BREEDING AND FORAGING ECOLOGY OF THE CACTUS WREN IN A VARIABLE ENVIRONMENT

The breeding and foraging ecology of the Cactus Wren Campylorhychus brunneicapillus was studied during 1978, 1979, and 1980 on a site in the Chihuahuan Desert of southern New Mexico. Significant differences in clutch size, breeding success, and timing of clutch initiation were observed among years. Differences were related to annual differences in the emergence and abundance of the major food of nestlings (grasshoppers), which, in turn, were related to annual differences in precipitation and temperature patterns. Results show that the Cactus Wren is capable of evaluating existing food supplies and adjusting its clutch size accordingly; no evidence of brood reduction was found.;Rate of feeding of nestlings was highest during approximately the first hour after sunrise and dropped-off sharply thereafter. Two factors appeared to be responsible for this pattern of activity: the catchability of grasshoppers and temperature-dependent water balance of parent birds.;Hypotheses from two different optimal foraging models (Orians and Pearson 1979; Schoener 1979) were tested. In general, my data support both models, but with regard to the selection of prey load size at intermediate distances, support only the Orians-Pearson model. This is because the two models were developed for different types of predators. A variable not considered in either of the above models, time spent not foraging between feeding trips to nestlings, potentially is an important one affecting foraging events of birds in a central place foraging situation.;The number of feeding trips to nestlings by the sexes during the first hour of the day was significantly different among years. This variation appeared to be associated with variation in the quality and distribution of food patches on territories. The concept of required foraging efficiency was used to interpret the observed differential participation in the feeding of nestlings by the sexes and to predict other differences in associated activities.;Long-term temperature patterns were analysed for variability and predictability. The breeding season is the most variable time of the year. Cactus Wrens initiated clutches during a period when high temperatures predicted favorable temperatures for the period when they would be feeding nestlings. Several factors that appear to affect clutch initiation in the Cactus Wren are discussed.