When is too much rain a bad thing? In eastern Kansas, we know that rain drives the productivity of crops and other plants, but we also understand that raging floodwaters can cause major damage. However, some ecosystems rely on enormous amounts of rain. In tropical rainforests, life thrives while receiving as much as seven times the average rainfall in Manhattan, Kansas. So can these tropical wet rainforests get too much rain?
Prevailing wisdom has always held that as rainforests get more rain, the plants grow more and produce more food, benefiting organisms all the way up the food chain. If this is the case, the small, fruit-eating birds that Dr. Alice Boyle has studied in Costa Rica for much of her career should be some of the main beneficiaries of epic rain events. Yet Dr. Boyle, an Assistant Professor in the Kansas State Division of Biology, has found that this is not always the case. Instead of benefitting from the rainiest part of the rainy season, these tiny birds, called White-ruffed Manakins, sometimes flee the downpours by migrating down the mountainside to drier environments.
Migration, the seasonal movement of animals from one region to another, is a defining trait of many bird species, especially some of our favorite Kansan birds. Many species make incredible journeys between and across continents twice a year, but the movement of these manakins is a little different. For one, their movement is a vertical one, up and down mountains instead of across flat land. This altitudinal migration is also intriguing because not all manakins make the journey—some individuals stay in the very rainy high-altitude regions, while others fly down the mountain to where it is drier. What can this strange behavior teach us about these birds and the effects of rain?
Dr. Boyle’s work provides some insight. She knows that staying in the rainy uplands can be dangerous for birds, perhaps because it is hard to find fruit in a downpour. She also has evidence that male manakins are more likely to migrate than females, a distinction that is especially interesting because of the displays male manakins make to attract mates.
These tiny birds perform an intricate dance on logs in the forest, wooing females with hops, flips, aerial displays, and calls. Females choose between different males, judging based on the qualities of their display. From her long-term research on individual males, Dr. Boyle knows that males that migrate do not have as much success as their brave counterparts in wooing females when they return to the breeding grounds up the mountain. She hypothesizes that there is a trade-off: males in poor condition can increase their likelihood of survival by fleeing the dangerous rains, but they are less likely to find a willing female and reproduce the next year. Why might female migration be different? Females, who do not have to spend the time and energy displaying, may be able to weather the storm without facing such consequences.
Too much rain appears to drive a trade-off between survival and reproduction in the White-ruffed Manakins, and Dr. Boyle suspects that this is the case for many birds in tropical wet forests. She proposes that many bird species in the tropics have a range of rain conditions to which they are best suited—even birds adapted to the rainiest of the wet forests. Understanding these rainfall ranges and the effects of rain on tropical birds will help researchers understand not only the biology of these understudied species but also anticipate the effects of changing weather patterns in the tropics.
For more information on Dr. Alice Boyle’s work (and the related work of her Ph.D. student Elsie Shogren) check out her website at www.aliceboyle.net or her Twitter feed @birdfiddler.
This post is part of our new series covering Science on Tap. This monthly event, a part of Sunset Zoo’s Behind the Science initiative, is a gathering at the Tallgrass Taphouse during which a featured scientist shares their research and engages with the audience in a lively conversation about the topic. Hopefully you are able to join in on the next conversation; if not, we’ll be here to provide a summary of the research that was shared!
This post was written by Sarah Winnicki, a second-year graduate student in Dr. Boyle’s lab. Sarah studies how cowbirds (which lay their birds in other birds’ nests) affect the growth and development of threatened grassland songbird nestlings (for more information check out www.sarahwinnicki.com/research).