Subject: Wildlife Monograph No 132 (Least Terns)
Date: Feb 8 20:22:31 1996
From: "Jon. Anderson and Marty Chaney" - festuca at olywa.net


Hi again folks,

A second wildlife monograph from the friendly people at The Wildlife =
Society, in which I thought some of us might be interested. Now, I'll =
be quiet for a while... :-)

Jon. Anderson
Olympia, Wash.
festuca at olywa.net


Habitat Selection and Productivity of Least Terns on the Lower Platte =
River, Nebraska. Eileen M Kirsch. January, 1996. Wildl. Mono. No. =
132. 48 pp.

Abstract: Least terns (Sterna antillarum) were studied on the lower =
Platte River, Nebraska, where this endangered population nests on =
natural sandbar habitat and on sandpit sites created by gravel dredging =
adjacent to the river. Theoretically terns should select habitats =
according to habitat suitability. However, the introduction of sandpits =
and conversion of tallgrass prairies along the river banks to =
agriculture, residential, and wooded areas may have affected terns' =
abilities to distinguish suitable habitat or the suitability of nesting =
habitats in general. I examined habitat selection and productivity of =
least terns to determine if terns selected habitat according to =
suitability (as indicated by productivity), what factors affected =
habitat selection and productivity, and if estimated productivity could =
support this population.
Available habitats of both types were characterized and quantified =
using aerial videography (1989-90), and habitat use was assessed from =
census data (1987-90). Productivity of adults and causes and correlates =
of egg and chick mortality were estimated (1987-90). Population trend =
was assessed with a deterministic model using my estimates of =
productivity and a range of survivial estimates for Laridae reported in =
the literature.
Terns tended to use river sites with large midstream sandbars and a =
wide channel, and large sandpit sites with large surface areas of water =
relative to unused sites on both habitats. Number of sites and area of =
sand available were estimated using discriminant function analysis of =
variables quantified from video scenes of both habitats. Terns =
apparently did not use all potentially available sandbar and sandpit =
sites because discriminant function function scores for used and unused =
sites overlapped broadly for both habitats. Terns did not prefer 1 =
habitat over the other. Although proportions of available sites used =
were greater on sandpits than on the river, proportions of available =
sand used did not differ between habitats. Proportion of terns using =
each habitat was similar to proportion of available sand on each =
habitat. The distribution of nest initiation dates and rates of =
colony-site turnover also were similar on both habitats.
Productivity did not differ between habitats but varied significantly =
among sites. Nest success, fledging success, and fledglings per pair =
averaged 0.54, 0.28, and 0.47, respectively. Key factor analysis =
revealed that chick survival had a greater influence on production of =
fledglings (on both sandbars and sandpits) than did failure to produce a =
maximum clutch size or egg mortality. Most egg mortality was caused by =
predation on sandpits and flooding on sandbars. Predation was suspected =
as the major cause of loss for chicks on both habitats. Path analysis =
revealed no strong or consistent correlations among mortality, numbers =
of nests and chicks, track trails of intruders into colonies, and =
habitat variables at colonies on either habitat.
Theoretically, terns should not prefer a habitat when habitats are =
equally suitable if terns have had time to respond to habitat changes. =
Although sandbars and sandpits appeared equally suitable and terns did =
not prefer either habitat, local productivity will not support this =
population unless annual postfledging survival is higher than current =
estimates for the species. Population trend estimated with fledglings =
per pair =3D 0.50 was negative for all but the highest (ca 0.90) rates =
of annual postfledging survival. Furthermore, deterministic models like =
the one used in this study overestimate trend.
Productivity insufficient to support the local population, in spite of =
habitat use that reflects habitat suitability, could be due to increased =
predation caused by habitat alteration adjacent to the river that may =
have changed the predator community. Alternatively, terns in this area =
could persist in spite of prevailing low productivity because they are =
relatively long-lived birds, if highly productive years occasionally =
occur or if this population is augmented by immigrants from elsewhere.