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Shorebirds and benthic fauna of tidal mudflats in Estero de Punta Banda, Baja California, Mexico.

Abstract.--We studied habitat use by shorebirds as related to tide and benthic invertebrates on three mudflats at Estero de Punta Banda, Baja California, Mexico between January and April, 2000. We recorded 15 shorebird species and 7974 individuals. The most abundant birds were marbled godwits (Limosa fedoa), small sandpipers (Calidris alpina, C. mauri, and C. minutilla), and willets (Catoptrophorus semipalmatus). The three sites were different in their shorebird assemblages, and shorebird density was significantly greater on the site closest to the mouth of the estuary. The benthic fauna in our samples included 14 polychaete and 1 cumacean families; 8 bivalve species, 7 gastropod species, 7 amphipod species, 4 decapod species, and 1 species of isopod. Benthic invertebrate abundance was significantly greater at the site closest to the mouth of the estuary in winter, and at the central site in spring. Abundance of shorebirds was clearly inverse to tide height. Shallow and deep probers responded differently to the tide cyc le at two sites. The most used feeding microhabitat, among four studied, was the waterline, although benthic invertebrate abundance was not different among habitats. The benthic fauna in our samples was potential food for the shorebirds present.

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Intertidal mudflats are the most important feeding habitats for shorebirds on their migratory routes (Schneider 1978; Evans et al. 1979; Gersternberg 1979; Burger et al. 1997). Far from being constant, such sites are constantly changing in response to the tide cycle. The energy gain that shorebirds derive from foraging on mudflats is influenced by temperature, wind, and rain, among other factors (Evans 1976; Burger 1984). Tides affect foraging intensity (Heppleston 1971; Connors et al. 1981), as well as the length of time and area in which shorebirds can forage (Puttick 1984). Indirectly, tides affect the distribution and activity of shorebird prey (Evans 1979; Puttick 1984). Overall, tides determine the daily patterns of activity of shorebirds (Heppleston 1971; Burger et al. 1977; Kelly and Cogswell 1979), and the number of shorebirds foraging generally reaches its highest values around low tide (Heppleston 1971; Burger et al. 1977).

Intertidal mudflats are heterogeneous in respect to the sediments that compose them, and in the distribution and abundance of benthic invertebrates (Trush et al. 1989; Trush 1991). Sediments can differ in their type (Burger et al. 1977), size of particles (Quammen 1982; Grant 1984; Hicklin and Smith 1984), and vary in their moistness and penetrability (Myers et al. 1987; Grant 1984; Mouritsen and Jensen 1992). Also, near the waterline, there are a number of microhabitats created by the level of water, or lack of it, and the presence of water pools in small depressions of the floor. Selection of a particular spot for feeding is the result of such characteristics, as well as of the morphology of different species (Ashmole 1970; Baker 1979). Furthermore, the distribution and abundance of birds reflects not only these physical caracteristics but also density and composition of benthic invertebrates, on which they depend during the non-breeding season (Goss-Custard et al. 1977; Bryant 1979; Hicklin and Smith 1984) .

Estero de Punta Banda is one of five large coastal wetlands along the Pacific coast of the Peninsula of Baja California and is important for migrating shorebirds (Palacios et al. 1991; Massey and Palacios 1994; Page et al. 1997). Although some inventories and monitoring of shorebirds have been carried out, and the local dynamics of western sandpipers (Calidris mauri) has been studied (Fernandez-Aceves 1996; Buenrostro et al. 1999), specific differences among sites have not been studied, nor has the relationship of this group with benthos been analyzed. The objective of this project was to obtain some insights into the use of different areas of mudflats within this coastal lagoon by shorebirds and to gather initial information on the relationship of shorebirds with benthic invertebrates.

Study Sites

Estero de Punta Banda (31[degrees]40'-31[degrees]48' Lat N, and 116[degrees]34' Long W), is located within Bahia de Todos Santos, and is about 15 km south of Ensenada (Fig. 1). This coastal lagoon is a 21 [km.sup.2] water body bordered on the seaside by a 7.5 km barrier beach. Tides are semidiurnal and have a mean amplitude of 1.04 m. In winter the Estero receives occasional fresh water through arroyos San Carlos and La Grulla. The most abundant intertidal habitat in the Estero is mudflats, and for this study we selected three such sites along the inner side of the barrier beach (Fig 1). These were:

1) Tony's Camp North (TCN). This site, on the northern barrier beach and bordered by sand dunes, was subject to strong tidal currents during Spring tides. The site measured about 6.1 ha at mid-tide level (as measured on an aerial image).

