The yellow-rumped warbler is moving around: on and off the branch

I went to check on my senior thesis project down at the University of Houston Coastal Center, and made sure to bring my binoculars and field notebook to check out some birds, which I was sure would be in the area.  As I drove in I saw some vultures and some shore birds I could not quite make out.  As I drove in some more I came across a group of feral hogs that were as surprised to see me as I was them; I have to admit I was pretty pumped to see them.  On a side note, I believe everyone should do their part to help control the feral hog population, and it is one animal I have no qualms about hunting (they taste really good too).  When I reached my plot I saw the leaves on the tall Chinese tallow trees (Sapium sebiferum) surrounding the area weren’t out from winter and I couldn’t find any of my trees, so I couldn’t collect any data.

Luckily, there were a lot of birds singing and moving around in the bushes and trees adjacent to the field where my plot is located.  I saw an eastern phoebe (hopefully this is correct -I’ve been known to label any flycatcher I see as a phoebe, or describe them with reference to phoebes), a mockingbird, a female northern cardinal, and then I saw a little bird jumping around from branch to branch and set my binoculars on it.  I was practicing the birding spotting technique where you keep your eyes focused and “nonchalantly” pull your binoculars up to your eyes to keep your spot, which is harder than it seems, especially for small jumping birds.

It was a nice light-gray bird, with a sharp black beak and a little yellow on the flank of its chest.  I was excited because I had a good idea of the bird thanks to a previous bird list we studied in class.  I pulled out my Birds of Texas book, which I like better than the Sibley guide, and all but confirmed my guess at a yellow-rumped warbler (Dendroica coronate), until it turned its back to me and I spotted the yellow on its behind, and I was able to cement the identity.

The bird kept hopping off the branch and then returning, and then moved to some other branches as if it were keeping a rotation going.  I was really excited, so I took out my camera and I was determined to get as close as I could to get a picture on my very amateur pocket-sized camera.  One of the most exciting parts of watching the bird was seeing its slight, but rapid movements.  At one point it landed on the ground, and I inched closer to try to get a better view and a picture, and boom, it flew away; it was then I realized I really need to invest in a good camera.

There are two subspecies of the yellow-rumped warblers in the United States: eastern, called Myrtle warblers, and western, called Audubon warblers.  They used to be considered two distinct species, but are now recognized as one.  In addition, there are two more populations of yellow-rumped warblers, one in Guatemala (Goldman’s warbler) and another in Mexico (Black-fronted warbler).  The grouping of the species has been an ongoing topic of discussion in many birding communities, and until recently not very many papers looked at the genetic similarities of the populations.  Two papers, one by Mila et al (2007) and another by Brelsford and Irwin (2009) have argued the four populations of Yellow-rumped warblers are distinct and should be recognized as separate species.  Mila and colleagues, used mitochondrial DNA of all four populations of warblers to construct a phylogeny, which shows how similar each population is to each other based on differences and similarities in their DNA.  Mila found the Goldman’s and Black-fronted warbler are more closely related to each other, but they diverged within the last million years, and since they form their own monophyletic groups should be considered distinct.  In addition, the Audubon’s and myrtle warbler were found to be most related due to three shared haplotypes.  This was interesting because most people believed the black-fronted and goldman’s warblers were more closely related to Audubon’s warbler due to common phenotypic characteristics, like all three having a yellow throat.  The researchers believe the two American populations are in the early stages of speciation and on separate evolutionary paths due to the distinctness of their characteristics, and absence of sustained interbreeding beyond a hybrid zone located in Canada.  In 2009, Alan Brelsford and Darren E. Irwin studied Audubon and myrtle warblers in the hybrid zone where they interbreed.   Using nuclear DNA they found the two warblers are actually more genetically distinct than previously shown, and the hybrid forms are being acted on by selection to some extent.  They also found the hybrid zone is fairly narrow and according to guidelines set by the American Ornithologists’ Union (AOU), the American governing body for birds, the populations meet the criteria as separate species.

Due to the two papers, in February 2010, the International Ornithologists’ Union (IOU), formerly International Ornithological Congress, officially recognized each of the four populations as distinct species.  Even though the IOU has already made the species split official, the American Ornithologists’ Union has not yet passed the yellow-rumped warbler species split proposal; the proposal lists the reasoning behind a number of splitting options the union is considering, and is a pretty interesting read.

Yellow-rumped warblers are migratory birds and have an extended stay in Texas, being seen from early fall through late spring.  Two authors, Michael Baldwin and Warren Conway and their colleagues looked at Chinese Tallow as a food source for tallow-rumped warblers.  Chinese tallow is a very invasive tree that is among one of the gravest threats to many ecosystems in the southeastern United States.  Baldwin et al, found that yellow-rumped warblers were able to use a high amount of energy from the Chinese tallow tree fruit, most likely due to its highly specialized digestive tract which allows consumption of saturated fatty acids; most birds in its order (Passeriformes) are not able to gain much benefit from consuming fatty acids.   This trait allows the yellow-rumped warblers to have a food source in its migratory path and wintering grounds.  Conway et al showed yellow-rumped warblers are among the most frequent foragers on Chinese tallow tree in a study in Texas, and while they did not increase the germination rate of the seeds, the authors suspected the birds may be able to actively disperse viable seeds, which could enhance the already persistent spread of Chinese tallow.  I was actually surprised the seeds the warblers foraged on did not produce higher germination rates than the controls because in the lab we always remove the waxy outer coating of the seeds before germinating.

