Lanius ludovicianus

Constitutes a superspecies with L. excubitor and L. sphenocercus (AOU 1998).

Still widespread and common in some areas but has been declining throughout North America since the 1960s, and perhaps earlier. The decline is particularly severe in the northeastern and north-central regions. The species is now extirpated from most of the Northeast, and is nearly extirpated from Minnesota, Wisconsin, and Michigan. Part of the decline can be attributed to reforestation and loss of open habitat and thus represents a return to pre-settlement conditions when shrikes were probably absent from much of the heavily forested northern states. However, the decline has proceeded beyond what can be explained by habitat loss, as much suitable habitat remains unoccupied in most northern states. Further, decline has been recorded in all regions of the country, even those with much open habitat. Thus, the decline remains unexplained. Pesticides, loss of wintering habitat quality, and/or dependency on roadside habitat with high predation pressure have been suggested as possible causes. Of most urgent importance is research to unravel the cause of decline, and to identify critical habitat features.

BREEDING: California, eastern Oregon, eastern Washington, and central Alberta eastward across southern Canada to southwestern New Brunswick and Nova Scotia, and south to southern Baja California, throughout Mexico to Oaxaca and Veracruz,the Gulf Coast, and southern Florida (AOU 1983). Recently has been disappearing from the northeastern portion of the breeding range. In the northeastern U.S., breeds in in western Maryland, extreme eastern West Virginia, and Virginia (perhaps several dozen pairs); extirpated elsewhere (Bartgis 1992, R. W. MacDonald pers. comm.). NON-BREEDING: central Washington, eastern Oregon, California, southern Nevada, northern Arizona, northern New Mexico, and (east of the Rockies) the southern half of breeding range south to the Gulf Coast, southern Florida, and Mexico (AOU 1983).

This species is represented by a large number of occurrences (subpopulations). It has an extremely large range and population size (Birdlife International, 2014) and Partners in Flight (2013) estimates a global population of almost six million individuals.

PESTICIDES: Since shrikes are high on the food chain, pesticides have been implicated as a potential cause of the decline (Fraser and Luukkonen 1986). DDE, a metabolite of DDT, has been detected in eggs from Illinois and Virginia (Anderson and Duzan 1978), but crushed eggs associated with eggshell thinning have not been reported. While there is evidence of some eggshell thinning in Illinois, there is no apparent eggshell thinning in California and Florida (Hands et al. 1989). The relatively high nesting and fledging success rate in the areas of marked decline indicate that pesticides have not reduced reproduction (Brooks 1988, Gawlik and Bildstein 1990, Kridelbaugh 1982, Luukkonen 1987, Novak 1989). Young exposed to dieldrin have been shown to attack and kill prey more slowly than unexposed birds (Busbee 1977). Wide-scale use of organochlorides was curtailed in the U.S. in the 1970s, yet the population decline continues. Blumton et al. (1989) reported that necropsies on six Virginia shrikes showed traces of pesticide contamination, but drew no conclusions on the relationship between contamination and mortality. Pesticides may pose a greater threat in reducing food availability; clutch and brood sizes declined after the introduction of organochlorines; and significant declines on the Canadian prairies corresponded with dieldrin treatment of grasshoppers, which make up 30-75% of the diet (Yosef 1996, Yosef 1994, C. Campbell in Cadman 1985).

FOOD AVAILABILITY: Pesticides may affect populations through reducing food availability (see comments under PESTICIDES above). In Virginia, Luukkonen (1987) reported several nests with malnourished nestlings and developmental variability within a brood, both evidence of limited food availability at some sites. However, on a broader scale, early breeding, brood reduction, and multiple broods should off-set any local problems from food shortages, at least for the nesting season (Luukkonen 1987). In the Midwest, excessive winter mortality may be an important cause of the decline; reduced food supplies may weaken shrikes and cause them to move into woodlots occupied by raptors (Byrd and Johnston 1991).

PREDATION: Predation is the leading cause of nest failure, but nest predation does not appear to constitute an important limiting factor (Bartgis 1992).