2) Tony's Camp South (TCS). This site was in the central part of the barrier beach, and was bordered by pickleweed (Salicornia spp.) marsh. Its tidal currents were less strong than at TCN. The site measured about 10.55 ha at mid-tide level.

3) Boss Pacific (BP). This site was located in the head of the estero and was bordered by a narrow strip of pickleweed marsh, and a raised dyke with a singlelane paved road. Tidal currents were much more moderate than at other sites. The site measured about 9.7 ha at mid-tide level.

Methods

Shorebirds were monitored on eight visits between January and April 2000 (Table 1). Each visit consisted of three days, one at each site, and were carried out only on days of Spring tides. At each site we established a fixed point of observation, from where all counts were made. A total count of birds was made every 15 minutes, while numbers per species were obtained every 30 min. Every hour we noted whether the birds were feeding, resting, or were engaged in some other activity. Observations were carried out for six hours, begining 4 hours before the lowest predicted tidal level at TCN and TCS. At BP, counts were carried out from 3 hours before lowest level to 3 hours after, because before this time the mudflats remained inundated and unavailable for the birds. We considered four categories of feeding substrate: mud, tideline, shallow water (up to the tibio tarsustarso metatarsus joint of most birds feeding), deep water (above the tibio tarsustarso metatarsus joint of most birds feeding).

Counts and identification were carried out with a 15X-45X spotting scope and 7X35 binoculars. Dunlin (Calidris alpina), western sandpiper (C. mauri) and least sandpiper (C. minutilla) were grouped as "small peeps," and both dowitchers (Limnodromus griseus and L. scolopaceus) were not differentiated. Whenever it was possible we noted the food being consumed.

Benthic invertebrates were sampled in the winter (20 & 21 January 2000) and spring (19 & 21 March 2000) in each site, for a total of 70 samples: 10 at each site in winter, and 16 at TCN and TCS, and 8 at BP in spring. We used a 10.5 cm diameter by 15 cm high sampler (86.59 [cm.sup.2] sampling surface). All organisms retained by a 1-mm mesh were saved in a mixture of saltwater and a 10% formaldehide solution. Once fixed, they were preserved in 70% alcohol. Specimens that were complete were measured.

After testing for normality and homogeneity of variances, data were analyzed through 2-way analysis of variance and Tukey tests, or through Friedman's test (Zar 1996). We calculated Shannon's diversity index, based on the sum of the maxima for each visit, and compared the values of the three sites through pairwise comparisons, with Hutcheson's test (Zar 1996).

Results

Our maximum daily tallies summed up to 7974 individuals throughout the study, of 15 species (Table 1). The most abundant birds were marbled godwits (Limosa fedoa, a = 2985), small peeps (2361), willets (Catoptrophorus semipalmatus, 922), dowitchers (891), and black-bellied plovers (Pluvialis squatarola, 252). Ruddy turnstones (Arenaria interpres), American avocets (Recurvirostra americana), long-billed curlews (Numenius americanus), whimbrels (N. phaeopus), semipalmated plovers (Charadrius semipalmatus), and greater yellowlegs (Tringa melanoleuca) were rare, while snowy plovers (Charadrius alexandrinus) and lesser yellowlegs (Tringa fiavipes) were sporadic. The abundance of the different species exhibited peaks on different dates.

All species were found at the three sites, except the ruddy turnstone which did not occur on BP. Overall TCN had more shorebirds than BP, while there were no differences between either of them and TCS, but individual species exhibited different uses, and the densities of some were not different among the different sites (Table 1). Whenever there were differences among the sites, TCN had the highest densities of birds. Diversity values (respectively 0.71, 0.67 and 0.65) were not significantly different among sites.

Number of shorebirds counted was clearly inverse to height of tide (Fig. 2). Maximum counts were obtained at lowest water level in TCN and BP, but 1.5 hrs before this level in TCS. Maximum counts were obtained for about 1.5 hrs. Shallow and deep probers were different in their pattern of increase and decrease with the tide at the three sites ([chi square] = 13.0, p; lt. 001; [chi square] = 6.23, p = 0.013; and [chi square] = 8.333, p = 0.004, respectively, Friedman's test).

