Bird and Mammal Activity at California Condor Feeding Stations

NOTE: Michael D. Silbernagle and I prepared the following paper in April 1979, following several years of providing supplemental food for California condors. It wasn't published because there was no way to statistically treat the information gathered. Although it may lack scientific rigor, I think it has some worthwhile information in it, particularly since it is one of the last studies dealing with the original wild condor population.

ABSTRACT: Between January 1976 and June 1978, carcasses of wild and domestic mammals were distributed among five feeding sites, as part of a supplemental feeding program for California condors (Gymnogyps californianus). Time-lapse cameras recorded that the sites were used regularly by condors, turkey vultures (Cathartes aura), golden eagles (Aquila chrysaetos), ravens (Corvus corax), coyotes (Canis latrans), and black bears (Euarctos americanus). All species followed definite use patterns each year. Condors most often used sites between January and April; turkey vultures used the sites only in late winter and spring; eagles occurred most frequently in October; and ravens were present all year. Coyotes also used feeding sites much of the year, but were especially prevalent in early summer. Bears visited the sites sporadically in summer.
Need for food is only one of the factors that attract animals to a station. Site location, seasonal flocking patterns, and activity of other species are also important. This feeding program did not result in population increases, or in modified distribution or behavior patterns of the species involved.

From February 1971 to July 1978, carcasses of various animals were distributed at selected sites in Ventura County, California, as part of a program to preserve the few remaining California condors. In January 1976, automated surveillance cameras were installed at the feeding sites, to more accurately monitor condor use and to document the amount and type of use by other species. This paper presents the results of the camera study.
We thank John C. Borneman, National Audubon Society, for assistance with the feeding program, and Sally H. Wilbur for help with data analysis. Personnel of the Los Padres National Forest radio shop constructed photocells for use with the surveillance cameras. The California Department of Fish and Game helped provide animal carcasses. The manuscript was reviewed by Ray C. Erickson, Stana Federighi, and Harry M. Ohlendorf.

Feeding sites were located on the Hopper Mountain National Wildlife Refuge, and in nearby portions of the Sespe Condor Sanctuary, Los Padres National Forest, both areas being ca 10 km north of Fillmore, Ventura County, California. Elevations within the study area range from 730 to 1,220 meters above sea level. The terrain is very steep and rugged. The climate is arid, with limited winter precipitation and long, dry summers. Dense brush characteristic of the California chaparral community (Jepson 1963) predominates on the refuge and national forest. However, the southern slopes of Hopper Mountain are vegetated with annual grasses. There are also small natural clearings in the chaparral (potreros). The Hopper Mountain grasslands have in the past been regularly used as a feeding area by California condors (Koford 1953:55-65, references to E. Percy and the Hopper Ranch).

Five feeding sites were used during the study, four in the grassland areas on the south slopes of Hopper Mountain, and one on an isolated potrero in the Sespe Condor Sanctuary. The Hopper Mountain sites were at elevations between 730 and 900 meters above sea level, and were 0.3 to 2.1 kilometers apart; the sanctuary site (1,220 meters above sea level) was about 4 kilometers from the others. Once each week, animal carcasses (mainly domestic goats, but also deer, cattle, and domestic sheep) were placed at one or more sites. Often two sites were baited simultaneously, so we could record differences in animal use and behavior when more than one food source was available. Sometimes we placed two carcasses at the same station to see if this affected animal use patterns.

Sites were monitored with 8 millimeter time-lapse movie cameras that took pictures at approximately 2 minute intervals throughout daylight hours. A battery-operated photocell turned cameras off at sunset, and reactivated them at sunrise. The two minute intervals were too long to record detailed animal behavior at the sites, but did record all but the most casual feeding visits, while allowing the cameras to run a full week without servicing.
Times of animal activity were determined by recording when the cameras were turned off and on, counting the exposed film frames between activity periods or during feeding sequences, and multiplying by the time interval between frames. This was only an approximate reading of actual time because periods of camera operation varied with weather conditions, operating longer on bright sunny days than on rainy, foggy, or overcast ones. Because some unresolvable discrepancies of up to one hour occurred, activity was analyzed in five general time blocks: the first hour after sunrise, one to two hours after sunrise, midday (6 hours in midwinter, 11 hours in midsummer), one to two hours before sunset, and the last hour before sunset. Information on nocturnal feeding was obtained by comparing the condition of carcasses at the end of the day with conditions when the cameras turned on the following morning.

