Academic journal article The American Midland Naturalist

Nut Caching by Blue Jays (Cyanocitta Cristata L.): Implications for Tree Demography

Academic journal article The American Midland Naturalist

Nut Caching by Blue Jays (Cyanocitta Cristata L.): Implications for Tree Demography

Article excerpt

ABSTRACT.-We examined jay caching patterns over 3 yr in a woodland-prairie landscape in S-central Iowa. Three aspects were examined: jay habitat preferences for caching, jay caching patterns before and after fire, and the influence of predation on nuts by small mammals on tree recruitment in jay territories.

Blue jays cached nuts in a wide range of habitats, from open, grassy patches to mature forest. Jays cached preferentially in regenerating woodland and edge habitats while usually avoiding grassland habitats. Caching increased in grassland following a controlled burn. Detection by small mammals of artificial nut caches placed by the investigator was extremely high.

The results support a keystone role for blue jays in oak forest ecosystems based on their habit of caching large numbers of most acorn species in the ground in habitats generally suitable for germination and establishment. Their long-distance transport and caching activities would be most significant to tree population dynamics during climate change or in contemporary fragmented landscapes.

INTRODUCTION

The blue jay (Cyanocitta cristata L.) has been proposed as a keystone species for oak forest ecosystems Johnson and Webb, 1989) because it may be the sole species that disperses acorns over long distances (1 km or more) and caches them in the ground where they may germinate, establish, and reproduce. Numerous other organisms are known to utilize acorns (Van Dersal, 1940; Cypert and Webster, 1948; Kilham, 1958;Jensen and Nielsen, 1986; Stapanian and Smith, 1986; Webb, 1987; Ostfeld et aL, 1996), but they either do not cache (e.g., deer, turkeys), cache seeds aboveground (e.g., woodpeckers using tree cavities), or cache in the ground but near source trees (e.g., squirrels). Blue jays may be especially important in maintaining oak species and associated biodiversity in landscapes during climate change when dispersal is crucial to population survival (Davis, 1981; Peters and Darling, 1985; Webb, 1988) or where forests have been fragmented by clearing for agriculture or occur naturally as islands (Burgess and Sharpe, 1981; Forman and Godron, 1986).

The importance of blue jays to oak ecosystems depends on the extent to which they facilitate oak recruitment. While jays have been observed to cache large numbers of softshelled nuts (primarily Fagaceae) in many regions (Darley-Hill and Johnson, 1981;Johnson and Adkisson, 1985; Johnson and Webb, 1989), no systematic studies of jay caching or recruitment from jay caches have been published. Caching by jays would not facilitate recruitment if the nuts cached were nonviable, damaged by insects, all consumed by jays and other animals, or cached in a manner or in habitats that would prohibit germination or survival.

The goal of this research was to determine the prospects for successful oak recruitment from the jays' caching activities by addressing three questions at a S-central Iowa field site: (1) What are blue jay caching patterns in natural vegetation?-including identification of the range of habitats in which jays cache nuts; whether caching is random among habitats or is primarily in habitats where oak recruitment is highest; (2) Does fire affect jay-caching patterns?; and (3) How safe are jay caches from predation by small mammals?

STUDY AREA

We conducted the study in a privately owned, 10-ha area in an E-W trending valley adjacent to Stephens State Forest (3725 ha) in Lucas County, S-central Iowa. The study area was a mosaic of grassland, regenerating woodland in an old pasture, and mature forest, with extensive and diverse transitions. This site was chosen because of its ample oak recruitment, high diversity of oak species, large resident population of blue jays, and wide variety of potential caching habitats. Crow et al. (1994) conducted a companion study of oak recruitment and survival at the same site.

Eleven vegetation types (potential caching habitats) were recognized within the study area (Fig. …

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