Shorebirds exhibited differences among the four microhabitats in the three zones (Table 2). Shorebirds were mostly engaged in feeding, but we could identify only a few prey items from feeding birds. Marbled godwits included polychaetes in their diet, long billed curlews and whimbrels ate crabs, especially a fiddler crab, Uca crenulata, willets ate bivalves (Macoma spp.) and crabs, and black-bellied plover ate bivalves and gasteropods, especially Bulla gouldiana.

Polychaetes were by far the most abundant invertebrates in our samples (Table 3, Appendix 1). Our innermost site (BP) was devoid of crustaceans, and exhibited the lowest density of invertebrates, although in March it was not statistically different from TCN. The four feeding substrates were not statistically diferent in their abundance of invertebrates (F = 2.743, df = 3, p = 0.058, 2-way ANOVA).

In addition to the species in our benthic samples, there were fiddler crabs (U. crenulata), abundant gasteropods (especially B. gouldiana) and, in eelgrass (Zostera marina) banks at the lowest intertidal reaches, many bivalves (Argopecten spp.). Also, whereas our samples contained few amphipods, isopods, and carideans, these were abundant in eelgrass banks.

Discussion

With the exception of dowitchers, all species that exhibited differences in their abundance at the different sites preferred TCN over BP, while TCS was intermediate. Both small (Calidris, Limnodromus, Arenaria, Charadrius and Pluvialis spp.) and large shorebirds (Limosa, Catoptrophorus, Recurvirostra, Numenius and Tringa spp.) were about equally represented (47.5, and 52.5%, respectively). That the most abundant species was the marbled godwit, followed by the small peeps, is comparable to previous studies (Palacios et al. 1991). Despite differences in the force of water currents, and in substrate texture, there were no significant differences between TCN and TCS. The overall higher abundance of birds in TCN than in BP (Table 1) was due to different responses by individual species to differences in the habitat.

Ruddy turnstones commonly fed on bivalves asociated with eelgrass, which were absent from BP. Black-bellied plovers were more abundant at TCN and only sporadic at BP, in agreement with its preference for harder substrates (Pienkowski 1983a). Willets and the small peeps were also more abundant at TCN. This site had more non-benthic insects, which can form a substantial part of western and least sandpipers diets (Reish and Barnard 1990). On the other hand, dowitchers were more common on BP, probably because its softer substrate allowed for more efficient feeding of these species.

As shown by others (Waumann 1998; Sinicrope-Talley et al. 2000) polychaetes dominated the bethic fauna in the three zones (Table 3). TCN, in winter, and TCS, in spring had significantly more benthic fauna than the two remaining sites. Although these two areas exhibit differences in the force of tidal currents, in both areas these currents are subtantially stonger than at BP, causing better oxygenation and higher polichaete abundances (see Farreras and Villalba 1980; Wolff 1983). The reasons for seasonal differences among TCN and TCS, and between them and BP, are not clear to us. In addition to the species in our benthic samples (Table 3), eelgrass banks at the lowest intertidal reaches accounted for even further differences between TCN, TCS, and BP, since they were present only at the first two sites, where they allowed for abundant bivalves (Argopecten spp.), amphipods, isopods, and carideans.

By affecting the amount of exposed mudflats, tidal level was the main factor affecting the local movements of shorebirds, as has been reported elsewhere (Recher 1966). As water levels lowered, shorebirds increased, reaching their maximum numbers around the lowest water level (Fig. 2). At TCS the maximum numbers of shorebirds were reached an hour and an half before lowest tide, when the tideline reached the mid-lagoon tidal channel. Afterwards, receding tides caused the exposure of no additional habitat. However, not all species reached their maxima at the same time, as individual response varied (Burger et al. 1977). Notably, semipalmated plovers always reached their peak numbers during lowest tidal level, when exposure of eelgrass was maximized. The closer tidal curve at BP, was caused both by its lower steepness and by the fact that we always sampled it under slightly higher tidal levels than the other two sites.

Although there were some specific microhabitat preferences by the shorebirds (Table 2), the tideline was frequently used. This is not surprising, as benthonic invertebrates are stimulated by changes in hydrostatic pressure when the tide begins to rise (Enright 1965).

Shallow water offers an alternative to the tideline. The small depressions, by maintaining pockets of water, offer a more penetrable substrate than adjacent mudflats that do not hold standing water (Grant 1984), and stimulate higher activity of benthic prey (Evans 1979; Pienkowski 1983b), reducing foraging costs of birds that feed by probing (Mouritsen and Jensen 1992). However, in our study this microhabitat was little used.