We analyzed film covering 1,043 days of camera operation, divided between sites as follows: Site 1, 243 days; Site 2, 340 days; Site 3, 202 days; Site 4, 178 days; and Site 5, 80 days. Carcasses were present on 679 days: 249 (37%) in winter (December-February); 210 (31%) in spring (March-May); 107 (15%) in summer (June-August); and 113 (17%) in the fall (September-November).
Animals recorded using the sites were turkey vulture, California condor, golden eagle, raven, coyote, black bear, and mountain lion (Felis concolor). Single bobcats (Lynx rufus) were recorded on three occasions at Site 2, but they were not observed feeding and apparently were just walking through the area.

Amount and Timing of Use.
Use of feeding sites by different species varied seasonally both in amount of time spent feeding (number of time blocks when a species was present), and in the number of individuals represented. In general, use of feeding sites was greatest in fall and winter, least in summer.
Turkey vulture: Turkey vultures only used the feeding sites between February and June, although we observed them in the vicinity at other seasons. Peak numbers (up to seven birds) occurred in February and March each year, soon after turkey vultures began arriving back in the area after being absent for several months. Peak numbers were similar in 1976 and 1977, as was total use, but both were lower in 1978. The weather in February and March 1978 was rainier and more overcast than in those months in previous years, so less feeding time was available to birds that depend on fair weather for food searching.
The majority of vulture use (66%) occurred during midday, with no use during the first two hours of the day. Turkey vultures have the lowest wing-loading ratio of the cathartid vultures, and can take advantage of minimum soaring conditions such as occur in early morning, but they usually stay at the roost for several hours after sunrise (Leach 1929, Nauman 1965, Davis 1974).

California condor: Single California condors were recorded on four occasions in November 1976. All other use by this species occurred between January and April in 1976 and 1977. No use occurred in 1978. The largest group recorded by the cameras contained three birds, but the senior author observed eight condors circling near one of the feeding sites on 24 February 1976. Also, D. Moody and J. Rangel (personal communication) observed eight condors feeding at Site 4 on 6 January 1976, when no camera was operating there.
The timing of condor use coincided with the usual peak of condor activity in Ventura County (Koford 1953, Wilbur 1978b). Long periods of inclement weather might explain the limited reports of condors at feeding sites in 1978, but we observed that all condor activity in Ventura and Los Angeles counties in 1978 was lower than usual.
On one occasion, a condor fed at a carcass during the second hour after sunrise. All other records were later in the day, a reflection of very little movement of condors away from their roosts before 0900 in winter and early spring (Koford 1953:47-48).

Golden eagle: Eagle use of feeding sites was definitely seasonal. The greatest activity was recorded in October each year; the lowest activity occurred in March and April or August each year. Highs and lows were of similar magnitude each year. They do not coincide with any known fluctuations of the natural food supply locally, nor do they relate in an obvious way to events in the annual breeding cycle.
Usually one or two eagles were at a feeding site simultaneously. Three eagles (two adults and an immature) were present on ten occasions in January 1976. A three-eagle group was recorded once in August 1977 (one adult, two immatures), and once in September 1977 (two adults, one immature).
Eagle use was evenly distributed throughout the day, except in the first hour after sunrise, when there was only occasional use (9% of the total). Often, eagles were still feeding at carcasses when the cameras shut off at sunset. Because eagles are more active flyers than condors or vultures, and less dependent on the midday buildup of thermal updrafts, they have a much longer foraging day.

Raven: Although the magnitude of raven use varied, their activity throughout the year was highly predictable. Major peaks occurred in February, with lesser ones in October. Annual lows in use occurred in January and in summer (June-August). At most seasons, raven feeding groups included one to three birds. Between December and April, flocks of up to 15 ravens occurred regularly. Large flocks were recorded occasionally in September and October, also.

