U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3038

2009

A Neotropical Migrant Bird’s Dilemma—

Where to Stop for a Good Meal

In cooperation with The University of Arizona and Northern Arizona University

o learn how migrating birds Tdetermine where to stop and find 

food, scientists from the U.S. Geological 

Survey, Northern Arizona University, and 

The University of Arizona studied the 

behavior of 28 species of neotropical 

migrant songbirds—warblers, flycatchers, tanagers, and vireos—along the 

lower Colorado River from 2001 to 2004. 

They found that, like interstate travelers greeted by restaurant billboards, 

songbirds flying over Cibola National 

Wildlife Refuge, Arizona, relied on the 

flowering of honey mesquite (Prosopis 

glandulosa) to detect the availability of 

insects that they prey on. Understanding 

where and why migrant birds stop will 

help land managers better protect key 

habitats used by these tiny travelers.

For migrant songbirds that breed in the 

United States or Canada and winter in 

Central or South America, migration is 

an energetically demanding part of their 

annual cycle. Because of their small size, 

these neotropical migrants cannot carry 

sufficient reserves to migrate nonstop. 

Choosing appropriate locations to stop 

and refuel is important to ensure that 

birds survive migration and reach their 

destination in suitable physical condition to 

breed. Songbirds that feed on insects, such 

as the Pacific-slope flycatcher (Empidonax 

difficilis) and Wilson’s warbler (Wilsonia 

pusilla), prefer locations with an abundance 

of insects, yet the “cues” birds use to 

determine food availability as they travel 

remain largely a mystery.

Scientists know that birds actively 

choose when and where to stop during migration; however, until recently relatively 

little was known about how individual 

birds judge the quality of potential stopover 

locations. One possibility was that birds 

determined food availability from the types 

or condition of local trees. The species of 

tree and their phenological state (seasonal 

changes in flowering, leaf flush, fruiting, 

leaf loss) may present birds with obvious 

and reliable clues to insect abundance that 

could be easily assessed at a distance. To 

test the hypothesis that migrating birds use 

the species and phenology of trees to determine food availability, U.S. Geological 

Survey, Northern Arizona University, and 

University of Arizona researchers repeatedly monitored birds and trees on two 1-kilometer transects at Cibola National Wildlife 

Refuge, Arizona. The refuge was selected 

because it contains choice riparian habitat 

(habitat near a waterway) and is a stopover 

location for numerous insectivorous neotropical migrant birds, including many critical 

and sensitive species.

Walking preestablished transects 

weekly during the spring migration period (March to May), the scientists were 

able to estimate the abundance of local 

tree species and follow the phenological 

progression from bare branches, to buds, 

to leaves, to flowers, and finally to fruit. 

At the same time, the scientists estimated 

bird diversity and abundance and cataloged the habitat preferences of species 

that they encountered. By comparing 

when and where birds were found with 

the availability of each tree species and its 

corresponding phenological progression, 

the scientists were able to relate patterns 

of migration with changes in local tree 

phenology. To ensure that the changes in 

 These photographs show (top to bottom) the 

Pacific-slope flycatcher (Empidonax difficilis) and 

the yellow-rumped (Dendroica coronata), orangecrowned (Vermivora celata), and Wilson's warblers 

(Wilsonia pusilla). The birds are some of the many 

migratory songbirds that pass through the Cibola 

National Wildlife Refuge, Arizona, and stop along 

the Colorado River during their annual treks from 

winter habitat in Mexico to breeding grounds in the 

United States and Canada.

©Stephen Minter/ASDM Sonoran Desert Digital Library ©Stephen Minter/ASDM Sonoran Desert Digital Library ©S. Mlodinow/VIREO ©Arlene Koziol/ASDM Sonoran Desert Digital Library
For more information contact:

U.S. Geological Survey

Southwest Biological Science Center

Sonoran Desert Research Station

Tucson, Arizona

520-621-5508 or

charles_van_riper@usgs.gov

This Fact Sheet and any updates to it are available 

online at http://pubs.usgs.gov/fs/2009/3038/

Joseph J. Fontaine and 

Charles van Riper III

phenology really informed the birds about 

food availability, they recorded the diversity and abundance of insects on each tree 

species throughout the spring. Scientists 

also experimentally removed the flowers 

on trees to determine whether leaves or 

flowers were ultimately the cue the birds 

used to select foraging locations.

Data gathered for the five most common 

tree species—Fremont cottonwood (Populus fremontii), Goodding’s willow (Salix 

gooddingii), screwbean mesquite (Prosopis 

pubescens), honey mesquite (Prosopis 

glandulosa), and tamarisk (Tamarix sp.)—

revealed that only the flowering of honey 

mesquite coincided with the spring peak 

in diversity and abundance of migrating 

neotropical songbirds moving through the 

study area. Migrant birds preferred honey 

mesquite to all other tree species in the 

study area. Moreover, they preferred honey 

mesquite trees with more flowers because 

these trees harbored more of the insects that 

they preferred to eat. 

The experiment that reduced the number of flowers on trees clearly demonstrated that birds showed a preference for 

honey mesquite with more flowers, independent of leaf phenology. Given that the 

experiment did not change the number of 

insects present, these findings indicate that 

the flowers were the signals the birds used 

to determine how much food was available. The fact that the flowering of honey 

mesquites coincides with the peak in 

abundance of migrant birds suggests that 

the timing and routes used by migrants 

may have evolved to take advantage of the 

readily available food resources that the 

flowering trees advertise.

Although our knowledge of how birds 

select locations to rest and refuel remains 

limited, this study demonstrates that the 

flowering phenology of a common tree 

species acts as a reliable cue for neotropical migrant birds. That the flowering of 

honey mesquite presents a reliable indicator of actual food availability highlights 

the importance of this visible signal for 

migrant bird populations, particularly 

along the Colorado River.

Avian migration along the Colorado 

River appears to be closely tied to the phenology of honey mesquite, but it remains 

uncertain how the phenology of honey mesquite or other tree species may influence 

migration along other important migratory 

corridors that differ geographically or in 

elevation. Those corridors may have different migratory peaks and support different 

bird populations or species assemblages. 

For these reasons, more work is needed to 

clarify how tree phenology differs among 

Western migration corridors, how these differences affect migratory bird populations, 

and how climate change and invasion by 

exotic trees may alter settlement cues used 

by migrant bird populations.

In the Western United States, neotropical migrant birds and the riparian corridors on which they depend are topics 

of conservation concern, and where these 

avian travelers stop to rest and refuel can 

have important conservation implications. 

For example, migratory birds may play an 

important role in ecosystem function in 

sensitive desert riparian corridors, because 

healthy bird populations may control insects that feed on local agricultural fields 

and native plants. Understanding where 

and why migrating birds stop during their 

annual travels can help land managers 

better care for key habitats for these longdistance avian travelers. 

Summer tanagers (Piranga rubra) were 

among the 28 species of neotropical songbirds studied by U.S. Geological Survey 

scientists and their cooperators at Cibola 

National Wildife Refuge, Arizona.

©Stephen Minter/ASDM Digital Library

Scientists found that migratory songbirds 

that feed on insects relied on the bright and 

showy flowers of the honey mesquite tree 

to determine where best to stop for food. To 

reach this conclusion, U.S. Geological Survey, 

Northern Arizona University, and University 

of Arizona scientists experimentally removed 

flowers and measured insect abundance on 

mesquite trees in the Cibola National Wildlife 

Refuge, Arizona, to determine if flowering 

affected where birds choose to feed.