To examine behavioral mechanisms of fruit selection, American Robins (Turdus migratorius) were studied foraging on Crataegus monogyna fruits in western Oregon. Robins faced a hierarchical decision—making process of fruit selection: (1) in which shrub to feed (decisions among shrubs); (2) which fruits to pick (visual decisions among fruits); and (3) which fruits to shallow once picked (within—bill decisions). Field observations of natural and experimentally manipulated populations of C. monogyna shrubs demonstrated that choices made by robins among shrubs were correlated with three plant traits (decision cues):fruit abundance, fruit size, and fruit pulpiness. These cues were themselves used hierarchically, so that when higher—ranking cues were held constant by manipulating shrubs, the next available cue became important. Once shrubs were selected, field and aviary work revealed that robins then made visual choices among fruits correlated with fruit size. However, no evidence was found to suggest that robins made within—bill decisions; once fruits were picked, additional discrimination did not occur. I suggest that a hierarchical process of fruit selection may have evolved to enable frugivores to cope with environmental uncertainty that is typical of fruiting systems. Such flexibility in foraging behavior may, in itself, promote variability within and among fruiting plants and therefore help to explain why so few general patterns have been identified in fruiting plant—frugivore relationships.

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The fate of fruits from a population of European hawthorn (Crataegus monogyna) in western Oregon, USA, was examined over a two-year period. Only one frugivore, the American robin (Turdus migratorius) foraged on the C. monogyna fruits, making this an unusually straightforward fruit-frugivore system. Dispersal efficiency was low, with an average 21% of seeds being dispersed (carried away from parent plants) each year; the most common fate of fruits was to simply fall. Robins dropped 20% of the fruits that they picked, and defecated/regurgitated 40% of the fruits (seeds) that they swallowed, beneath parent plants. One trait, gruit abundance, strongly affected the probability of bush visitation by robins, bushes with larger fruit displays being preferred. Both absolute dispersal success (number of seeds) and dispersal efficiency (proportion of seeds; success per propagule) were also found to be correlated most strongly with initial fruit abundance. Individual plant fecundity and fruit quality were found to vary little between years; as a result, dispersal efficiencies for individual plants were also annully consistent. Larger (older) plants produced more fruits and therefore had higher fitness. These results suggest that the optimal fruiting strategy for C. monogyna is therefore to get as big as possible as quickly as possible by delaying fruiting until later in life.

Currently, the most common strategy when managing forests for biodiversity at the landscape scale is to maintain structural complexity within stands and provide a variety of seral stages across landscapes. Advances in ecological theory reveal that biodiversity at continental scales is strongly influenced by available energy (i.e., climate factors relating to heat and light and primary productivity). This paper explores how available energy and forest structural complexity may interact to drive biodiversity at a regional scale. We hypothesized that bird species richness exhibits a hump-shaped relationship with energy at the regional scale of the northwestern United States. As a result, we hypothesized that the relationship between energy and richness within a landscape is positive in energy-limited landscapes and flat or decreasing in energy-rich landscapes. Additionally, we hypothesized that structural complexity explains less of the variation in species richness in energy-limited environments and more in energy-rich environments and that the slope of the relationship between structural complexity and richness is greatest in energy-rich environments. We sampled bird communities and vegetation across seral stages and biophysical settings at each of five landscapes arrayed across a productivity gradient from the Pacific Coast to the Rocky Mountains within the five northwestern states of the contiguous United States. We analyzed the response of richness to structural complexity and energy covariates at each landscape. We found that (1) richness had a hump-shaped relationship with available energy across the northwestern United States, (2) the landscape-scale relationships between energy and richness were positive or hump shaped in energy-limited locations and were flat or negative in energy-rich locations, (3) forest structural complexity explained more of the variation in bird species richness in energy-rich landscapes, and (4) the slope of the relationship between forest structural complexity and richness was steepest in energy-limited locations. In energy-rich locations, forest managers will likely increase landscape-scale bird diversity by providing a range of forest structural complexity across all seral stages. In low-energy environments, bird diversity will likely be maximized by managing local high-energy hotspots judiciously and adjusting harvest intensities in other locations to compensate for slower regeneration rates.

Song repertoire size is the number of distinct syllables, phrases, or song types produced by an individual or population.Repertoire size estimation is particularly difficult for species that produce highly variable songs and those that produce many song types. Estimating repertoire size is important for ecological and evolutionary studies of speciation, studies of sexual selection, as well as studies of how species may adapt their songs to various acoustic environments. There are several methods to estimate repertoire size, however prior studies discovered that all but a full numerical count of song types might have substantial inaccuracies associated with them. We evaluated a somewhat novel approach to estimate repertoire size-rarefaction; a technique ecologists use to measure species diversity on individual and population levels. Using the syllables within American robins' Turdus migratorius repertoire, we compared the most commonly used techniques of estimating repertoires to the results of a rarefaction analysis. American robins have elaborate and unique songs with few syllables shared between individuals, and there is no evidence that robins mimic their neighbors. Thus, they are an ideal system in which to compare techniques. We found that the rarefaction technique results resembled that of the numerical count, and were better than two alternative methods (behavioral accumulation curves, and capture-recapture) to estimate syllable repertoire size. Future estimates of repertoire size, particularly in vocally complex species, may benefit from using rarefaction techniques when numerical counts are unable to be performed [Current Zoology 57 (3): 300-306, 2011].