The Loggerhead Shrike in Southwest Idaho
Type of Culminating Activity
Master of Science in Raptor Biology
Tom J. Cade
In 1991 I initiated a study to determine ecological characteristics of the Loggerhead Shrike (Lanius ludovicianus gambeli) breeding in southwest Idaho's semi- arid cold desert ecosystem. I found the Loggerhead to be a widely distributed and often locally abundant summer resident throughout lower elevations in the sagebrush (Artemesia tridentata) habitat characteristic of the southern portion of the state. Shrikes were on breeding territories by 21 March in 1992, and clutch initiation dates ranged from 3 April through 29 June. Mean and modal clutch sizes was 6.1 ± 0.8 and 6 eggs, respectively (n = 82), but clutch size decreased as breeding seasons progressed. The incubation period averaged 17.1 ± 0.5 d, and the median hatch date was 30 May. Most nestlings left the nest at 17 d of age, with departure ranging from 13 May to 5 August. Larvae of the blow fly Protocalliphora braueri infested some nestlings, but none had more than three larvae, and the infestations probably did not significantly hinder the shrikes. Differences in spring weather between 1991 and 1992 correlated with an 11 to 18 d difference in the median date of various stages of the breeding cycle between those years.
Most (106/162) nests were constructed in sagebrush, although bitterbrush (Purshia tridentata) and greasewood (Sarcobatus vermiculatus) were also used frequently. Height of nest shrubs averaged 162 ± 41 cm (n = 162), and the mean height of nests was 79 ± 24 cm. No nest site characteristic distinguished successful from unsuccessful nests. I found that many shrikes maintained an "impaling station" - a particular shrub where prey was frequently taken to be impaled and dismembered, then eaten or fed to the incubating female or nestlings.
Nest success, the percentage of nests which produced any fledglings, was 60.7% (68/112 nests). Owing to renests following failure, however, 76.4% of all pairs (68/89 pairs) nested successfully. Predation accounted for 54% of all nest failures. Productivity was 5.1 ± 1.5 fledglings per successful pair, and 3.9 ± 2.5 per nesting pair. Rate of territory reoccupancy was 71.7% (81/113 territories), and adult return rate was 30% (6/20 banded adults). Natal dispersal distance for four yearlings averaged 8 km, but three bred within 5 km of their natal site. Nesting densities varied from lone shrike pairs nesting in large stands of sagebrush to one pair/8.9 ha and one pair/25 ha on two high density sites. Nearest neighbor distance on those sites averaged 276 ± 133 m, and ranged from 90 to 670 m.
I used dead lab mice (Mus musculus) to develop a technique for providing supplemental food to breeding shrikes. Productivity of supplemented pairs was greater than that of controls, although food supplementation did not substantially increase mass or feather growth in supplemented nestlings.
Shrikes in this study played a role in determining the presence or absence of some other passerine species breeding in their vicinity, and overlap in diet between shrikes and other shrubsteppe passerines, and potential foraging interference, imply shrikes can benefit by excluding other birds from their territories.
High reproductive rate and breeding densities suggest that the Loggerhead is well-adapted to life in southwest Idaho scrub and, furthermore, that this population may be more viable than many other populations of this predatory passerine.
Woods, Christopher P., "The Loggerhead Shrike in Southwest Idaho" (1994). Boise State University Theses and Dissertations. 702.