Ravens fed at all times of the day, but use was heaviest in the morning and early afternoon. Forty-five percent of total use occurred in the first two hours of the day; only 20% occurred in the last two hours.

Coyote: Much coyote feeding activity occurred at night, but they also fed regularly during daylight hours at all times of year. Obvious peaks in daytime use occurred each year in June and July, and in September. A lesser peak occurred each year in April. Lowest use periods were in March (1976 and 1978 only), and in November and December (1976 and 1977). There was no season when packs of coyotes were more prevalent than in another. Two or three coyotes fed regularly almost every month. Groups of four and five coyotes were recorded on four occasions in September, October (two records), and May.
Variations in coyote use of feeding sites correlated fairly well with availability of live pretty. Small mammal activity is highest during and just after winter rains, so carrion is just one of several food sources available from December until about April. In summer, particularly during hot, dry periods, ground squirrels (Otospermophilus beecheyi) aestivate and are not as readily available as food for coyotes.
Coyotes were recorded feeding at all times of day, but they appeared more likely to feed in the early morning (38% of total records) than in late afternoon (26%). Coyotes do much of their feeding at night, and the morning records are probably the end of their nocturnal activities.

Black bear: Black bears were recorded in 37 separate time blocks. Except for one record in December, all daylight activity occurred between April and October each year. Many carcasses disappeared at night during that same time period, indicating that April-October was a major season of nocturnal use as well.
One of 37 sightings was of an adult bear and one cub; all the other records were of single adults. Use occurred at all times of day, with no pattern evident.
Although winters are usually mild in Ventura County, bears apparently become dormant for periods of time (Ingles 1965). They may partially explain the seasonal nature of bear use at feeding sites, but it may also be true that natural food is especially scarce in summer. During summer, local bears frequently move downslope into settled areas, doing damage to orchards and apiaries.

Mountain lion: A lion was recorded once at Site 2 on 25 September 1976. It stayed at the goat carcass from about 1700 to 1720. The lion was actively feeding in two of the ten movie frames. The rest of the time it stood or sat close to the carcass.
We know of one other instance of apparent mountain lion activity at a feeding site. On 10 January 1975 at Site 1, the senior author and John Borneman observed that a deer carcass had been partially covered with grass scraped from a wide circle around the carcass. The carcass had not been fed on, and the lion apparently did not return to it.

Change in Amount of Use by Species over the Course of the Study

Concerns arising from supplemental feeding are that animals might become habituated to a feeding site and dependent on it, or they might increase to unnatural levels because of the easy, dependable food supply. We had determined during earlier feeding studies (Wilbur et al. 1974, Wilbur 1978a) that condors do not significantly change their activity patterns to take advantage of feeding stations. That appeared to be the case during this study, as well. During the two and one-half years of the present monitoring, no species showed any obvious changes in either total amount of use or in the peak numbers of individuals using the sites.

Differential Use of Feeding Sites
A rough measure of feeding site preference was obtained by comparing time-blocks of animal use per station and "opportunity days" (days when carcasses were present at the site). Site 5 was excluded from analysis because it was only in operation a short time. Results for the other stations were not treatable statistically, but do suggest that there was preferential use of certain sites.
For example, Site 3 had twice as much use by coyotes in the fall as the other stations, even though it provided fewer "opportunity days." Winter coyote use at Site 2 and Site 3 was double that of Site 1 and Site 4. Site 3 was little used by eagles, except in fall. Site 2 seemed to be preferred by ravens in winter, and by bears and turkey vultures whenever they were present in the vicinity.
There are a number of possible reasons for these differences. Site 1 and Site 2 had a somewhat longer history of use as feeding stations than the others, and also were baited more frequently. As pointed out above, there wasn't any substantial increase in the numbers of animals using these sites during the course of the study, but habituation to them may have developed in certain individuals. Also, Site 2 was apparently located on a regularly used bear trail, and bears may have passed that station more often than they did the others.
Differential use by birds is less readily explained. An eagle or other bird circling high over Site 2 could simultaneously see carcasses at sites 1, 2, and 3. Site 4 was farther away, but still in the same general area. Why one should be "chosen" over another is unclear.

