EPOW - Ecology Picture of the Week

The Amazing Mistletoebird

Mistletoebird (Dicaeum h. hirundinaceum), Family Dicaeidae
Litchfield National Park, Northern Territory, Australia

Explanation:  This week we are still exploring wonderful Litchfield National Park in Northern Territory, Australia.  Last week it was magnetic termites.  This week it is a bird that does a lot more than meets the eye.

Meet the Mistletoebird, a member of the flowerpecker family, but it doesn't peck on flowers.  Found in Australia and Indonesia, the Mistletoebird is an obligate seed disperser.  This is the story of a most interesting symbiotic relationship between bird and ... you guessed it ... mistletoe. 

The mistletoe plant is itself a parasite of trees -- well, more specifically, a hemiparasite, meaning that it takes some nourishment from its tree host plant but it also photosynthesizes directly from sunlight.  Mistletoes produce a seed pod, shown copiously in the main photo, above and to the right, that, in some species, dehisces with a minor explosion, shooting the seed out to germinate elsewhere.  

However, enter the Mistletoebird that has evolved a highly specialized diet.  They commonly eat mistletoe seeds (and occasionally can take other foods including insects).  They first squeeze the outer cover of the seed (the epicarp) with their bill until the sticky (viscous) diaspore can be swallowed.  

Then the evolutionary magic begins.

Their digestive system has evolved to pass more easily-digested foods such as mistletoe seeds past the blind-sac stomach that would otherwise destroy the seeds.  This is because the bird lacks the well-developed muscular gizzard of other birds; it is greatly reduced.  Its main digestive system is an even duct which bypasses the gizzard and through which large numbers of stripped mistletoe berries quickly pass.   

Some of this fleshy material is digested and a string of sticky seeds is egested that adheres to branches, thus dispersing and "planting" the mistletoe seed onto another host tree. 

Moreover, the birds perch along the long axis of branches instead of transverse to them, as shown in the next photo, below, and have evolved specialized perching feet to do so.  This posture helps ensure that, as the bird excretes its seedy food waste, it gets deposited onto the branch and does not fall to the ground.  
 

Note the posture of this mistletoebird, perched along the
axis of the branch so that when it excretes the sticky
mistletoe seed mass, it gets deposited onto the branch.

The mistletoebird consumes ripe parasitic mistletoe berries as the main food and thus the birds are important agents in dispersal of the seeds and viability of the mistletoe species.    

Many of these seeds germinate quickly and thus the mistletoe plant is able to thereby parasitize another host ... thanks to the bird that just ate its seeds.  

Moreover, there are functional vicariates of the Mistletoebird in other forest ecosystems around the world, that is, other unrelated bird species that have evolved other specific adaptations of physiology and behavior to disperse other mistletoe species!  These other birds include the Phainopepla (Phainopepla nitens) in the arid deserts and scrublands of southwest U.S. and northern Mexico; the Mistle Thrush (Turdus viscivorus) in southern Asia; and the Puerto Rican Tanager (Nesospingus speculiferus) of the highlands of Puerto Rico's humid forests, second growth forests, dense brushlands, and palm forests.  

So there you have it ... another amazing bit of evolutionary symbiotic magic between bird and plant, and examples of evolutionary functional convergence among unrelated birds across the globe!