from the Field
The First Verified Summer Tanager Nesting In San Diego County, or, Flying Tomatoes Arrive In Force
After years of tantalizing breeding-season encounters, Summer Tanagers have finally been found nesting in Anza-Borrego Desert State Park, San Diego County. Although exciting, this should not be a great surprise since the species breeds in nearby Imperial, Riverside, San Bernardino, and Kern counties and is seen regularly in San Diego County riparian woodlands. Now, after eight pairs have been confirmed this year at two locations in Anza-Borrego, the obvious question is: Have they nested here all along, or is the discovery a result of recent expansion? Or is it both?
During a 7-hour Least Bell's Vireo survey, biologist Jeff Wells and I encountered a pair of Summer Tanagers in tall cottonwoods and willows along San Felipe Creek on 21 June 2000 (San Diego County Bird Atlas square J22). Jeff told me about finding several males in the area the week before. Soon, a lone male was seen carrying what looked like a berry, but such a flimsy sighting was not going to be submitted as evidence of San Diego's first breeding record. Later, we staked out the pair for about an hour and a half in an attempt to confirm breeding but saw nothing but foraging. I returned the next day, and during a 3-hour vigil saw male and female several times, including a 2-second copulation, but no confirmation.
On 23 June, having given up on the mating pair, I went to the best-looking habitat I could find, a large stand of 50-foot cottonwoods along the creek at the park's north boundary. Arriving at 0600 and moving quietly under the canopy, I soon heard the staccato "chit-up" call I'd heard both sexes give the day before. By 0620 I was watching a bright red male accompany a female as she carried food into a nest high in the canopy. I felt exhilarated.
After feeding young in the nest the female brooded for an hour and a half straight. The nest was attached at the trunk, supported by a pencil-sized branch, about 45 feet up, near the top of a spindly 50-foot-tall, 6-inch-diameter (at breast height) cottonwood. This was unlike the descriptions in references, which described nest placement as far out on a horizontal limb. This nest was dark in color, compared to its surroundings. When the female was off you could see through the nest in places. Male and female gave the "chit-up" call frequently but did not sing.
On the way home I ran into park ranger Bob Thériault; I was eager to show him the find. We hurried to the nest and were rewarded by seeing the female carrying food to the nest with father close by. Bob wanted to go to his atlas square in Sentenac Cienaga (J23) to search for the endangered Willow Flycatcher, so I joined him. After over an hour of fruitless search we heard a Summer Tanager call. Retraining our attention upward we soon saw a pair foraging. Within a few minutes we both saw the male carrying food, and, as we followed it, Bob saw the male enter a nest. The first two Summer Tanagers confirmed for San Diego County came within 4 hours of each other. This nest resembled the first nest in almost every detail: it was roughly the same shape and color, it was similar in location in the canopy and placement along a spindly trunk, but was about 35 feet up in a 40-foot-tall red willow, Salix laevigata. The nest site is about ½ mile from the site where Bob found a singing pair in 1999. Elated, I left Bob to his search for the flycatcher.
Since that day there have been a total of seven pairs found on park land along 2.5 miles of San Felipe Creek in atlas squares J22 and J23, five confirmed nesting, including three active nests, and two probable nesting. (Note: At no time were taped calls or songs used). James Zimmer and I found a probable used nest upstream in I21, where we also heard one "prrrrt" call on separate occasions in July. Between 30 June and 3 July 2000, Lori Hargrove found a female carrying food in upper Borrego Palm Canyon, while she bravely endured a 3-day sheep count.
I calculate the nesting density at San Felipe Creek at 7 pairs/24 hectares of suitable habitat (12 pairs/40 ha). This is based on the extent of tall dense cottonwood/willow habitat along the section of creek surveyed. In comparison, Robinson (reference below) reported nationwide densities most commonly ranging from 6 to 12 pair/40 ha.
The park's database reveals a number of other locations in or near Anza-Borrego Desert where Summer Tanagers have been reported in the breeding season but not confirmed nesting. Most promising are Banner Canyon, Vallecito Creek, and Lower and Middle Willows in Coyote Canyon. In addition, there are at least eight other sites in the county where Summer Tanagers have been reported during the nesting season since the San Diego County Bird Atlas began in 1997.
