Nest Site Selection And Nest Thermal Properties Of Common Nighthawks On The Tallgrass Prairie Of Kansas

摘要

My research uses a behavioral ecology approach to understand Common Nighthawk (Chordeiles minor) declines and to inform nest-site restoration efforts in urban and suburban landscapes. The objectives of this study were to determine if nighthawks are choosing to nest non-randomly with respect to the distribution of types of vegetation and substrates within a tallgrass prairie landscape on Konza Prairie in Kansas, and to establish the thermal properties of nest sites for biologically informed restoration efforts. These two objectives were designed to identify nest-site characteristics that could be used to design suitable nesting substrates in urban and suburban landscapes. In the summer of 2007, I investigated nest site selection. Common Nighthawks typically lay clutches of one to two eggs either directly on the ground or in a shallow scrape. Most nests that have been documented were on patches of rock, gravel, or bare ground, but I wanted to determine whether this was due to differences in detectability between open gravel patches and areas with more vegetation cover. In early May, prior to the breeding season, we identified 10 target watersheds on Konza Prairie Biological Field Station. I then surveyed 120 randomly-placed vegetation plots in those areas. The circular plots were 4 m in diameter and I recorded percent cover of 5 cover class types within each plot (forbs, grass, shrub, dead vegetation, and rock/gravel/bare ground). During the breeding season from mid-May to late July, I systematically searched for nests within the targeted watersheds. I found 27 nests in these areas. As soon as possible (zero to three days) after finding each nest, I surveyed the area directly around the location of the eggs or chicks, recording percent cover in the same manner as the vegetation plots (4m diameter circle with percent cover of 5 cover classes). Post-breeding season, in early to mid-August, I re-surveyed the same 120 vegetation plots. At the same time, I also re-sampled the vegetation at all the nest sites. When I compared the summer re-survey of the nest plots to the summer re-survey of the random vegetation plots using a Mann-Whitney "U" test, I found that the birds were selecting areas with bare ground and rock at a significantly greater frequency than they occur in the surrounding landscape. I also observed that they were avoiding shrubs. Overall, 96% of 27 nests occurred on bare ground and rock. In 2008, I used a trained dog to find nests; 100% of the nests discovered by the dog were on bare ground and rock (n=9), a percentage similar to the percent in my 2007 sample. To investigate the thermal properties of nest sites, from July 9 - July 29 of 2008, I placed iButton thermal data loggers at nests. The iButtons recorded the temperature, time, and date every five minutes. At each nest, I placed one iButton directly on the nesting substrate away from the eggs or chicks (on rock/bare ground), and I placed an additional iButton in the next nearest cover class (grass or forbs). At each nest, the iButtons were approximately .5m apart. I recorded the temperature at 6 nests for1660 hours at each. I found that within a single month, the nesting substrate can experience temperatures that range from 14.5 degree Centigrade to 60 degree Centigrade. When I compared the vegetation versus the nest site I found that the average daily temperature and the high daily temperatures were significantly cooler in the vegetation during the day. Based on behavioral observations, these cooler areas serve as refuges for the young nighthawks during the day. Based on these observations of nest-site characteristics I recommend that natural areas be managed to increase rock and gravel patches and that urban restoration efforts test patches that maximize the amount of shade available during the day.