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Spatial ecology of breeding Least Bitterns in northwest Missouri.

Article Excerpt
The Least Bittern (Ixobrychus exilis) is currently classified as vulnerable in Missouri (Missouri Natural Heritage Program 2006) and a Region 3 priority species by the U.S. Fish and Wildlife Service (USDI 2002). Much of this species' wetland habitat has been lost over the past 80 years as a result of dmining and filling wetlands across the state, yet Least Bitterns still occur in relatively large numbers in the floodplain marshes of the Missouri and Mississippi rivers (Robbins and Easterla 1992). Least Bittern research has been limited due to the highly secretive nature of the species; however, previous research on Squaw Creek National Wildlife Refuge provided a basic understanding of Least Bittern nesting ecology in Missouri (Nelson 2003). Research focusing on spatial ecology has been conducted in western New York (Bogner and Baldassarre 2002), but little is known about the spatial ecology of Least Bitterns in Missouri. Past research suggests Least Bitterns maintain a somewhat small home range (Bogner and Baldassarre 2002), pair bonding is seasonal (Weller 1961), and renesting and double brooding may occur (Kent 1951, Bogner and Baldassarre 2002).

Our objectives were to: (1) measure home range size, examine habitat use, and describe nest success of breeding pairs; (2) identify pair bond length; and (3) examine renesting and double brooding capabilities. We predicted that Least Bitterns in both Missouri and New York had similar spatial ecology patterns and breeding behaviors.

METHODS

Study Site.--Our study was conducted on the 3,012-ha Squaw Creek National Wildlife Refuge (SCNWR) within the Missouri River floodplain in Holt County, Missouri (Fig. 1). The refuge consists of six main habitat types including old field (24 ha); developed land consisting of roads, parking areas, levees, and dminage ditches (102 ha); agricultural cropland (234 ha); forest (558 ha); prairie (643 ha); and wetland (1,451 ha). The Refuge is within the North American summer range of the Least Bittern (Peterson 2002), and prior nest searches conducted by SCNWR biologists and past researchers demonstrated the area supports a healthy breeding population (Jacobs and Wilson 1997, Nelson 2003).

All radio-marked Least Bitterns were initially captured during 2005 in Pelican Pool (239 ha) in the southwest portion of the refuge (Fig. 1). We initially captured and radio-marked Least Bitterns in 2006 on two pools: Pelican Pool and Eagle Pool (245 ha) (Fig. 1).

Nest Searches and Capture.--We conducted nest searches primarily from an airboat, although we also made periodic searches on foot and nests were occasionally discovered during vegetation surveys. We assigned each nest a unique number and flagged each site with a 3-m willow pole placed 5 m to the north. We recorded coordinates for each nest using a global positioning system (GPS). Nest searches occurred in all pools containing suitable nesting habitat.

[FIGURE 1 OMITTED]

We conducted recurrent checks of marked and active nests until at least one adult was observed on the nest. We captured the bird on the nest or as it flushed from the nest into the vegetation by hand. This method caused some disturbance due to the presence of the airboat, but was highly effective and ensured the quick capture of nesting pairs without spending undue time in the vicinity of the nest. Capture attempts began on 24 May 2005 and on 1 June 2006. Efforts to capture nesting pairs during 2006 were abandoned as birds were extremely illusive, as compared to the 2005 capture season. Most individuals in 2006 were captured while foraging and were not associated with a marked nest. All Least Bittern captures and handling followed guidelines provided by Gaunt and Oring (1997).

Radio Transmitter Attachment.--All birds captured during 2005 were assigned as males or females (Weller 1961), weighed to the nearest 0.10 g, and fitted with a 2.5-g radio transmitter (Model A2460, Advanced Telemetry Systems Inc., Isanti, MN, USA). Transmitters were attached to birds with leg bands. We used epoxy to attach two, size 4A leg bands (National Band and Tag Company, Newport, KY, USA) to...

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