There is little in the way of recent studies of the species, especially the southwestern and assumed local nesting subspecies, Piranga rubra cooperi. Terri Gallion conducted a life-history study several years ago at Kern River Preserve. Her work is unpublished but is summarized on the preserve's website. A note of caution: Although the subspecies reported nesting in the southwest is P. r. cooperi, the subspecies encountered in migration in the mountains and coast of San Diego is the more eastern P. r. rubra.
The definitive monograph for the species: Robinson, W. D. 1996. Summer Tanager (Piranga rubra), in The Birds of North America, No. 248 (available in the museum's library). Among other facts, I learned from this source that I wasn't hallucinating when I heard and saw a female sing repeatedly along with a male on 15 July at a newly discovered nest. Robinson and Gallion also report females singing.
Describing sounds is usually hopeless, but here is an attempt to describe those I have heard: a very fast staccato "chit-up" or "chit-uh" given by both adults at or near two nests, possibly elsewhere. To me, this is the call in Peterson's Western Bird Songs CD, written as "pi-tuk". While listening closely with little outside noise, I did not usually hear a middle syllable as in the often reported "pit-i-tuk". I have heard a "pit-i-tuh" call in different contexts but not as often as the "chit-up". Jim Zimmer and Larry Hendrickson are the ones who pointed out that the call of the local birds they have heard does not have the hard upward syllable at the end, as in "pit-i-tuk", but trails off or down--more like "pit-i-tuh" or "chit-i-duh" or 'de-de-deh". I concur, especially after listening more closely to a female on 27 July. Two birds have given a repeated "prrrrrt" call similar to the one found on the Stokes four-CD set, Birds of the Western Region. During my observations, only three of the nesting pairs ever sang. By my ear the song is slightly louder than the Blue Grosbeak's song and call, whereas the "chit-up" call given by feeding birds is quieter than the song, especially near the nest.
Circumstantial evidence, in the form of numerous historical breeding-season reports, strongly suggests that Summer Tanagers have nested in San Diego County before, probably in small numbers. Mission Gorge had the most persistent records some years ago. But why had such a noticeable bird, the so called "flying tomato," one whose nest is not particularly hard to find, gone undetected? And, what about the rather large newly discovered San Felipe Creek population?
Could seven pairs have been overlooked? Maybe. To my knowledge, the best-quality cottonwood/willow habitat along San Felipe Creek had never been surveyed as thoroughly or as late in the season as it has this year. Atlaser Ed Hall makes a point that could explain why the San Felipe population may have been overlooked. The Summer Tanager is easily missed in the desert because it is most easily detected in midsummer, late June-July (hence the name "Summer" Tanager?), when few birders want to stop in the sweltering heat at a place like Scissor's Crossing. Also, unless you are focused on the call you'll probably miss it. In July, fewer than one-third of the males I encountered were singing. To top it off, you usually have to look straight up to find nesting activity.
Hints for confirming nesting: (1) Recognizing the calls and the song is very important. (2) Search tall, dense cottonwood/willow habitat in late June/early July and listen. (3) Walk under or next to the canopy and follow males or, if lucky, females. Do not hesitate to follow the male because he is easy to see, often chases or follows the female, and helps feed young. (4) Be aware that courting males feed females, apparently often. I saw it twice. (5) Seek chiropractic therapy for a stiff neck, and muster up the patience to sit for several hours--no reading allowed.
Three cheers to the Anza-Borrego Foundation, California State Parks, and others who waged the two-year campaign resulting in the purchase of 1700 acres of San Felipe Valley. The Summer Tanager is a neotropic migrant and California Species of Special Concern, which, in the southwest, nests only in high-quality riparian habitat. Such species are known as riparian obligates--that is, they are only found where there is sufficient riparian habitat. As a measure of the high quality of this new purchase, all seven Summer Tanager pairs discovered along the creek are within this magnificent addition to the state park system.
Paul Jorgensen, Resource Ecologist, California State Parks
Summer Tanager photo by Ken Fink.