Abstract. In a cloud forest at Monteverde, Costa Rica, we examined pollen loads received by self-compatible flowers of two pairs of plant species pollinated by hummingbirds: Hansteinia blepharorachis and Razisea spicata (Acanthaceae), and Besleria triflora and Drymonia rubra (Gesneriaceae). Each pair consisted of one species (Hansteinia or Besleria) pollinated by short-billed hummingbirds and a related species (Razisea or Drymonia) pollinated by long-billed hummingbirds. At three different times per species, separated by 1-3 mo, we examined flowers on 28-40 focal plants from a wide variety of floral neighborhoods, ranging from plants isolated from conspecifics, either by distance or by other flowering species pollinated by the same hummingbirds, to plants surrounded by conspecifics.Because short-billed hummingbirds often restrict foraging to areas ofhigh flower density, and because short-tubed flowers adapted for hummingbirds often have similar pollen placement, we predicted that short-tubed flowers isolated from conspecifics would receive fewer conspecific grains and more heterospecific grains than short-tubed flowers surrounded by conspecifics. Because long-billed hummingbirds often forage over large areas and because long-tubed flowers adapted for hummingbirds tend to diverge in pollen placement, we predicted that pollination of long-tubed flowers would be relatively unaffected by floral neighborhood.Effects on pollen loads of floral neighborhood (nearness to or isolation from other flowers) followed few patterns consistent with our prediction or with conventional theory. (1) There were no consistent effects of floral neighborhoods on numbers of heterospecific grains deposited on stigmas; in all four species, regardless of corolla length, effects of particular neighborhood variables (as determined with stepwise multiple regression) were as likely to run exactly counter to conventional models as to corroborate models. (2) In none of the 12 sampling runs did increases in absolute densities of neighboring heterospecific flowers adversely affect pollination. (3) However, in two runs, loads of conspecific grains increased with increases in the absolute density of neighboring conspeci:fic flowers, and/or (in three runs) with increases in their relative density (proportion of conspecifics among neighboring flowers). These runs all involved short-flowered species rather than long-flowered species, tending to confirm our initial prediction, but half the sampling runs, even of short-flowered species, failed to show any density-dependent effects from neighboring flowers pollinated by the same hummingbirds.Flowers frequently received fewer conspeci:fic grains than they had ovules to be fertilized. Therefore, the potential existed for floral neighborhoods to affect seed set and fitness of plants. Nevertheless, even though neotropical hummingbird-pollinated flowers have been cited as examples of species whose flowering peaks are displaced through competition for pollination, competitive effects from neighboring heterospeci...

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Abstract. In cloud forest at Monteverde, Costa Rica, two guilds of bird-pollinated plants exist; one guild pollinated by long-billed hummingbirds, primarily the Green Hermit (Phaethornis guy), and one guild pollinated by short-billed hummingbirds, primarily the Purple-throated Mountain-gem (Lampornis calolaema). Plants were assigned to guilds based on hummingbird visit patterns documented during >4000 plant-hours of field observations, and on identities of pollen grains collected from 600 mist-netted hummingbirds. Other studies indicated that pollination in these plants is often insufficient for maximum seed set. Each guild was examined for character displacement expected within a stable assemblage of plants structured by competition for pollination. (1) By comparing observed flowering phenologies with those obtained through a randomization procedure, we determined whether each species' phenology minimized overlap with the remainder of its guild.(2) We also examined complementarity between phenological displacement and morphological displacement in reproductive structures.Neither guild exhibited pronounced character displacement.(1) In most cases, flowering phenologies were indistinguishable from those generated at random; the few statistically significant departures mostly indicated aggregation, rather than displacement, of flowering seasons. (2) In most cases, morphological similarity was independent of phenological similarity. The only statistically significant result among the studied species was a positive correlation, among long-flowered species only, between rarity and uniqueness of flowering season.We do not conclude that this absence of expected pattern indicates that competition never occurs or that competition is an inconsequential ecological event. Rather, we attribute absence of pattern to the following aspects of biological variability, two of which we have demonstrated in other studies. (1) Within any one year, density-dependent competition for pollination is sporadic, and is not clearly related to flowering season or morphological similarity. (2) The nature of interspecific interactions varies among years, as neither the relative intensities of flowering nor the flowering seasons themselves are consistent from year to year. (3) The nature of interspecific interactions varies with changes in species composition, which occur over short distances. (4) The assemblage of species is probably not stable over long time spans; the species have Gleasonian ecologies that change distribution and abundance faster than natural selection or diffuse competition can screen out improper phenotypes or species, respectively.