BY ANNELIESE MOLL
For the UAS Whalesong

Spring! The time of the year when nature surprises us with a few days that are slightly warmer than the others, when flowers begin to break through, trees develop buds, and birds start to return.

Migratory birds, depending on the species, travel thousands of miles. For example, National Geographic recorded sooty shearwaters migrating almost 40,000 miles a year, which one would not suppose is an easy feat, because they fly from New Zealand across the Pacific Ocean in search of food. Next in line for longest migration are the arctic tern with 22,000 miles and the pectoral sandpiper with 18,000 miles. There are several other bird species with migrations routes in the thousands of miles.

When birds return they can often carry pieces of their winter residences along with them. They carry seeds or spores in their feathers and/or digestive tracts. Long-distance dispersal has recently been proposed as a new mechanism for the distribution of many plant species (Viana et al. 2016).  So, with global climate change allowing the range of some migratory bird species to expand, it is likely that we will be observing a change in the range of associated plant species. Through long distance dispersal seeds and spores have the ability to be carried to remote or otherwise isolated habitats.

Besides the transport of plant species beyond their typical range birds have the ability to carry a number of other organisms. The Journal of Infectious Diseases published a study in 1996 discussing the role of migratory birds and their ability to transport the deer tick (Ixodes dammini), which are known to carry Lyme Disease, over long distances (Smith et al. 1996). Another study conducted on birds and their role in long distance dispersal of other organisms focused on waterbirds and aquatic invertebrates. This particular study presents evidence that the birds transport the invertebrates internally as well as externally (Green and Figuerola 2005).

Birds are not the only ones moving plant seeds and matter around. Mammals are also huge contributors, but on smaller scales, because most mammals do not have migrations quite as lengthy as birds.  Also, without the aid of birds and mammals in dispersal, plants must weigh the pros and cons of moving the next generation themselves, and the majority of the time the cons do outweigh the pros. It boils down to how energetically expensive the investment of movement machinery, such as wings or other structures which could cause the seed to be lifted into the air. If they land in an area that unfavorable the whole effort goes to waste. When birds and mammals are introduced into the equation, the plants can spend less energy developing structures for movement. When it comes to moving large distances there a very few factors that are as effective as birds. Ultimately, birds and other animals have a surprisingly large role in the expansion of plant species as well as other organisms, and the blossoming of new life in spring.

References:

Green, Andy J., and Jordi Figuerola. “Recent advances in the study of long‐distance dispersal of aquatic invertebrates via birds.” Diversity and Distributions 11, no. 2 (2005): 149-156.

Smith, Robert P., Peter W. Rand, Eleanor H. Lacombe, Sara R. Morris, David W. Holmes, and Diane A. Caporale. “Role of bird migration in the long-distance dispersal of Ixodes dammini, the vector of Lyme disease.” Journal of Infectious Diseases 174, no. 1 (1996): 221-224.

Viana, Duarte S., Laura Gangoso, Willem Bouten, and Jordi Figuerola. “Overseas seed dispersal by migratory birds.” In Proc. R. Soc. B, vol. 283, no. 1822, p. 20152406. The Royal Society, 2016.