (tanto en número de gremios como en el de especies dentro de los mismos) durante la primaveraverano. Estos resultados constituyen una evidencia de la importancia de las estrategias oportunistas entre los organismos de las zonas áridas (Wiens y Rotenberry 1979, Rotenberry 1980b, Polis 1991, Boobyer y Hockey 1994) y, a la vez, sugieren que la estructura gremial puede no ser el resultado de procesos organizativos como la competencia interespecífica (Capítulo 1), sino más bien la expresión epifenomenológica , MS) de la existencia de grupos de especies que responden de manera oportunista a las variaciones en la abundancia de distintos recursos (Jaksic et al 1991). Estos aspectos serán abordados en los restantes capítulos.…”

“…En consecuencia, la aproximación depende de la corroboración de una relación entre las características morfológicas de las especies y las características del alimento que consumen (Hespenheide 1971, Wiens y Rotenberry 1980b, Wiens 1984a, Dayan et al 1989, Wainwright 1996, Jones 1997. A pesar de que su importancia es evidente, este supuesto raramente fue considerado y mucho menos puesto a prueba, restringiéndose la mayor parte de los estudios a un examen de las relaciones morfológicas entre las especies (pero véanse Rotenberry 1980b, Wiens y Rotenberry 1980b, Grant 1986b, Jones 1997.…”

Food abundance does not determine bird use of early‐successional habitat

“…Kilgo (2005), working in our study site, suggested that arthropod abundance might not be a limiting factor in determining avian habitat selection in some regions or habitat types, and Rotenberry (1980) and Rosenberg et al (1982) concluded that in some situations, the energy demands of birds are small compared to available food resources. In addition, the flexibility of avian foraging strategies (Alatalo 1980, Rotenberry 1980, Hutto 1981) may have allowed birds to adapt to reduced food availability. Because birds are capable of changing both their foraging behavior and the foods they select, they may locate sufficient food without having to leave areas of low insect abundance.…”

Characterization of the Hanford 300 area burial grounds. Task IV. Biological transport

“…7 Burial Grounds. The dependence of the sage sparrow on sagebrush shrubs (Rotenberry, 1978) for nesting s i t e s limits i t s nesting t o these burial grounds which have a remnant o f sagebrush cover t h a t was not removed during the burial operation. The other three species often s e l e c t nest s i t e s i n areas with l i t t l e or no s h r u b s present (Fitzner, J. N .…”

“…The other three species often s e l e c t nest s i t e s i n areas with l i t t l e or no s h r u b s present (Fitzner, J. N . , 1978;Rotenberry, 1978). All other species l i s t e d in Table 19 probably use the burial s i t e s for feeding .…”