Cerruti R2 Hooks$ and Anahí Espíndola*
$Associate Professor and Extension Specialist, CMNS, Department of Entomology
*Assistant Professor, CMNS, Department of Entomology. Twitter: @Analyssi
Note: This is the fifth article of our series on pollinators. The initial articles can be found at the vegetable and fruit headline news June and July special editions and Maryland Agronomy News Blog.
Introduction
Birds are key global pollinators of wildflowers, and ornithophily (bird pollination) occurs in more than 60 flowering plant families. In truth, birds that frequent flowers to obtain nectar are the most species-rich vertebrate pollinators in the world. Relative to this, there are 2,000 bird species globally that feed on nectar, insects and spiders found within nectar bearing flowers. Bird pollinators include the American oriole (Icteridae), honeycreepers (Thraupidae), sunbirds (Nectariniidae), honey-eaters (Meliphagidae) and hummingbirds (Trochilidae). The latter three families include most of the species we know to visit flowers on a regular basis. Within continental US, hummingbirds are the key avian pollinators of wildflowers and will be the main focus of this article, honeycreepers and honeyeaters (Fig. 1) are vital pollinators in Hawaii and Australia, respectively, while brush-tongued parrots (Fig. 2) and sunbirds are tropical carriers of pollen.
Hummingbirds
Hummingbirds are distributed throughout the Americas and is among the most diverse and specialized group of avian pollinators. Hummingbirds can be extremely small, with the bee hummingbird (Mellisuga helenae, Fig. 3, top) recognized as the smallest bird in the world, weighing 0.06 oz (1.7 g) and measuring about 2 in (5.1 cm). The largest hummingbird, however, is still pretty small. Indeed, the magnificent or giant hummingbird (Patagona gigas, Fig. 3, bottom) can weigh as much as 0.85 oz (24.1 g) and measures about 8.5 in (21.6 cm). All of the more than 300 known species of hummingbirds are found only in the western hemisphere, from southeastern Alaska to southern Chile. Although hummingbirds subsist in a wide range of climates and environments, far less species are found in temperate than in tropical zones. It is suspected that hummingbirds originated from an Eurasian ancestor and evolved in South America 22 million years ago. From South America, the availability of a large variety of habitats along the Andes may have allowed them to diversify and eventually colonize Central and North America. Today, roughly 26 and 17 species visit and breed in the US, respectively, sometime during the year. Among these species, the eastern US is populated by the ruby-throated hummingbird, and common species in the west include broad-tailed, rufus and black-chinned hummingbirds. North American hummingbird species are migratory, many of which visit North America to breed in the summer and migrate south to overwinter in Mexico. Because of the large energetic needs of hummingbirds, such migrations can only occur if sufficient food is available to them throughout their trip. For this reason, the timing of these migration events matches the flowering time of nectar resources at their destination, which makes sense if we understand that the journey between overwintering and breeding grounds may involve an 18- to 22-hour, non-stop flight. One of the most extreme hummingbird migrants in North America is the pugnacious rufous hummingbird (Selasphorus rufus), which breeds farther north than any other hummingbird, traveling about 3,900-miles (one-way), the longest documented hummingbird migration.
Food preferences and energy requirements
Hummingbirds feed by day on nectar from flowers, including annuals, perennials, trees, shrubs and vines. They feed while hovering or, if possible, while perched. Hummingbirds lap up nectar with their long tongues deploying their beak as a straw, and can lick 10-15 times per second while feeding. Indeed, hummingbirds have long beaks (Fig. 4) and even longer tongues (Fig. 5), which allows them to feed on flowers that are too long and thin for other pollinators. With the exception of insects, hummingbirds (while in flight) have the highest metabolism of all animals, with heart rates that can reach as high as 1,260 beats per minute (as a reference, an adult human has a heart rate of 60-100 beats per minute) and wing beating rate of 70 beats per second. As most of their energy is spent flying, they must constantly feed, consuming several times their body weight in nectar to meet their high energetic requirements. Although nectar is not the only food source used by hummingbirds, it provides most of the carbohydrates that supply the energy they need for continuous flight and survival. Interestingly, hummingbirds can remember where every flower in their territory is and how long it takes it to refill with nectar after they have fed. Their great memory allows them to avoid revisiting recently sampled flowers and consequently wasting energy on a flower whose rewards have already been depleted. To obtain enough energy for their survival, these mostly nectarivorous birds must forage all day long, visiting hundreds of flowers. A hungry hummingbird might visit between 1,000 and 3,000 flowers a day to maintain an adequate caloric intake. Along with feeding on nectar, hummingbirds also consume small insects, such as fruit-flies and gnats to supplement their sugary diet, and may consume tree sap when available. For the latter, they exploit sap wells drilled in trees by sapsuckers and other hole-drilling birds and insects, since they are unable to drill these wells themselves.
