Where Are All The Birds? New Observations and Further Insights

By Tim Helentjaris

My last post detailed how the 2020 drought has had devastating impacts on a number of our birds and how that was continuing into 2022. And yet I was recently birding along the Santa Cruz River near Tumacacori and the bird song was deafening and the density of birds apparently as high as ever? So what gives, how can we reconcile this?

Most birds don’t show any impact from the 2020 drought, it’s limited to certain species. It appears that whole habitats were virtually unaffected: riparian, wetlands, urban, and maybe surprising, higher elevation areas. Lower elevation and dry canyons, deserts, and grasslands exhibit many more affected species. There is quite a bit of variation in precipitation in Southeast Arizona, which shows a strong correlation to elevation (see Graph 1). Average annual precipitation varies from ~3” per year in low areas like Yuma, to ~11” around Tucson, and then up to ~30” at higher elevations in our Sky Islands. In fact using just elevation as a variable, we can explain almost 80% of this variation in average annual precipitation across Southeast Arizona. Compared to lower elevation foothills areas, high elevation forests normally get a lot more rainfall and they also lost proportionately much less during the 2020 drought (data not shown). These much higher and wetter environments, along with wetlands and riparian areas, are “buffered” against the drought and probably explains why few species there were affected by the 2020 drought.

I next compared the precipitation levels to the eBird data from the last ten years. Not surprisingly, most of the severely affected species exhibit a very close correlation of their frequency with variation in precipitation from year-to-year, even in “non-drought” years. In Graph 2, you can clearly see that the frequency of four desert species show quite similar changes in frequency, and all of them track with variation in precipitation from year to year, with about a one year delay. After the wet years of 2018 and 2019, all observations rose dramatically only to fall in 2021 after the drought of the previous year. Similar effects can be seen in other habitats, such as the fluctuations in Grasshopper and Cassin’s Sparrows and White-tailed Kites in grasslands, or Montezuma Quail and Black-chinned and Rufous-crowned Sparrows in foothills. So, the 2020 drought effects upon these species were not that unusual, just exaggerated given the extremes of that year. Might this hint at a recovery if we get normal precipitation in 2022?

A further look into the eBird data for 2021:

• Most of the affected species are residents and breeders here, such as Elegant Trogons. They have partially rebounded during our 2022 surveys but are still not near our previous highs. Species that are migrating through were mostly unaffected.
• Most of the very affected species were also smaller in size, such as gnatcatchers, wrens, and sparrows. These smaller residents have smaller territories, are less likely to wander, and also have shorter lifespans. Any detrimental impacts are likely to show up with these species first.
• Many of our Mexican pioneer species expanding their ranges and numbers in Arizona were also in the affected group—trogon, Rose-throated Becard, Five-striped Sparrow, and Black-capped Gnatcatcher—precisely the birds most visiting birders are coming here to see.
  
  

Costa's Hummingbird by Lois Manowitz

One conundrum with these conclusions: Why were hummingbirds unaffected even while the lower elevations were seemingly devoid of wildflowers? Perhaps these observations are buffered as they mostly come from areas with feeders, where the drought had no effect on this sugar source, i.e. Paton Center, Santa Rita Lodge, and our yards. If so, did hummingbirds in strictly wild areas suffer? This is an interesting idea to examine.

In general, I think we understand a little more about the sharp decreases in some Southeast Arizona species that we saw after the 2020 drought. Looking at past variations in precipitation and species observations, we can be hopeful to get back to higher numbers for these species as long as our precipitation rebounds this year as well.

Tim Helentjaris is a retired biologist with a keen interest in the life histories and occurrence of our resident birds in Southeastern Arizona.