Species Interactions
Although several species were frequently at a feeding site simultaneously, usually only one species fed. The usual order of decreasing dominance among birds at a carcass was as reported by Koford (1953:74): golden eagle, condor, turkey vulture, and raven. In both our and Koford's studies, condors almost always yielded to eagles at a carcass, and turkey vultures and ravens were both subordinate to the two larger species. Turkey vultures and ravens sometimes fed side by side. Coyotes appeared to be dominant over all birds.
Nine of 17 records of more than one species feeding at the same time were of turkey vultures and ravens. One out of 17 included an adult condor and a golden eagle; one was an adult condor and ravens; two were an immature condor and ravens; two were an eagle and ravens; and two were a coyote and ravens.
Koford (1953:64) noted that condors pay little attention to livestock roaming near them. The cameras recorded a similar lack of interaction between livestock and other species. On four occasions, up to three head of cattle walked within 2 or 3 meters of a feeding coyote without causing obvious disturbance. Twice, ravens fed within a few meters of horses. However, in one instance an adult eagle apparently left a carcass when a cow walked up to it.
On 8 January 1976 a bobcat walked within 5 meters of two condors (an adult and an immature) feeding at Site 2. In the next picture (2 minutes later) the bobcat was gone and the condors were feeding as before, so apparently there was no interaction.

Effects of Having Two Carcasses at a Site
When two carcasses were placed at the same site, wired in place about 5 meters apart, one carcass was frequently completely consumed before the second was touched. Some individuals sat around the site unfed while others ate, even though the second carcass was available to them. However, the camera recorded 82 instances in which both carcasses were used simultaneously. Fifty-five of these records involved members of the same species feeding at both carcasses: eagles (21 instances), ravens (16), coyotes (15), turkey vultures (2), and condors (1). There were 14 records of ravens feeding on one carcass, while an eagle fed on the other. Other combinations were: vulture and raven (4), coyote and raven (2), eagle and condor (1), and condor and raven (1). On two occasions, an immature condor and several ravens fed on one carcass while an eagle fed on the other. Three records of turkey vultures and ravens feeding side by side on two carcasses simultaneously support Koford's observations (1953:64) that there is little or no dominance relationship between these species.

Effects of Baiting Two Sites Simultaneously
Although two stations were baited simultaneously about 50% of the time, there were only 33 instances of activity at both sites concurrently. In 11 cases, one of the stations was Site 5, which is located about 4 kilometers from the other sites and might be expected to have a different animal activity pattern. It appears that in many cases animal activity at one feeding site had a stronger attraction to other animals than did the food at the nearby site. Twenty or more individuals of various species congregated at one site, even though only a few actually fed.
The most common simultaneous records (five examples each) were of eagles at both sites, or coyotes at both sites. Twice, single eagles fed at one site while a large assemblage of turkey vultures and ravens fed at the other. Only once was there substantial activity at both sites at the same time: on 26 February 1977, when ravens and turkey vultures were at both Site 2 and Site 3. There was also a condor and an eagle at Site 2.

As was shown in previous studies (Wilbur et at. 1974, Wilbur 1978a), this feeding program did not modify basic behavior patterns of California condors. Condors used feeding sites if the sites were located where condors normally occurred. Condors did not congregate at the sites, and did not remain in a feeding area beyond normal seasonal departure times. While a program such as ours might be used to enhance the food supply of locally resident condors, it would not be likely to modify long established behavior of the population.
The same seems to apply to other species. Feeding sites were readily used, and some individuals may have learned to expect food at a given site. Nevertheless, there was no evidence of changes in numbers, seasonal status, or behavior as a result of the two and one-half year program.
Certain feeding hierarchies and dominance relationships were evident, as they have been in other studies of scavenger feeding relationships (Koford 1953, Kruuk 1967, Houston 1974, Konig 1974 and 1976). There was no evidence of aggression that would cause bodily harm, nor was there evidence that dominance or competition was depriving species or individuals of necessary food.
Use of the feeding sites was apparently only partially prompted by actual need to eat. The presence of many individuals of several species at a single carcass, even though other food was available nearby, suggests that dominance hierarchies are partially modified by social attractions.

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Sanford Wilbur 2021