Musings on Site Fidelity

Buckle up, this post may be what my stats professor at Indiana would call “chewy.”

The wildlife surveys are still going strong—pronghorn fawns started showing up about a month ago, and I’ve collected quite a few data points on nesting and chick-rearing for some of the BLM and Wyoming Game and Fish’s birds of concern.

American avocet (Recurvirostra americana), trying to distract us from its chick farther down the road.

Although I haven’t actually run any numbers yet, the distribution patterns I’m noticing make me much more confident about the conservation prospects of some species than I am about others. Part of that seems to depend on basic habitat requirements—I am increasingly suspecting, for example, that the sage thrashers I mentioned a couple of months ago are more adaptable to habitat variations than has previously been recognized—but how different animal species respond to habitat changes in one location can also be very important.

If a disturbance is extreme enough to affect a species’ viability in a particular area (e.g. fire, invasion by a radically different plant community, conversion to farmland or subdivisions), one of two things typically happens. Species with little instinctive attachment to that area, or low site fidelity, will generally emigrate to more suitable habitat (if they can find it); if the site returns to an approximation of its previous appearance, they often recolonize it fairly quickly. (For species with restricted ranges or highly specialized habitat needs, this can raise a whole string of other issues, but most of the animals I’ve been observing aren’t subject to that threat.) Species with high site fidelity—an instinctive tendency to use the same territory year after year, regardless of its suitability—appear to be more vulnerable to habitat shifts. Although they will remain in their old home for as long as they can, and may have some breeding success, they are rarely able to raise young at a replacement level in disturbed habitat, and their reluctance to move out of it often means that the population dies out before the habitat recovers.

In the existing literature on sagebrush ecosystems, there seems to be a pretty clear pattern in different species’ responses to disturbance—particularly loss of sagebrush due to fire or “range improvement” efforts. Within a year or two after the disturbance, sage thrashers and Brewer’s sparrows disappear, while sage sparrows tend to hang on for several years before petering out. If the sagebrush is able to reestablish, the first two species often reappear fairly quickly, while sage sparrows may take decades to recolonize the site. Accordingly, site fidelity has been recognized as a potentially important factor in sage sparrow population trends, but not studied in depth—and I’m not sure that the issue has received enough attention outside the scientific community. Although I don’t know the site histories of my survey points well enough to weigh in with any confidence yet, I’ve been finding them far less frequently than the other two species, in more scattered populations, which is at least circumstantial evidence. Some researchers have also suggested that thrashers have high fidelity, but what little information I’ve found on that species, combined with my own observations this season, suggest the opposite. The upshot is that I’m getting genuinely concerned about the long-term prospects of sage sparrows in the Bighorn Basin—especially if fire frequency or severity increase, due to climate change or continued cheatgrass invasion—but expect the other two species to persist at fairly healthy levels, as long as at least some sagebrush or other shrub habitat remains available.

Still with me? Have another photo.

If, as is often the case, grasses and forbs take over a site in the short term after disturbance, grassland species (including prairie dogs and mountain plovers) often follow, but I suspect that those species vary in site fidelity as well. For example, the Bighorn Basin saw an unusually wet winter and spring this year, and grasses (native and invasive) are apparently growing taller and more densely in a lot of areas than they have in the past. Vesper sparrows, lark buntings, and meadowlarks are practically ubiquitous as a result; I’ve been observing curlews and lark sparrows regularly, but not frequently, while grasshopper sparrows and McCown’s longspurs only seem to be present in small corners of the basin. The more widespread species may be the ones who were best able to capitalize on this year’s increase in grass cover, while the rarely-seen ones may be less inclined to explore new habitat due to higher site fidelity. (On the other hand, the grasses that have sprung up this year may simply not meet the needs of some of these species—particularly grasshopper sparrows, which are stereotyped as a tallgrass bird.)

Common nighthawk (Chordeiles minor), clearly not happy at being woken up.

Since high site fidelity can make species more vulnerable to habitat disturbance, or less able to capitalize on expansion of suitable habitat, how could it have ever evolved in the first place? My guess is that it allows birds to develop greater familiarity with their home ranges, which provides an advantage over competitors (both conspecifics and members of other species) in finding food, shelter, nesting sites, etc. It would follow, of course, that site fidelity is most beneficial under stable habitat conditions—and that the impacts of human-caused habitat changes are likely to be especially severe for these species.

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