Calibrating the data logger…
Explores how hummingbird bill morphology, floral traits, and cognitive ecology shape pollination interactions and drive floral evolution across Rocky Mountain wildflowers including columbines, bluebells, and sacred datura.
Flowering plants and their pollinators are bound together by one of the most visible partnerships in mountain ecosystems. In the Gunnison Basin, hummingbirds, bumblebees, and a diverse cast of wildflowers — columbines (Aquilegia), bluebells (Mertensia), scarlet gilia, larkspurs (Delphinium), and many others — illustrate how flower shape, nectar reward, and animal behavior have been shaped by one another over evolutionary time. Understanding this partnership matters because pollination underlies seed set for most subalpine wildflowers, and small shifts in pollinator behavior, abundance, or morphology can ripple through plant reproduction and ultimately the composition of meadows around Gothic, Colorado.
Three ideas anchor most of this research. The first is bill morphology, which simply means the measurable shape of a hummingbird's beak — its length, width, depth, and curvature. Because flowers vary in tube length and shape, bills that match a flower well can extract nectar more efficiently, while mismatched bills may waste energy or fail to transfer pollen. The second is sexual dimorphism, the consistent difference between males and females in size, shape, or behavior that arises when the two sexes face different selective pressures. In hummingbirds, males and females sometimes use flowers differently, and their bills can reflect that. The third is cognitive ecology, the study of how a pollinator's decisions — when to leave a flower, which plant to visit next, how to remember a profitable patch — shape both its own energy budget and the reproductive success of the plants it visits.
Together these concepts frame a central question: how do the choices and physical traits of small, fast-moving animals translate into evolutionary pressure on flowers, and vice versa? The high-elevation meadows around RMBL, with their compressed growing season and well-studied plant and pollinator communities, have served as a natural laboratory for answering this question for more than four decades.
Much of the conceptual scaffolding for pollination ecology at RMBL was built in the late 1970s and early 1980s through a remarkable series of studies on optimal foraging — the idea that bees and hummingbirds should behave in ways that maximize net energy gain. Graham Pyke's work established baseline rules for how bumblebees move between and within inflorescences (Pyke, 1978)(Pyke, 1979) and applied the marginal value theorem to hummingbirds visiting clusters of flowers, showing that birds decide when to leave a patch based on the nectar they just received and the number of flowers available . Parallel work on bumblebees showed that they often leave nectar behind in rich flowers in ways consistent with optimal foraging predictions and that they sometimes adjust effort based on the expected quality of the next plant .
Differences between males and females in morphology, physiology, and/or behaviour resulting from differences in strength and shape of selection acting...
Quantitative measurements of beak characteristics including length, width, and curvature
Study of how cognitive mechanisms govern decision-making by floral visitors and shape pollination and plant fitness
Hummingbirds are captured using a gravity-based drop-net system that releases around a suspended feeder onto a platform when triggered.
Standard morphological measurements of hummingbird bill dimensions, mandible, and wing chord using digital calipers with duplicate measurements by dif...
These behavioral studies were quickly connected to plant fitness. Experimental nectar manipulations in Delphinium nelsonii showed that plants producing more nectar attracted longer pollinator visits and set more seed, demonstrating that pollinator decision rules feed directly back on plant reproduction (Zimmerman, 1983). Companion work revealed that nectar resources are patchy at the landscape scale, with "hot" and "cold" spots that bees can learn and exploit (Pleasants & Zimmerman, 1979)(Zimmerman, 1979). By the early 1990s, attention also turned to events inside the flower: pollen tubes from different donors interact and compete within a single style, with local and outcross pollen experiencing different fates (Cruzan, 1990).
A first major thread running through this body of work is that pollinator behavior is finely tuned to nectar economics. Bumblebees and hummingbirds adjust how many flowers they visit per inflorescence, how long they linger, and where they fly next based on the rewards they encounter, broadly matching predictions from optimal foraging theory (Pyke, 1978)(Pyke, 1978)(Hodges & Wolf, 1981). These adjustments are not perfect — bees sometimes deviate from strict energy-maximizing predictions, suggesting that simple models cannot capture every aspect of real foraging (Zimmerman, 1981) — but the general principle that cognition and energetics shape visitation has held up.
A second thread connects behavior to plant fitness. Because pollinators visit more flowers and stay longer on plants offering more nectar, plants that invest in higher nectar production can gain measurable increases in seed set (Zimmerman, 1983). The spatial patchiness of nectar across a meadow means that neighboring plants influence one another's attractiveness, creating local hot and cold spots that structure pollinator movement (Pleasants & Zimmerman, 1979). Inside the flower, post-pollination processes add another layer: styles can selectively favor some pollen donors over others, so the identity of a pollinator's previous visit affects which pollen ultimately sires seeds (Cruzan, 1990).
More recent work has turned to the morphology of the pollinators themselves. In Broad-tailed Hummingbirds (Selasphorus platycercus) around RMBL, females have significantly longer, wider, and more curved bills than males (Hsu, 2016), and are larger than males in gape, culmen, bill width, bill depth, and mandible width, though not in bill curvature or wing chord (Hsu, 2017). This sexual dimorphism in bill shape suggests that males and females may exploit different flowers, a form of niche partitioning within a single species. Flower traits matter too: in two Mertensia bluebells, M. brevistyla holds its flowers more upright while M. fusiformis flowers hang downward, and these orientation differences interact with both pollinator access and protection from rain (Eckhart et al., 2019).
Early work in the late 1970s and 1980s established the behavioral and energetic rules of pollination at RMBL. Recent studies since 2015 have shifted focus in two directions. One is morphological: digital caliper measurements of bills, combined with mist-net sampling, are revealing fine-scale variation within hummingbird species and raising questions about how that variation maps onto flower use (Hsu, 2016)(Hsu, 2017). The other is functional and comparative: researchers are asking how floral traits like orientation mediate the joint pressures of attracting pollinators and surviving weather (Eckhart et al., 2019), and cognitive ecology is being pushed to consider how realistic memory demands — remembering location, timing, and reward of many feeders or flowers — shape what counts as adaptive foraging (Nicolson, 2017).
Several important gaps remain. How will warming temperatures, earlier snowmelt, and shifting flowering times alter the match between hummingbird bills and the flowers available to them? Does sexual dimorphism in bill shape translate into measurably different pollination services delivered by male versus female hummingbirds, and does this affect plant evolution? How do cognitive limits — memory, attention, the cost of learning — interact with the increasingly variable nectar landscapes that climate change is producing? And how do floral traits beyond nectar, such as orientation, color, and scent, integrate with pollinator behavior to determine reproductive success in the rapidly changing meadows of the Gunnison Basin? Answering these questions will require linking the classic behavioral and morphological tools developed at RMBL with longer-term monitoring of both plants and pollinators.
Cruzan (1990). Pollen-pollen and pollen-style interactions during pollen tube growth in Erythronium grandiflorum. American Journal of Botany. →
Eckhart et al. (2019). The function of floral orientation in bluebells: interactions with pollinators and rain in two species of Mertensia. →
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Hsu (2017). Variation in bill morphology and pollen prevalence in the Broad-Tailed Hummingbird. →
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Pyke (1981). Optimal foraging in hummingbirds: rule of movement between inflorescences. Animal Behaviour. →
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Zimmerman (1981). Optimal foraging, plant density and the marginal value theorem. Oecologia. →
Zimmerman (1983). Plant reproduction and optimal foraging: experimental nectar manipulations in Delphinium nelsonii. Oikos. →