Hummingbirds may also be territorial and this is suspected to be a consequence of their high energetic requirements. As such, flowers that display large amounts of easily-accessible nectar can lead to territorial tendencies while the rewards are available. In our region, this aggressive behavior may be more intense during late spring or early summer when hummingbirds are claiming those important resources while defending and providing for their offspring. However, hummingbirds may still display aggression well into the fall as they defend prime feeding areas while they prepare for migration.
Hummingbirds as pollinators
Hummingbirds are nectarivorous (nectar-feeding) birds that are amazingly adapted pollinators. They have long, slender bills and tube-like tongues used for drinking nectar from brightly-colored flowers (Figs. 4 and 5). Nectar provides them energy required to fuel their high metabolism. For protein, they eat insects caught on the fly or present in flowers that they visit. Hummingbirds may drink up to two times their body weight per day; and as they visit various flowers, pollen is deposited on their bodies and is thus transported among plants.
Though hummingbirds are not considered vital for pollinating plants that provide us food, they play a crucial role in the pollination of many different flower types, particularly tropical flowers in the New World. Approximately 8,000 plants in North, Central and South America depend on hummingbird pollination services. Further, thousands of flowering plant species have evolved specific features to attract hummingbirds, as they use them as pollen carriers. Even though few plants rely on only one species of hummingbird for pollination, many are exclusively pollinated by hummingbirds. For instance, some Neotropical plant families rely predominantly on hummingbirds for pollination, including Bromeliaceae (pineapple family of flowering plants). Relative to this, a study found that when insects were allowed to visit the flower of two species of Bromeliaceae (Aechmea nudicaulis and Vriesea neoglutinosa) and hummingbirds were excluded, fruit set was considerably reduced in both species. These findings indicate that although some hummingbird-specialized plants are frequented by other floral visitor groups, they are dependent on hummingbirds for reaching their maximum reproductive potential. In habitats where such plants exist, hummingbirds play a central role in supporting the ecosystem.
Because of their direct effect on plant populations, hummingbirds are responsible for the rapid speciation of some groups of flowering plants. As said before, these birds are particularly important in tropical ecosystems, where hummingbirds are the main pollinators of several plant groups. In particular, the evolution of some of these abundant tropical plant groups, such the family Gesneriaceae and Bromeliaceae, is suspected to have been directly affected by their interactions with hummingbirds. In reference to this notion, a study was conducted to investigate whether Gesneriaceae association with hummingbirds affected their diversification. The study showed that plants pollinated by hummingbirds had a twofold higher speciation rate compared with insect-pollinated plants of the same group, and suggested that establishment of hummingbird pollination in the Early Miocene (23-16 million years ago) triggered rapid diversification in this plant family.
Floral choice and characteristics
Because of their extreme energetic requirements, hummingbirds need nectar resources throughout their entire lives. However, most plants do not flower throughout the year, meaning that hummingbirds may visit varying flower types, depending on each plant’s flowering time. As their floral choices change, they may pollinate different types of flowers. Over time, however, many flowers evolved elaborate traits to attract hummingbirds and consequently encourage more frequent visits and better pollination. Flowers pollinated by hummingbirds and other birds flower during the day, display abundant nectar, and traits such as exerted reproductive structures contributing to further specialization. A very constant trait displayed by hummingbird pollinated plants is the sugar concentration and abundance of their nectar. Indeed, hummingbird-pollinated flowers produce large volumes of relatively dilute nectar, which facilitates rapid nectar harvesting but may exclude other less-efficient floral visitors. These specialized floral characteristics are thought to facilitate pollination by birds while filtering out other pollinator groups.
