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Not Just Passing Through - For The Birders
Wilson's Warbler in a catclaw acacia by Marshal Hedin from Flickr CC

Not Just Passing Through

A Wilson’s Warbler’s Journey Through Southern Arizona

Wilson's Warbler in a catclaw acacia by Marshal Hedin from Flickr CC
Wilson’s Warbler in a catclaw acacia by Marshal Hedin from Flickr CC

This article by Jennie MacFarland originally appeared in the April-June 2014 issue of the Vermilion Flycatcher, the quarterly magazine of the Tucson Audubon Society.

As a birder in southern Arizona, it is always a treat to see a Wilson’s Warbler. The appearance of this species in our region is brief, and when we see Wilson’s Warblers they are on the move. These colorful and charismatic warblers do not winter or nest here, so to us they seem to be merely passing through. They will spend a small amount of time here before picking up and traveling again. It is tempting to think of such periods as being the most brief and transitory in a bird’s yearly cycle, but an individual Wilson’s Warbler probably doesn’t think of it in this way. Migration is the most dangerous period in a bird’s life and more individuals perish during their bi-annual journeys than during any other part of their lives. That beautiful Wilson’s Warbler you glimpse through your binoculars this spring isn’t “merely passing through,” it is undertaking a perilous journey to reach a patch of habitat as far north as Alaska where he will find abundant food resources for he and his mate to successfully raise their family. To understand fully what this tiny, energetic bird is undertaking, we must see his journey through southern Arizona on a number of scales. This requires “big picture” thinking all the way down to focusing on an individual mesquite tree. Of the many Wilson’s Warblers that travel through Arizona during spring migration, let’s follow one on his journey: a bright yellow male with a black crown. As environmental cues such as day length fall into place, this bird prepares to head north from his winter home in central Mexico towards the promise of a seasonal food abundance that will fuel the growth of his future young. With his fat reserves topped off, he begins his long trip and crosses the international border into Arizona east of Nogales. Our Wilson’s Warbler needs to follow routes where he can find food to maintain his energy levels during the exhausting ordeal of migration. The Santa Cruz River creates a good route to travel as it heads north. It is lined with riparian vegetation in stretches that provide the insect prey
that a migrating bird needs. The Upper Santa Cruz River Important Bird Area (IBA) encompasses one such stretch of riparian habitat near Tubac that will help him to safely navigate this leg of the journey north. This area’s designation as an IBA draws attention to the habitat vital for native birds and ensures the travel route is available and productive for this bird when he needs it the most. Once our bird has navigated this distance, he finds himself in Tucson where there are many great opportunities for him to rest and refuel before a big push in his journey.
Migration is a big picture concept where we humans look at continental and even hemispherical maps and track the routes birds take on unimaginably long trips in search of endless summer. But we also have to zoom in on each day of this bird’s migration, as he could perish at any time if he cannot find the suitable microhabitat. Once he reaches Tucson, he will find many resources in our landscaped human habitat. It is fortunate for him that many people choose to plant native mesquite trees and palo verde trees, as one of these flowering trees can support him for several days to weeks. Research conducted over years by University of Arizona’s Charles van Riper III has shown that this one Wilson’s Warbler will likely find a flowering mesquite tree and spend several days feasting on insects and resting. This is a stage in our bird’s journey where the impact of one person’s actions directly helps a migrating bird. Perhaps it will be a native tree in your yard or neighborhood that is flowering right when our bird arrives in Tucson, and the tree serves as his oasis of food and shelter in this critical stage of migration. Our Wilson’s Warbler will stay in his flowering mesquite tree for several days or more and fill up his energy reserves. When he is feeling up to long-distance flight, he will rise out of his tree each evening to test conditions: if the breeze isn’t exactly right, he will settle back into his tree and check again the next night. When all the conditions line up perfectly he will take off in a dramatic way. Field research has shown that at this stage, he will not fly north as one might expect, but due west over a vast stretch of arid desert. This is where we need to change the scale of our thinking once more: this bird isn’t planning on hopping from small oasis to oasis all the way north—he clearly knows the lay of the land and is bee-lining it to the Colorado River. He will likely make the trip from Tucson to Yuma in one night and burn much of the energy he built up in Tucson to make this stage of the journey. This bird is betting that the lush riparian habitat that lined the Colorado River last year is still there, and with a string of Global IBAs along the Colorado, we are helping his bet be a winning one. Once our Wilson’s Warbler reaches the mighty Colorado River, he will find ample food and shelter on this next stage of his journey north, and safely make it through Arizona to continue on even further north. If we were to undertake such a trip using nothing but our own power to get there, it would be daunting. This warbler weighs between 6 and 7 grams and is less than 5 inches long but still makes this
journey twice a year. His determination and resilience help him to achieve this staggering accomplishment and serve as inspiration for the humans who fight for and advocate conservation projects that ensure this one Wilson’s Warbler and his descendants can continue to make this epic journey year after year.

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