CORDILLERAN FLYCATCHERS MAKE A COMEBACK ON MT LEMMON

 By Dr. Charles van Riper III

Active Western Cordilleran Flycatcher nest on Mt Lemmon, all photos by Henry Johnson, Mt Lemmon AZ Images

The Western Cordilleran Flycatcher (Empidonax difficilis occidentalis) is an inconspicuous member of the southern Arizona summer breeding bird community. This Neotropical migrant breeds in higher elevation habitats of the Sky Islands, where flying insects constitute over 95% of their diet. Our study has been focused on the Mt Lemmon population, just north of Tucson, since 2016. Because the Cordilleran sexes look virtually identical to human observers, we attempt to capture and color-band all breeding flycatchers. You can find these color-banded birds in our Mt Lemmon study areas on Turkey Run Trail (from Summerhaven to the ski area) and at Camp Lawton (behind the US Forest Service Visitor Center at Palisades) and the Boy Scout Camp area. We have documented annual return rates of banded birds, time of egg laying, incubation behavior, nestling feeding, and overall nest success (productivity).

In 2022, it looked like the following year might be a good breeding season for the Western Cordilleran Flycatcher because of abundant precipitation and the heavy snowpack that would enhance insect abundance, and thus increase food resources. All the information collected at our nesting locations on Mt Lemmon has now been analyzed, and indeed, 2023 was a banner year, especially for the production of flycatcher young. During 2023, more flycatcher eggs were laid per nest, more hatched, and more young were fledged than in any of the five previous years of our study (Fig 1).

Figure 1. Cordilleran Flycatcher reproductive rates from 2019 through 2023 on Mt Lemmon, AZ. The mean annual nests success rate is depicted by the line, with the percent annual success given on the right-side vertical scale. 

Nesting began a little later in 2023 than in previous years due to cooler spring conditions that existed on the wintering grounds around Guadalajara, MX, and along the migratory route to Arizona. We found the first active Western Cordilleran Flycatcher nest in the third week of May. In comparison, nesting began the first week of May in 2019, while in 2020 and 2022, nesting started during the second week. The later arrival of migrant flycatchers in 2023 resulted in a truncated breeding season, but one that was very productive.

There were several factors that contributed to the very productive 2023 Cordilleran Flycatcher nesting season, with the most important being a high abundance of food resources. We operated three malaise flying insect traps along Turkey Run road, just past the Guthrie intersection on Mt Lemmon, where we collected and weighed captured flying insect biomass every two weeks. Since the Cordilleran Flycatcher diet consists of over 98% flying insects, the malaise data provide us with a good relative index of available flycatcher food resources (Fig 2).

Figure 2. Flying insect biomass (dry weight grams/day) from malaise traps in 2019 through 2023 on Mt Lemmon, AZ.

Insect levels were higher in 2023 (the last light-blue bars in the Figure, at the right of each two-week sampling period), than during any previous year that we have monitored flying insects on Mt Lemmon. This was particularly true in late June, when most young flycatcher nestlings are being fed by adults.

The more synchronous start of breeding resulted in a lower flycatcher egg predation rate, possibly through predator swamping, and this resulted in a higher 2023 nesting success. Steller’s Jay (Fig.3) predation rates of Cordilleran Flycatcher eggs are usually quite high on Mt Lemmon, averaging 34.25% of all eggs laid. When analyzing our mist-net capture data, we found that during 2023, we caught fewer Steller’s Jays than during 2019-2021. The decreased number of jays in 2023 resulted in fewer nests with flycatcher eggs being lost to predation. With the significantly larger clutches in 2023, this resulted in a greater number of eggs that could hatch and eventually fledge young. To put this in perspective, only 25 Cordilleran Flycatcher young fledged in our study area in 2021, 60 young fledged in 2022, while this year 83 young fledged. The average number of young flycatchers leaving each nest is even more dramatic, with only 1.1 young fledged from each active nest in 2020, an average of 1.5 per nest in 2021-22, and an average of 3.38 young/nest in the summer of 2023. 

Flycatcher eggs in nest

Figure 3. Steller's Jay

In summary, the larger number of flycatcher eggs laid in 2023, resulted in more eggs hatched, and with lower predation rates, more flycatcher young were produced. The 2023 Cordilleran Flycatcher breeding season was quite successful, and the large number of fledged young should help enhance breeding of flycatchers on Mt Lemmon during 2024.

We would like to thank those Mt Lemmon residents who took part in the Cordilleran Flycatcher Research Project, and thank those who provided permission to catch birds and monitor nests on their private lands. Teo Melis and Serena Mankiller have provided continuing support of our research that is within the US Geological Survey, Southwest Biological Center’s Emeritus Program. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Dr. Charles van Riper III is Emeritus Professor and ST Scientist, School of Natural Resources and the Environment & US Geological Survey, Southwest Biological Science Center, University of Arizona, 520 N Park Ave, Tucson, AZ 85719 – charlesvanriper3@gmail.com