Shape and position. Most hummingbird species feed from many kinds of flowers, but there are some specialized plant-hummingbird partnerships. For example, to accommodate hummingbirds, flowers are often positioned so that hummingbirds can comfortably feed while hovering without their wings touching adjacent stems or leaves. Further, flowers pollinated by these birds lack an insect landing platform and typically contain a tubular corolla, which improves pollination while making it more efficient. The long, narrow central tubes are engineered to force hummingbirds to stick its long beak inside, consequently brushing its head or body against flowers’ reproductive organs. These flowers often droop or hang pendulously and have modified petals that accommodate the hovering feeding activity of birds. Some hummingbirds contain bills that are the perfect length or shape to fit a particular flower. Relative to this, the South American sword-billed Hummingbird has a four-inch beak exactly suited to fit some species of passionflowers (Fig. 4). This has been explained by the fact that plant-hummingbird associations are interactions from which the two partners (the plants and the hummingbirds) mutually benefit. For instance, studies have shown that hummingbirds that best match flower length improve nectar intake, and that plants whose flowers match hummingbird bill lengths are better and more precisely pollinated. Hence, reciprocal matching leads to fitness benefits for hummingbirds and plants they pollinate. Relative to this, differences in bill length are thought to be associated with trade-offs in foraging abilities. For example, longer-billed birds are able to extract nectar from longer flowers more efficiently (less handling time), whereas shorter-billed birds are able to feed more successfully within narrow flowers.
It has also been shown that larger flowers are also correlated with nectar volume, and as such, hummingbirds tend to select larger over smaller flowers. Relative to this, a study conducted on hummingbird-visited flowers in the Brazilian Atlantic Forest found that nectar volume is positively correlated with corolla length, and that this provided better rewards for long-billed hummingbirds. Plants that have evolved floral traits closely matching their hummingbird pollinators have been shown to be more efficiently pollinated by hummingbirds than by other pollinators (e.g., insects, bats). For instance, trumpet creeper (Campsis radicans) contains tubular flowers with large quantities of nectar. When the pollination of this species was studied, it was shown that the ruby-throated hummingbird (Archilochus colubris) deposited 10 times more pollen per visit than honey bees (Apis mellifera) and bumblebees (Bombus spp.), and that fruit set was the highest where ruby-throated visitation was most frequent.
Color. Flowers that attract hummingbirds typically contain contrasting and showy coloration. Hummingbirds have extremely good vision, and are able to also perceive UV light. Hummingbirds are seen often visiting red-pigmented flowers (Fig. 6), but also visit pink, yellow, and orange ones. Even though it had been thought that hummingbirds have a natural preference for these colors, it has been shown that these preferences are not innate, but rather learned. Experiments have indeed shown that hummingbirds preferentially select flowers with large amounts of rewards (a lot of nectar), independently of their coloration. Since most other pollinators usually do not prefer/cannot see red colors, it follows that those flowers are usually the ones that have the most available rewards (Fig. 6). Further, by restricting flower visits to specific types of pollinators, flowers can increase pollination precision and seed production. Over time, this led to the evolution of those colorations in hummingbird-flowers, which has led to a perceived impression of color preference by hummingbirds.
Odor. Unlike flowers that attract bats and beetles, flowers that specifically target hummingbirds for pollen transfer usually do not have much aroma. The lack of odor prevents olfactory attraction of other pollinators. That said, some other pollinators do sometimes visit flowers that are mainly pollinated by hummingbirds. For instance, hummingbird moths (hummingbird mimics, Fig. 7) obtain nectar from odorless flowers and nectar feeders in the same way than hummingbirds do. Unlike these moth mimics, hummingbirds have a poor sense of smell, which explains why it is not useful for hummingbird-pollinated flowers to evolve floral scents for pollinator attraction.