In addition to the diverse foraging habits of yellow-rumped warblers, Paul Strode found the birds are able to time their arrival at a migrating stopover site perfectly with early bud sprouts on trees and the time caterpillars are most abundant.  Even though budding has occurred earlier each year due to climate change, the species has been able to adjust its arrival time and is also able to shift to new tree species and food source as spring progresses.

After learning about the foraging abilities of the yellow-rumped warbler, I think it’s safe to say it will be around for a long, long time.  One of the biggest surprises that has come from birding is how interesting each bird really is.  I feel like I would always take smaller birds for granted and never really paid attention to them or their importance in the ecosystem.  It was fun observing this lively bird and I’m interested in seeing how the AOU will sort out the species proposals in July.

Here are the references mentioned in the blog:

Baldwin, Michael J., Wylie C. Barrow, Clinton Jeske, and Frank C. Rohwer (2008):. “Metabolizable Energy in Chinese Tallow Fruit for Yellow-rumped Warblers, Northern Cardinals, and American Robins.” The Wilson Journal of Ornithology 120.3 525-30.

This paper looks at the ability of yellow-rumped warblers to use the apparent metabolizable energy in Chinese tallow fruits, and this helps them to have a large foraging resource in their wintering grounds.

Conway, Warren C., Loren M. Smith, and James F. Bergan (2002). “Avian Use of Chinese Tallow Seeds in Coastal Texas.” The Southwestern Naturalist 47.4: 550-56.

This article looked at a number of hypotheses concerning the foraging of yellow-rumped warblers on Chinese tallow fruits and how they affect germination.  They found the birds did not increase germination, but may be able to spread the tree species.

Brelsford, Alan, and Darren E. Irwin (2009). “Incipient Speciation Despite Little Assortative Mating: The Yellow-Rumped Warbler Hybrid Zone.” Evolution 63.12: 3050-060.

This paper looks at the breeding zone between audubon’s warbler and myrtle warblers in Canada and the relatedness of the two populations using DNA analysis.  The researchers found there are distinct genetic differences between them and the hybrid zone does not extend very far.  There may also be selection against the hybrids breeding among the two separate populations.  The authors suggest they meet the criteria for being distinguished as separate species.

Milá, Borja, Thomas B. Smith, and Robert K. Wayne (2007). “Speciation and Rapid Phenotypic Differentiation in the Yellow-rumped Warbler Dendroica Coronata Complex.” Molecular Ecology 16.1: 159-73.

This article looks at mitochondrial DNA of the four yellow-rumped warbler subpopulations to find their phylogenetic grouping.  The researchers found the goldman’s warbler and black-fronted warbler are distinct and the audubon’s and myrtle warbler are more closely related, but have enough differences to suggest the two are also distinct.

Strode, Paul K (2009). “Spring Tree Species Use by Migrating Yellow-Rumped Warblers in Relation to Phenology and Food Availability.” The Wilson Journal of Ornithology 121.3: 457-68.

The author observed yellow-rumped warblers at a stopover site in Illinois and found the birds are able to time their arrival as the first trees begin to bud and caterpillars are most abundant.  In addition, the birds move to different tree species as they begin to bud.  The author also showed climate change has affected budding time of trees, but the warblers are able to adjust to the earlier budding of trees.

A Myrtle warbler; notice the white throat, which is an easy way to distinguish it from the other yellow-rumped warbler populations (species).

Here is a yellow-rumped warbler in Houston showing the characteristic trait it’s named after.

Figure 1. Sampling localities (black dots), breeding and wintering ranges, and basic plumage characteristics of males of the four main yellow-rumped warbler groups: Dendroica coronata coronata (‘myrtle warbler’); Dendroica coronata auduboni (‘Audubon’s warbler’); Dendroica coronata nigrifrons (‘black-fronted warbler’); and Dendroica coronata goldmani (‘Goldman’s warbler’). Stippled areas on bird schematics represent yellow plumage. For a more detailed description of phenotypic characteristics of the complex, see Table 1 and Hunt & Flaspohler (1998). Hatched area corresponds to wintering range of D. c. auduboni, and area of empty circles represents wintering range of D. c. coronata. Thick-hatched area in the northwest corresponds approximately to the hybrid zone between coronata and auduboni.

This figure is taken from the Mila paper and shows the distribution of each of the four yellow-rumped warbler populations.  The black bars between the auduboni and coronata populations represent the hybrid zone.

Figure 4 Julian date of median Yellow-rumped Warbler (YRWA) observations in each of 11 tree species at Trelease Woods, Urbana, Illinois was correlated with tree species’ date of 100% bud break (r  =  0.50, P  =  0.003).

This figure taken from Paul Strode’s paper shows the correlation between the arrival date of yellow-rumped warblers and the bud break of 11 tree species.  You can see they are very good at achieving near optimal foraging.

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One Response to The yellow-rumped warbler is moving around: on and off the branch

  1. Pingback: Sapium salicifolium | Find Me A Cure

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