Relationship of Songbird nest concealment to nest fate and flushing behaviour of adults

Auk, The, Jan 2001 by Burhans, Dirk E, Thompson, Frank R III

Avoiding predation is an important consideration for any potential prey animal. Failure to escape from a predator results in loss of fitness, so there is strong selection for choices and behaviors that result in successful escape (Lima and Dill 1990). In their costbenefit approach to flight from predators, Ydenberg and Dill (1986) stressed that flight should be optimized rather than maximized, because there is a cost (usually cessation of feeding) incurred by fleeing from predation. Field studies have largely supported their predictions (see Bonenfant and Kramer 1996).

Whereas for foragers, flight from predators incurs an implicit cost in lost foraging time, birds confronted by a predator at the nest face an explicit choice between loss of current reproduction versus total reproductive loss. A bird flushing from the nest too early may escape, but reveal the nest location to the predator, resulting in loss of the current brood. However, flushing late from the nest could result in loss of both nest contents and the parent bird. Because nests are often hidden in vegetation (Martin 1992a), vegetative concealment could play a role in flight from the predator, much in the same the way in which cryptic body coloration may determine flight-- initiation distance for animals such as lizards (Heatwole 1968). A perplexing outcome of research done to date on nest concealment is that improved nest concealment is not always correlated with lower nest predation in studies of real songbird nests (Howlett and Stutchbury 1996, Burhans and Thompson 1998, Braden 1999). Studies have similarly shown that frequency of brood parasitism, which typically lowers host fitness, is often not influenced by nest concealment (Burhans and Thompson 1998, Clotfelter 1998).

If a view from the nest is important to adult birds, there should be a relationship between nest concealment and willingness of the adult bird to flush at the approach of a predator. We measured the relationship of flushing behavior to nest concealment for four songbird species having cryptically colored adult females. Our prediction was that nests with better concealment would result in females flushing at a closer distance from an intruder. We also examined the relationship between concealment and frequency of nest predation and avian brood parasitism. Two previous studies at our sites (Burhans 1996, Burhans and Thompson 1998) indicated that nest concealment was not related to nest predation, whereas one of two studies indicated that higher concealment was correlated with reduced frequency of brood parasitism (Burhans 1997).

Methods.-We located nests in old fields and adjoining forests from April through July 1998 at the Thomas S. Baskett Wildlife Research and Education Center near Ashland, Missouri. Those sites have been described previously in Burhans (1997) and have been the subject of songbird nesting and behavior studies since 1992 (Burhans 1997, Burhans and Thompson 1998, Dearborn 1998). We also searched for nests in a nearby agriculture field (30.8 ha). We used nests of Field Sparrows (Spizella pusilla), Indigo Buntings (Passerina cyanea), Northern Cardinals (Cardinalis cardinalis) and Yellow-breasted Chat (Icteria virens) because they are among the most abundant nesting species and females are cryptically colored when viewed on the nest. Although we have not measured nest defense responses specifically to humans of those species, they are not tame, and appear to respond to humans as they do to live or model predators. Response to humans near the nest generally includes rapid chipping or scolding and avoidance of the nest, which is similar to responses given to predators (Burhans 2000).

We searched sites daily for nests and marked them with plastic flagging at least 3 m distance from the nest. Nests were monitored every two to three days until fledging approached, after which we monitored them daily to document fledging. Although many studies use presence of an empty nest on expected fledge date as confirmation of nest success, video cameras at our nests (Thompson et al. 1999) indicate that snakes often depredate nests on or im- mediately prior to the expected fledge date. Fledging thus was documented either by video camera or by behavioral evidence during early morning visits on the expected day of fledging. We looked for confirmation of fledging by nestling begging calls, sight of nestlings, parents carrying food, or parents chipping rapidly nearby. Nests empty prior to that were considered depreciated; nests active up to the expected fledging date where we did not observe those activities were classified as unknown. We noted presence of eggs or nestlings of Brown-headed Cowbird (Molothrus ater) and categorized nests as parasitized or unparasitized. We are reasonably confident that nest-monitoring and other activities (see below) did not increase probability of predation. Other studies done at our sites indicate that nests visited daily for video filming had significantly lower daily mortality for some species (Thompson et al. 1999).