Central America

By Oliver Komar and John van Dort

Every winter, Yellow-rumped Warblers migrate south from the Canadian boreal forests to their wintering grounds in southern US, Mexico, and Central America. In Central America, Yellow-rumped Warbler is a non-breeding visitor from mid-October until mid-April (although the unique Goldman’s race is resident and breeds in the highlands of western Guatemala). The migratory races are more abundant in Belize and Guatemala than in southern Central America, but abundance in our region varies quite a bit from year to year. This month, some Honduran observers have sensed that there are more Yellow-rumped Warblers than in recent years, so we decided to see if eBird data, accessed through the public web interface, can support that hypothesis. The range map for this autumn, set to show only data from September to November of 2017, shows many red pins, at localities in every Central American country, indicating that the species has arrived throughout the region within the last 30 days (Fig 1).

Figure 1. Red markers on the species distribution map indicate localities with reports from the last 30 days (this map was generated on 25 November 2017, restricting the date range as September to November of the current year).

To compare relative abundance for 2017 to previous years, without being biased by variation in effort of eBirders, we looked at the abundance graphs accessible through the bar chart (just click on the Yellow-rumped Warbler species name from the bar chart graph). This can be done easily at the country level or finer geographic levels, such as department, or specific hotspot, through the menu options. It can also be done at the regional level, for example Central America, but that requires a bit of tweaking. We made graphs for the entire Central American region by first choosing a location from the menu, and then replacing the resulting graph’s locating code in the URL with the code for Central America, which is “ca.” The frequency graph clearly shows much higher frequency for Yellow-rumped Warblers throughout Central America in 2017 compared to the four previous years (Fig. 2). “Frequency” refers to the proportion of all complete checklists that include a species as present. The “change date” query box was used to specify graphing just the last five years, only the autumn months (September to December), and the option for separate years was checked. Several graphs were created at once. The greater abundance in 2017 was also visible on the abundance graph, which shows the average count of birds on all complete checklists (Fig. 3).

Figure 2. The frequency (percent of complete checklists that include Yellow-rumped Warbler) in Central America was notably higher in 2017 after 22 November, when compared to any of the previous four years.

Figure 3. The abundance graph shows the 2017 anomaly clearly, for Yellow-rumped Warblers in Central America in the last week of November (no data is available yet for December).

Yellow-rumped Warbler is one of those few species that have distinctive subspecies occurring together on the wintering grounds, and they are often easily separated in the field by noting the color of the throat. The western North America subspecies, auduboni, has a yellow throat, whereas the eastern North American subspecies, coronata, has a white throat. Note also the shape of the throat patch (smallest in auduboni) and the presence of a pale supercilium in coronata. These two forms were once considered separate species, Audubon’s and Myrtle Warblers. The locally distributed Guatemalan form, Goldman´s Warbler or Yellow-rumped Warbler (Goldman’s) has both yellow and white on the throat. Some taxonomists consider all three forms to be separate species, and we encourage Central American eBirders to carefully determine which form they are observing and report them as either Yellow-rumped Warbler (Myrtle) or Yellow-rumped Warbler (Audubon’s) (Fig. 4). The Audubon’s form is more common in the highlands, but sometimes flocks contain both forms. In the highlands of Huehuetenango, Guatemala, it is possible to find all three forms in a single flock.

Figure 4. Examples of Yellow-rumped Warbler subspecies, showing the differences in throat coloration and pattern. From left to right, Audubon’s, Myrtle, and Goldman’s (the latter is only known from Guatemala). Photos (left to right) © John van Dort, Oliver Komar & Luke Seitz / Macaulay Library.