BREEDING HABITAT LOSS/DEGRADATION: Limited evidence from most of the Northeast suggests that lack of suitable breeding habitat limits abundance in this region. Habitat loss has been caused by farmland abandonment, development, and widespread changes in farming practices (Novak 1989). Although acreage in pasture has decreased dramatically since World War II in New York and Virginia, there apparently is a substantial amount of unoccupied habitat remaining in both states (Luukkonen 1987, Novak 1989). There also seems to be considerable unoccupied breeding habitat in Maryland and West Virginia (Bartgis, pers. comm.). At least on a local level in Virginia, habitat loss may be the primary problem (Ridd, pers. comm.). Luukkonen (1987) expressed concerns about the fragmenting of potential habitat into islands in the Ridge and Valley of Virginia. Occasional use of marginal habitat may be partially to blame for the decline in Virginia. In southern Idaho, nearly 70% of original sagebrush steppe has been destroyed by agriculture and other development (Woods 1994). In the north-central states, however, habitat loss may explain some of the decline, but not all of it. In Missouri, declining populations coincided with regions with the highest proportion of lands being converted from pasture to row crops (Kridelbaugh 1982). Several reports have concluded that much suitable habitat remains unoccupied in Michigan, Minnesota, and Wisconsin, although shrikes now are nearly absent from these states (Brooks and Temple 1990, Robbins 1991). However, Luukkonen (pers. comm.) questioned the availability of adequately sized pastures for habitat in Michigan. Threats in western Canada (Telfer et al. 1989) include habitat loss such as the conversion of unimproved pasture to cropland (Telfer 1992). In eastern Canada, declines probably have been due to loss of breeding habitat to changing agricultural practices, industrial development, residential development, and vegetation succession (Cadman 1986, 1991). Yosef and Grubb (1992) suggested that a human-caused reduction in the number of hunting perches is at least partially responsible for the decline.

WINTER HABITAT LOSS: Populations that winter along the Gulf Coast have lost much habitat, and remaining habitats are often impoverished by red fire-ants and associated pesticide-control procedures (Lymn and Temple 1991). Because of relatively high reproductive success in southern Minnesota, Brooks (1988) concluded that the 20% mean annual rate of decline in the population in the region was "probably due to factors on their nonbreeding range" to the south. Brooks (1988) further summarized concerns that the decline in Minnesota is possibly caused by decreases in winter habitat in the breeding birds' wintering range. She stated that "if resident (southern) shrike populations are being limited by habitat availability, migrant shrikes wintering in the same area are almost certainly being forced to occupy marginal habitats that are not being held by territorial residents." However, a banding study in Missouri indicated that the winter and summer populations in that state are completely separate (Kridelbaugh 1982). Conclusive evidence that factors during the nonbreeding season are limiting is not available (Yosef 1994).

COWBIRD PARASITISM/NEST PREDATION: A study documented cowbird nest parasitism in shrikes in Iowa, but the frequency was very low, only 3 out of 261 nests (DeGeus 1991). The same study found a high incidence of nest predation among shrikes nesting along roadsides (86% of all losses), and only 35% nesting success overall. Predation is apparently more intense in roadside and other linear habitats (DeGeus 1990). If shrikes are utilizing roadside habitat extensively throughout their range, high nest predation may be one explanation for their decline.

OTHER MORTALITY: Locally, mortality from vehicle collisions may be significant. A high incidence of automobile-caused mortality was noted by Miller (1931). Shrikes typically fly low to the ground, sometimes across roadways, and often feed on roads. Inexperienced juveniles have been observed following adults across highways and learning from adults to feed on highways (Hershberger 1989, Novak 1989). In many areas, hedgerows, barbed-wire fences, and other habitat features utilized by shrikes are concentrated along roadways. Fledglings and other juveniles are frequently killed by automobiles. Automobile collisions killed all three fledglings produced at an Ontario nest and four of seven young fledged over three years at a New York site (Novak 1989). Juveniles killed by vehicle collisions in the summer have also been observed in both Virginias (Bartgis 1989, Luukkonen 1987) and Maryland (Hershberger 1990). Blumton et al. (1989) reported that automobiles accounted for 29% of the observed fall and winter mortality among Virginia shrikes.

FIRE ANTS: In Florida, Yosef and Lohrer (1995) found no evidence of an effect of imported red fire-ants on territory size and prey capture rate, but cautioned that insecticides used to control red fire-ants are likely to be detrimental (see PESTICIDES and WINTER HABITAT LOSS).

DISTURBANCE: Shrikes are not particularly alarmed by proximity to human activity. Brooks (1988) noted that nests near buildings had a success rate similar to those farther away from buildings. In Virginia, a shrike continued to incubate a nest in a tree after the top was trimmed off (Luukkonen 1987), although a Maryland nest in a tree was abandoned after a multiflora rose concealing it was killed with herbicide (Dean, pers. comm.). Some shrikes have nested less than 3 m from a road, but were not flushed by passing vehicles (Bartgis 1989, Luukkonen 1987). Food shortages may occasionally limit nesting success.