DUNKING OF PREY BY BREWER'S BLACKBIRDS: A NOVEL SOURCE OF WATER FOR NESTLINGS

WALTER D. KOENIG

Small birds lose relatively more water through evaporative respiration than do larger birds (Bartholomew and Cade 1963). As a result, nestlings can generally be expected to be under greater water stress than adults. One well known method ofsupplying nestlings with free water, in addition to that provided in their food, is for the adults to soak their belly feathers at watering sources; this behavior has been described in several species of sandgrouse (Pterocles namaqua and P. burchelli of the Kalahari Desert, as well as P. alchata and P. senegallus of lraq; Cade and MacLean 1967, MacLean 1968) and in the Egyptian Plover (Pluvianus aegyptius; Howell 1979), and may occur in other species as well. Here, I describe a novel and potentially important method of supplying free water to young by Brewer's Blackbirds (Euphagus cyanocephalus): dunking of prey.

My observations were made at "Blompond," a small stockpond, about 350 m2, built in 1970 adjacent to Hastings Reservation, Monterey County, California. This area has a Mediterranean climate with less than 2% of the average annual precipitation falling in June through September (Bradford 1974); during this season, the availability and occurrence of surface water are critical for much of the local avifauna (Williams and Koenig 1980). Brewer's Blackbirds have been recorded as nesting at Hastings in only five years since 1939, most recently in 1984. Nests have invariably been in a small colony of 5 to 10 pairs on or near a knoll, approximately 0.25 km away from what is now Blompond.

Every day between 24 June and 5 July 1984, during which time nestlings were being fed, I watched Brewer's Blackbirds in the vicinity of the pond. Their activities included drinking, bathing, and foraging in the grasses surrounding the pond, primarily for grasshoppers (Orthoptera: Acrididae). A large proportion of birds, however, apparently came to the pond to dunk grasshoppers in the water before flying to their nests and feeding the grasshoppers to their young.

I monitored the birds from 09:00-10:15, 12:00-13:45, and 15:00-16:15 on 30 June and from 14:00-15:30 on 2 July (5.5 h), and recorded their activities at the pond. A total of 48 visits by birds were observed: of these, two (4%) birds bathed, six (12.5%) drank, another six foraged at the pond's edge, and 35 (73%) arrived at the pond carrying grasshoppers and dunked them before returning to their nests (one after having foraged and caught prey along the edge of the pond). Birds often dunked prey repeatedly: the mean (±SD) number of dunkings was 3.4 ± 1.7 (n = 12). Both males and females engaged in dunking behavior: of the 35 cases recorded, 14 (40%) were performed by males. Birds were not seen to eat the grasshoppers following dunking, nor did they appear to swallow water while they were dunking prey. Thus, I infer that dunking behavior was directly associated with feeding of nestlings. Because these birds were not marked, I could not determine the frequency of dunking trips by individuals. The colony consisted of only about 10 nests, however, and it appeared as though individuals were repeatedly engaging in dunking behavior.

In order to measure the potential importance of this behavior, I captured 15 grasshoppers, weighed them, dunked them while holding them with a pair of tweezers, and then measured their weight gain (the amount of water picked up by dunking). The mean amount ofwater picked up was 0.062 ± 0.058 g per grasshopper, which was 34% of the prey's mean live weight (0.208 g). The average dry body mass of grasshoppers at Hastings was about 0.045 g (mean of 18 grasshoppers caught in insect traps at Hastings). If none of the water acquired by dunking blows off in transit to the nest, each dunked grasshopper fed to a nestling would contain an average of 0.225 g water, of which 28% would originate from the dunking behavior itself.

Dunking behavior may have been an important way of providing free water for nestlings in this colony of Brewer's Blackbirds. Other species that nest in seasonally dry environments with some source of free water relatively near their nests may also dunk their prey. (Wind and evaporation would largely deplete water accumulated through dunking if the water source were too far from the nest, as discussed for sandgrouse by Cade and MacLean [1967].) That such behavior has gone previously unreported is not surprising, given the ease with which it might be missed by researchers who focus on adults while the birds are feeding young rather than while they are foraging.

I thank R. Noyce and L. Berta for permission to work at Blompond; S. Albano and A. Peters for help in catching grasshoppers; and P. Williams and the reviewers for comments on the manuscript.

LITERATURE CITED

• BARTHOLOMEW, G. A., AND T. J. CADE. 1963. The water economy of land birds. Auk 80:504-539.
• BRADFORD, D. F. 1974. Water stress of free-living Peromyscus truei. Ecology 55:1407-1414.
• CADE, T. J., AND G. L. MACLEAN. 1967. Transport of water by adult sandgrouse to their young. Condor 69: 323-343.
• HOWELL, T. R. 1979. Breeding biology of the Egyptian Plover, Pluvianus aegyptius. Univ. Calif. Publ. Zoo!. 113:1-76.
• MACLEAN, G. L. 1968. Field studies on the sandgrouse of the Kalahari Desert. Living Bird 7:209-235.
• WILLIAMS, P. L., AND W. D. KOENIG. 1980. Water dependence of birds in a temperate oak woodland. Auk 97:339-350.

Hastings Reservation and Museum of Vertebrate Zoology, University of California, Star Route Box 80, Carmel Valley, California 93924. Received 10 December 1984. Final acceptance 19 April 1985.