John R. Probst

Relating Kirtland's warbler population to changing landscape composition and structure

The population of male Kirtlands warbler (Dendroica kirtlandii) in the breeding season has averaged 206 from 1971 to 1987. The Kirtlands warbler occupies dense jack pine (Pinus banksiana) barrens from 5 to 23 years old and from 1.4 to 5.0 m high, formerly of wildfire origin. In 1984, 73% of the males censused were found in habitat naturally regenerated from wildfire or prescribed burning. The rest were in plantations (11%) or in harvested, unburned jack pine stands stocked by natural regeneration (16%). Twenty-two percent (630 of 2,886) of the Kirtlands warbler males counted in the annual censuses from 1971 through 1984 were found in 26 stands that were unburned and naturally regenerated following harvest. From 1982 to 1987, suitable regenerating areas were barely sufficient to replace currently occupied maturing stands, so population growth was impeded. Ecosystems of suitable size and regeneration characteristics (wildfire and plantation) doubled in area by 1989. In response, the population of Kirtlands warblers increased from 167 to 398 males between 1987 and 1992, but they withdrew almost entirely from the unburned, unplanted barrens by 1989 when the area of more suitable regeneration types increased. Minimum (368 males) and maximum (542 males) population estimates for 1996 were calculated based on 1984 average density (1.9 males per 40 ha) and peak population in burns (2.8 males per 40 ha).

Breeding bird communities in regenerating and mature broadleaf forests in the USA Lake States

Probst, J.R., Rakstad, D.S. and Rugg, D.J., 1992. Breeding bird communities in regenerating and mature broadleaf forests in the USA Lake States. For. Ecol. M, anage., 49: 43-60. When Lake States aspen tree canopy is removed by clearcut harvest, bird species turnover is almost complete. Bird species richness and total populations were highest in mature stands with well-developed understories and in regenerating stands about 4 years after clearcutting. However, species composition in regenerating stands was different to that in mature stands. Most bird species that are of regional concern were restricted to more mature stands. The lack of a direct relationship between species richness and increasing stand age was related to compensatory trends in species populations among five foraging groups of birds.

Fire and Shade Effects on Ground Cover Structure in Kirtland's Warbler Habitat

Abstract Researchers and managers have suggested that a narrow range of ground-cover structure resulting from fire might be necessary for suitable Kirtlands warbler nesting conditions. Yet, Kirtlands warblers have bred successfully in numerous unburned stands and there is little direct evidence to indicate that ground cover structure is a limiting factor for nest sites or habitat suitability within appropriate landform-ecosystems. We documented the range of percent cover for dominant ground-cover structural components in burned and unburned habitat (stand ages 7–23 y) occupied by Kirtlands warblers. The mean percent cover for the dominant ground-cover structural components was lichen/moss (12.1%), blueberry (Vaccinium angustifolium) (9.5%), bare ground and litter (5.6%), sedge/grass (5.2%), deadwood (4.3%), sand cherry (Prunus pumila) (3.3%), sweet fern (Comptonia peregrina) (2.3%), coarse grass (1.8%) and bearberry (Arctostaphylos uva-ursai) (1.2%). Burned sites had significantly more deadwood, sweet fern and lichen/moss cover, while unburned sites had significantly more bare ground and sedge/grass. We also investigated how fire, shade-history (i.e., pre-fire tree crown cover approximated by tree height and density) and succession influenced the percent cover of the dominant ground-cover structural components from 1 to 5-y after wildfire disturbance. The magnitude of differences in percent cover among shade-histories changed through time for the ground-cover components sand cherry, deadwood, grass/sedge and coarse grass. The percent cover of sweet fern, bearberry and bare ground was significantly different between some shade-histories. All dominant ground-cover components showed significant difference between at least one shade-history when compared to an unburned harvested reference stand. This suggests that more similarities exist among the three burned sites than between the burned sites and the unburned reference site. Our results suggest that fire, shade-history and succession influence ground-cover, but that various ground-cover components are affected differently by these factors. Because of the complex role disturbance history plays in maintaining ground-cover in Kirtlands warbler habitat, optimal management prescriptions are difficult to specify, especially when aspects of Kirtlands warbler ecology other than nest location are also considered. Although suitable ground cover structure can result without fire, maintaining prescribed fire is still desirable because this is a historically fire-regulated system. However, the range of ground-cover structures accepted by the Kirtlands Warbler and its resilience to disturbance suggests that suitable ground-cover for Kirtlands warbler could be maintained in some stands without burning after every timber harvest.

Effect of mating status and time of day on Kirtland's warbler song rates

Knowledge of factors affecting song rates is important because biologists use song in estimating or monitoring bird populations. Perhaps for no other species have song censuses played such a major role in population assessment and management as for the Kirtland`s Warbler (Dendroica kirtlandiz). In 1951, this endangered songbird was first censused within the known nesting range, which is restricted exclusively to Michigan (Mayfield 1953). The entire population was censused again in 1961 (Mayfield 1962) and every year after 1970. This information has been used to assess the response of this species to various management techniques (e.g., Brown-headed Cowbird [Molothrus ater] control). Recruitment to the population has been estimated by assuming that all singing males were paired (Mayfield 1975, 1983; Walkinshaw 1983; Probst, in press) and using known or inferred values for mortality rates, number of young fledged, and other demographic variables. Recently, Probst and Hayes (unpubl.) showed that a significant percentage of these singing males were probably unmated.