Pollen. Hummingbird-pollinated flowers open during the day and tend to have significantly more pollen than insect-pollinated flowers. Their anthers are usually placed specifically on the inner top side of the flower, which facilitates deposition of pollen at a very specific position. Relative to this, hummingbird flowers have evolved to dust the bird’s head and/or back with pollen as they forage for nectar. Hummingbirds thrust their long slender bills deep into flowers for nectar, subsequently withdrawing their pollen-dusted faces. Further, when they insert their beaks into flowers to drink the nectar, sticky pollen grains cling to their beak (Fig. 8) or head and is transferred to the next flower bloom visited. In relation to this, it has been shown that hummingbirds can carry extremely large amounts of pollen, and that they can do so even after more than 20 flower visits. This makes them extremely good pollen dispersers. Further, just knocking pollen grains around inside a single flower or nudging a flower with its bill may lead to successful pollination. Moreover, when a hummingbird creates a breeze with its wings, that action can sometimes be enough to pollinate flowers.
Floral specialization on specific hummingbird species. It is apparent that hummingbirds are able to recognize certain plant traits (e.g., color, shape and size) when deciding (and learning) which flowers to visit while searching for nectar. It has been recently shown that some plants respond differentially to visitation by dissimilar hummingbird types. On this topic, a study found that the tropical understory herb, Heliconia tortuosa (Fig. 9) evolved an effective behavior that allows it to discriminate among floral visitors and consequently recognize specific (and supposedly more efficient) hummingbird pollinators. It then uses this information to regulate its reproduction. Basically, it was found that successful pollination of H. tortuosa occurred frequently when plants were visited by long-distance traplining hummingbird species with specialized bills, but was reduced by 5.7x when visited by straight-billed territorial birds or insects. Follow-up experiments found that H. tortuosa used the nectar extraction capacity of tropical hummingbirds, a positive function of bill length, as a cue to turn on reproductivity (induce pollen tube growth/increase). More precisely, visits by hummingbirds with long curved bills led to the extraction of larger amounts of nectar which, in turn, was used as a cue by the plant to increase its stigma receptiveness (i.e., facilitate pollen tube growth). Although this was discovered in one plant species, this is now believed to be more widespread among plants.
Conservation
Similar to all organisms, hummingbirds are adapted to their environment and food source. As a result, many hummingbird species are incredibly sensitive to environmental change and dependent on the continued availability of their preferred habitat. In fact, many hummingbird species currently listed as vulnerable or endangered are mostly threatened by habitat loss and destruction. Planting native nectar plants is a good means of supporting hummingbirds as well as other pollinators. Hummingbird feeders may also support resident and migrating hummingbird populations. However, their contribution to hummingbird conservation is likely negligible when other proper practices are followed.
Because hummingbirds rely on feeding and nesting resources, National Parks and other protected areas can serve as hummingbird refuges. Indeed, they provide large expanses of forest for breeding and nesting. Further, the pesticide-free cultivation of native plants on private and public lands also provides excellent food sources for hummingbirds. This can also play a central role for migratory hummingbird species, which require energetic resources along their migration routes, as well as in their breeding, nesting and wintering areas. To this point, long, narrow pieces of habitat such as utility corridors, field edges and roadsides can provide important connections among larger habitats. Simply put, the presence of small habitat patches along their migratory path can be critical to birds by providing them places to rest and food to fuel their journey.
Summary
Though not required for the production of human food, bird pollination is central to the reproduction of many wild plants and maintenance of large ecosystems. Among all bird pollinators, hummingbirds are of key importance in the New World, where they are present from Alaska to Patagonia. In North America, hummingbirds are migratory, and in the East coast of the U.S., we can see them during warm months. Because of their extremely high metabolic rates, hummingbirds have very high energy requirements, and it is for this reason that they feed on nectar throughout the day. Even though their floral choices are learned, they usually visit red-colored flowers, where they can obtain abundant nectar. Flowers adapted to hummingbird pollination usually flower during the day and have tubular shapes, sturdy enough to support the pressure of the birds’ beak when it visits the flower. Hummingbird conservation can be improved by providing sufficient food and nesting resources, and by establishing habitats abundant in nectar-rich flowers. Although hummingbird feeders do not contribute significantly to their population protection, these devices allow us to appreciate these magnificent birds and admire their beauty from the comfort of our residence.
Financial support for the publication of this article is via USDA NIFA EIPM grant award numbers 2017-70006-27171.
