Environmental and Habitat Correlates of Pasture Use by Nonbreeding Shorebirds
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The Condor 9X337-344 0 The Cooper Ornithological Society 1997 ENVIRONMENTAL AND HABITAT CORRELATES OF PASTURE USE BY NONBREEDING SHOREBIRDS MARK A. COLWELL AND SARAH L. DODD~ Department of Wildlife, Humboldt State University, Arcata, CA 95521, e-mail: [email protected] Abstract. Throughoutcoastal regions of the world, pasturelandsoften augmentimportant and declining intertidal foraging habitats for nonbreedingshorebirds (Suborder Charadrii). Little is known, however, about factors influencingbird use of pastures.Hence, we examined the spatialdistribution of shorebirdsin coastalpastures near Humboldt Bay, California from October 1991-May 1992 and correlatedspatial distributionpatterns with environmentaland habitat variables. Shorebirdsused pasturesin a nonrandom (clumped) fashion. Pastureuse variedseasonally for Killdeer (Charadrius vociferus), Marbled Godwit (Limosa fedoa), and Common Snipe (Gallinago gallinago); Dunlin (Calidris alpina) and Black-bellied Plover (Pluvialis squatarola) use increased when it rained, and Dunlin, Black-bellied Plover, and Killdeer use increasednearer the time of a new moon. At the level of the individual pasture, the likelihood of encounteringmost species(5 of 6) increasedas vegetationheight decreased. The likelihood of pastureuse by Dunlin and Killdeer increased when shorebirdsused pas- tures the previous week. Extensive, nonrandom use of coastal pastures by nonbreeding shorebirdsindicates that conservationplanning for shorebirdsshould consider habitat char- acteristics. In the vicinity of Humboldt Bay and other coastal bays, use of pastures by shorebirdscan be increasedby practicesthat provide short vegetation. Although shorebird use did not correlate with use of pasturesby cattle, grazing by livestock is probably the means by which to achieve habitat characteristicsattractive to shorebirdswhile maintaining compatible human uses on private lands. Regular use of some pasturesby shorebirdsindi- cates that site faithfulnessmay be important and that “traditional” sitesneed to be identified and protected. Kev words: shorebirds. Charadrii. oastures, spatial distribution, logistic regression, bird-habitat relationships, Vegetation height. INTRODUCTION lance is facilitated by short vegetation. It also The value of coastal pastures to nonbreeding has been suggestedthat some waterbirds asso- waterbirds is well known (Townshend 1981, ciate with livestock because of foraging benefits Goss-Custard and Durell 1983, Colwell and (Thompson et al. 1982, Colwell and Dodd Dodd 1995), especially in areas where intertidal 1995). For some waterbirds, pasture use has habitats have been mostly “reclaimed” by hu- been shown to increase when standing water is mans. In coastal areas, loss of important inter- present (Colwell and Dodd 1995). However, less tidal foraging habitats has been partially offset is known about environmental conditions and by agricultural land use practices, which provide habitat characteristics associated with pasture habitats for waterbirds, especially shorebirds use by shorebirds. (suborder Charadrii). Although the importance Elsewhere, we showed that a waterbird com- of pasturelands to nonbreeding shorebirds is munity numerically dominated by shorebirds widely recognized, little is known about envi- had greater species richness and densities of ronmental and habitat features associated with most species in grazed pastures with short veg- shorebird distributions in these habitats. Several etation (Colwell and Dodd 1995). However, wa- studies have demonstrated that nonbreeding terbird species did not respond uniformly to shorebirds frequent short-vegetation pastures short vegetation. Here, we expand our analysis (Fuller and Youngman 1979, Milson et al. 1985, of variables influencing shorebird use of pas- Colwell and Dodd 1995), presumably because tures to encompass environmental conditions prey are easier to capture and/or predator vigi- prevailing at the time of use, as well as habitat characteristicsof pastures themselves. Our ob- jectives are to: (1) examine the spatial distribu- I Received29 July 1996.Accepted 17 January1997. 