Abstracts Category : Other

Influence of Vegetative Characteristics on Predation and Predator Assemblage of Bird Nests

by Helen Trice Davis

Predation is the primary cause of nesting failure in birds. Thus, understanding the drivers behind nest success is of paramount concern to land managers. In the southern United States, woody vegetation encroachment has altered vegetation structure available for nesting bird species, potentially impacting underlying predator assemblages and predator-prey dynamics in this region. To inform future management of grassland birds, I examined how vegetative characteristics drive predation and predator assemblage of bird nests in semiarid grasslands affected by woody encroachment. In 2015 and 2016, I monitored bird nests within four vegetation types occurring on the San Antonio Viejo Ranch of the East Foundation in South Texas. I also placed infrared cameras at a subset of nests to identify predators. I then collected vegetation measurements at each nest site, including metrics such as percent woody cover, concealment, and distance to the closest anthropogenic or natural edge. I also sampled potential predator activity within the four vegetation types using track plates and data from a concurrent, multi-species monitoring program examining small mammals and herpetofauna. I monitored 258 nests of 18 bird species and deployed 109 infrared cameras at nest sites. I then divided bird species into four nesting groups to control for variation due to nesting strategy. All nesting strategy groups exhibited similar percentages of success (~25%) in 2015 and 2016 with exception of exposed nests that decreased by ~50% between years. Woody cover at the nest site was a significant driver of nest success of my medium cup nest group, indicating that small increases in woody cover (~10%) could have substantial impacts on birds utilizing this nesting strategy. Snakes were my iii primary predator at camera monitored nests (57%) and were also driven by increased woody cover (~15%) at the nest site. Meso- and large mammalian predators were most active in vegetation types dominated by herbaceous cover, small mammals were most active in vegetation types dominated by woody cover, and herpetofauna activity was highly variable. Predator activity did not reflect predator identity at camera monitored nests, indicating activity may not be a valuable metric for quantifying predation risk.Advisors/Committee Members: Morrison, Michael L (advisor), Smeins, Fred E (committee member), Campbell, Tyler A (committee member).