Search Strategies of Wheatears (Oenanthe oenanthe) and Stonechats (Saxicola torquata): Adaptive Variation in Perch Height, Search Time, Sally Distance and Inter-Perch Move Length

Abstract

SUMMARY (1) The foraging behaviour of six pairs of wheatears Oenanthe oenanthe and three pairs of stonechats Saxicola torquata was studied in a dry thornscrub area of N. Spain during the breeding season. (2) Wheatears were more diverse in their foraging mode using two main techniques, perch-to-ground sallying and running ground-gleaning. Stonechats were sallying specialists. Wheatears on average used lower perches than stonechats. (3) The distributions of successful and unsuccessful search times for both species resemble log-normal distributions. Departures from both types of perches occurred independently of the time already spent on the perch. Stonechats, on average, stayed longer on perches, and had lower departure rates from them than wheatears. (4) There appears to be no direct relationship between capture probabilities and departures from perches. Capture probabilities were lower for stonechats. Giving-up times and successful search times increased with perch height, and were longer for stonechats at the same perch heights. (5) This suggests that these birds were able to estimate how long it would take to search the area viewed from each perch, and that the height of the perch and probably the complexity of the area determine these giving-up times. There appears to be a difference in the detection ability or in the selection of prey between species. (6) Distant prey were more easily detected from high perches, as shown by the positive correlation between perch height and sally distance. Birds appeared to search systematically covering nearby areas first and progressively looking further away, as shown by the positive correlation between sally distances and successful search times. Stonechats sallied the same distances from higher perches and, on average, later than wheatears. (7) The return-to-perch rate of stonechats was twice that of wheatears. There was a marked resource depletion after the first use of a perch for wheatears, but not for stonechats. The return rate was negatively correlated with search times preceding the previous sally for wheatears, but not for stonechats. Again, the difference between species could be due to different detection efficiencies. (8) The distributions of move lengths between perches were also skewed to the right. Move length was positively correlated with perch height for both species. On average wheatears moved longer between perches than stonechats. These trends indicate that the size of the detection area from a perch determines the subsequent distance moved after giving up.