2 Current address:South Carolina Departmentof tion of shorebirds within coastal pastures of Natural Resources,Samworth WMA, 420 Dirleton northern California adjacent to important non- Rd., Georgetown,SC 29440. breeding habitats (Colwell 1994), and (2) eval- [3371 338 MARK A. COLWELL AND SARAH L. DODD - I \ ACIFIC‘ : ICEAN ( 0 N FIGURE 1. Locationof 20 pastureplots within the largerstudy area in northernCalifornia. uate the relative contributions of variables ex- these study plots because cooperative landown- plaining nomandom shorebird distributions. We ers permitted access for research; hence, the include in our analysis variables representing study plots are not a random sample and may site faithfulness of birds to pastures in order to not be representative of local pasture habitats. understand whether or not birds use pastures Colwell and Dodd (1995) provide a complete consistently, at least over short periods of time. description of the area. Finally, we discussconservation implications of our findings, focusing on bird-habitat relation- BIRD SURVEYS ships that can be manipulated. Each week from 1 October 1991 to 1 May 1992, we collected data on shorebird distribution with- METHODS in pastures in two ways. First, during weekly STUDY AREA surveys (see below), we recorded opportunisti- We studied shorebirdsin coastal pastureslocated cally the location (t 100 m) and number of between the Mad River Slough of Humboldt shorebirds observed on the ground. Second, on Bay and the Mad River estuary in northern Cal- each of the 20 pasture plots we surveyed shore- ifornia (40”5’ N, 124”O’ W, Fig. 1). Much of this birds weekly by scan sampling (Altmann 1974) area was coastal salt marsh, but humans con- during daylight hours, beginning surveys within verted intertidal habitats to pastures nearly a 2 hours of mean high tide for Humboldt Bay. century ago (Haynes 1986); these pastures are Compared with intertidal habitats (Colwell currently grazed year-round by cattle (Bos tuu- 1993, 1994), pasturesappear to be supplemental TUS).Within the larger study area defined by Fig- feeding sites used by shorebirds during high ure 1, we examined shorebird use of 20 pastures tides (Hoff 1979), especially during winter. We varying in size from 2.3-13.6 ha. We selected used 8.5 X 44 mm binoculars and a 22X spot- SHOREBIRD USE OF PASTURES 339 ting scope to survey each pasture from the same location 1 m outside a perimeter fence, although on some occasions we surveyed at greater dis- tances if we thought birds would be frightened by our presence. Surveys lasted the time re- quired to scan pastures and record data on spe- cies, their abundances, and behaviors (not pre- sented in this article). After each survey, we walked a zig-zag path through each pasture to flush unseen birds (0.7% of all bird detections), which may have been hidden by vegetation. HABITAT AND ENVIRONMENTAL MEASUREMENTS We analyzed variables potentially influencing shorebird use of pastures at two levels-envi- ronmental and habitat (pasture) characteristics (see below). At both levels of analysis, variables varied temporally and spatially in a similar man- FIGURE 2. Perspective plot showing spatial varia- ner. tion in shorebird distribution across the larger study area. Grid cells are approximately 150 X 150 m. Data Environmental variables. Prior to each sur- representsummed observations for the 1 October 1991 vey, we recorded the following environmental to 1 May 1992 study period. variables: (1) maximum height of high tide co- incident with the survey based on National Oce- anic and Atmospheric Administration (NOAA) birds could penetrate the soil with their bills us- data (Tidelogs 1991, 1992), (2) time of day (sun- ing a device similar to that described by Myers rise-10:00; lO:Ol-14:OO; 14:01-sunset), (3) et al. (1980; see Colwell and Dodd 1995 for de- week of the study (1 October = week 1,27 April tails). We summarized habitat data by calculat- = week 29), (4) moon phase (new, quarter, half, ing means of 10 random samplesfrom each pas- three quarters, full; ignoring waxing and waning ture visit, yielding a single data point for each conditions) based on NOAA data (Tidelogs pasture per week. 1991, 1992), (5) wind speed estimated in km hri, (6) wind direction measured in compass DATA SUMMARY AND ANALYSIS bearing, (7) temperature (“C), and (8) whether or We examined the spatial distribution of shore- not it rained. Finally, we recorded the amount of birds in two ways. First, we collated shorebird precipitation in the 24-hr period preceding the locations (x, y coordinates) for the entire study survey date based on data collected approxi- period and graphically portrayed the distribution mately 2 km south of the study area (B. Alden, (Fig. 2) using geostatisticalmethods (Rossi et al. pers. comm.). 1992). Second, we summarized the average (2 Pasture characteristics. At the start of a sur- SD) density, as well as the prevalence (propor- vey, we tallied the number of cattle using a pas- tion of observations; n = 29 weeks) of shore- ture. To assessthe importance of shorebird site birds in each pasture (Fig. 3). faithfulness to pasture use, we also noted wheth- We used logistic regression(Glantz and Slink- er or not shorebirdshad used a pasture the pre- er 1990, Trexler and Travis 1993) to examine vious week. After each bird survey, we quanti- correlatesof shorebirduse (presenceor absence) fied vegetation height, water depth,