by Dave Zittin
Phoresy is the term used to describe the concept of one species using another species for dispersal. Dispersal can be for reasons of reproduction, or it can be the need to leave an area with dwindling resources to find a new, better area. Phoresy is known to be widespread in nature.
Species that use other species for transportation tend to be relatively immobile because they lack wings, swimming ability, or the legs required for long-distant movement. When resources become limited, they have to move or die. Winged creatures such as some birds and insects are excellent at providing transport services.
Several flower mite species are phoretic. Flower mites are tiny relatives of spiders and ticks. They eat nectar and pollen. Hummingbird flower mites use hummingbirds for transport. These mites walk from flower to flower on a given plant, but cannot move from one plant to another. Mite species are plant-specific, living on only a certain species of plant. Because of its plant dependency, the mite has to get off onto the correct plant or die. Although flower mites have been fairly well studied, there is still much research to be conducted to understand the interactions between hummingbirds, mites, and flowers. Flower mites are found in Mexico and south from there. One species, however, does occur on Indian Paintbrush in Southern California.
When a plant no longer provides sufficient food, mites actively seek hummingbird beaks when the bird feeds. The hummingbird species does not matter to the mite. Some mites use smell to determine if the hummingbird has recently visited a required flower species. If this is the case, then that hummingbird is more likely to revisit the desired plant species and is attractive to mites that need to move. When an attractive smelling beak comes near, mites run onto it and crawl into the nostrils (nares) of the hummingbird. Later, if the hummingbird pokes its beak into a flower that meets conditions required by the mites, they jump off into their new location.
Is the hummingbird harmed during the transportation of mites? Well, not directly because the mites pose no threat to the bird during carriage if one ignores the tiny extra weight. Any potential harm seems to come from the competition for nectar between the hummingbird and the mite. One study shows that the mites on a plant are voracious feeders and consume up to 40% of the nectar in a given flower, leaving a reduced amount available to hummingbirds. Is the hummingbird distributing a mite that is detrimental to its own success? Will the hummingbird have to work harder to acquire nectar and pollinate more flowers, or will it be discouraged by making it spend less time on nectar-depleted plants? Some studies show that the reduction in nectar caused by the mite reduces the frequency at which hummingbirds visit those flowers, suggesting that plant reproduction may be affected. Do fewer hummingbird visits mean fewer future plants will grow for both the hummingbird and the mite? Hopefully, studies will determine if this is good or bad for the hummingbird. The mite also consumes pollen which reduces the amount available to attach to the beaks and faces of hummingbirds, suggesting even further effect on the reproductive success of the plant: Does less pollen available for pollination produce fewer plant seeds? It’s complicated.
I will conclude with this wonderful story of phoresy we were told by a Native American many years ago when we lived on the West Coast of Vancouver Island. He explained that hummingbirds that returned to the island each summer did so by riding on the backs of the migrating Canadian Goose. Perhaps, just perhaps…phoresy?
References
Some of these references are inaccessible without subscriptions, in which case abstracts are provided. Some articles are in PDF format and require downloads. A few articles are long, but if you are interested in the topic, consider reading the abstract, if present, and the conclusion sections.
General descriptions of phoresy
An article mostly about phoresy among insects, some bird information is covered.
Studies of the hummingbird-mite relationship have been going on for a long time.
A comprehensive review of phoresy literature. The abstract and conclusions are worth reading.
Population dynamics of phoretic mites and effects on hummingbirds and plants
An interesting study on the consumption of nectar by mites vs. hummingbirds.
Effects of mites on seed set and implications for hummingbirds
More about how mites might affect the reproduction of the plants on which they feed.
Photos of mites on Hummingbirds
https://natgeo.nikkeibp.co.jp/nng/article/20140221/384755/ph_5.jpg
https://www.audubon.org/news/mites-take-flight-hummingbird-beaks