Influence of vegetation structure and landscape context on the occupancy of shrubland birds

Across the guild of shrubland birds, some species have been experiencing long-term population declines while other have been increasing. One potential reason for these differences is that various shrubland bird species prefer different types of shrubland habitat and some habitats are more common than others. However, we lack a clear understanding of the attributes of shrublands that shrubland birds prefer. Specifically, we lack information on how the percent shrub cover, the proportion of shrubs comprised of non-native shrubs, the patchiness of shrubs, and the surrounding landscape context influence occupancy (the probability that a given species will be at a site) of shrubland birds. To better understand these relationships I used bird survey and vegetation data from a long-term monitoring program that randomly monitored shrublands across the state of Illinois. I examined the influence of landscape and site-level variables on the occupancy of 22 shrubland bird species. Generally, the proportion of non-native shrubs at a site had little influence on shrubland bird occupancy. Shrubland birds responded positively to both the percent shrub cover and to the patchiness of shrubs; however the strength of the relationship differed between species. Over half (6/10) of the species that are experiencing population declines in the region (as determined by Breeding Bird Survey data) responded strongly to the patchiness of the shrubs, whereas, only 3 of the 12 shrubland species whose populations were either stable or increased responded to the patchiness of shrubs. The difference in population trends across the suite of shrubland birds may be driven by preferences for patchy shrublands. When creating or managing shrubland habitat, it is important to consider that the presence of shrubs is not enough and that the patchiness of shrubs may be important for a subset of shrubland birds.