In our study we documented that some individuals of the largest species would rest on very windy days, while none of the smaller shorbirds ever did so. Also, two hours after lowest tide, marbled godwits, semipalmated sandpipers, curlews, and whimbrels rested on the shore, whereas the smaller peeps and dowitcher continued to feed. This may reflect the greater energy needs of such smaller species.
Appendix. Abundance (individuals/0.1 [m.sup.2]) of benthonic
invertebrates that are potential prey for shorebirds, in samples
obtained at three sites on mudflats in Estero de Punta Banda, Baja
California, in Winter and Spring 2000.

 TCN TCS

 20 Jan. 20 Mar. 20 Jan. 19 Mar.

Polychaeta
 Magelonidae * 89 85 20 118
 Capitellidae * 101 95 27 36
 Cirratulidae * 17 10 22 93
 Obiniidae * 31 41 74 60
 Nereidae * 55 21 16 12
 Glyceridae * 10 7 12 3
 Goniadidae 8 7 7
 Opheliidae 7 20 1
 Lumbrineridae 1 2 2
 Sabellidae 7 1
 Nephytydae * 5
 Spionidae * 3
 Onuphidae * 1
 Polynoidae * 1
 Gastropoda
 Bulla gouldiana + 15 3 6 3
 Haminoea vesicula * 8 2 3 1
 Acteocina spp. *
 Nassarius tegula 1
 Ceratostoma spp. 1
 Cylichnella inculta 1 1
 Cerithidea california 1
 Crepidula coei
 Melampus spp. * 1
 Bivalvia
 Argopecten aequisulcatus 1
 Chione californiensis 1
 Macoma nasuta+ * 1 1
 Pitar spp. 1 2
 Lucina nutalli 2 1
 Tellina spp. 3 1 3
 Cooperella subdiaphana 4 2 4
Amphipoda
 Ampithoe polex * 2 4 2
 Parapoxus spinosus * 7 1 1
 Podocerus spp. 2 2
 Hyale frequens * 1 4 1
 Tethygeneia quinsana 1 4
 Monoculodes spp. 2 1
 Rudilemboides spp. 1 1
Cumacea *
 Dyastilidae 6 7 7
Isopoda
 Paracerceis spp. * 2 1 1 2
Decapoda
 Rhithropanopeus spp. 2 1
 Pachygrapsus crassipes * 1 1
 Hemigrapsus oregonensis * 1 1
 Hyppolyte californiensis 6 1 10 1

 BP

 22 Jan. 19 Mar.

Polychaeta
 Magelonidae * 9 28
 Capitellidae * 16
 Cirratulidae * 16
 Obiniidae * 3
 Nereidae * 49 98
 Glyceridae *
 Goniadidae
 Opheliidae
 Lumbrineridae 6 9
 Sabellidae
 Nephytydae *
 Spionidae *
 Onuphidae *
 Polynoidae *
 Gastropoda
 Bulla gouldiana +
 Haminoea vesicula *
 Acteocina spp. * 5 7
 Nassarius tegula 2
 Ceratostoma spp.
 Cylichnella inculta
 Cerithidea california
 Crepidula coei
 Melampus spp. *
 Bivalvia
 Argopecten aequisulcatus
 Chione californiensis
 Macoma nasuta+ *
 Pitar spp.
 Lucina nutalli
 Tellina spp. 2 3
 Cooperella subdiaphana 3
Amphipoda
 Ampithoe polex *
 Parapoxus spinosus *
 Podocerus spp.
 Hyale frequens *
 Tethygeneia quinsana
 Monoculodes spp.
 Rudilemboides spp.
Cumacea *
 Dyastilidae
Isopoda
 Paracerceis spp. *
Decapoda
 Rhithropanopeus spp.
 Pachygrapsus crassipes *
 Hemigrapsus oregonensis *
 Hyppolyte californiensis

+ Represent species that were observed to be consumed by shorebirds in
this study

* those reported as shorebird prey in other studies.


[FIGURE 2 OMITTED]
Table 1. Maximum shorebird density (individuals/ha) at three mudflat
sites within Estero de Punta Banda, Baja California, Mexico, between
January and May 2000. Values in column "p" are the probability of type I
error in Friedman's Analysis of Variance. Within a species, sites with
the same letter are not different under a non-parametric Tukey test.
Sites within a species are un-lettered when there were no statistical
differences at P [less than or equal to] .05.

 17-18 2-4 16-18 2-4 16-18 30 Mar-
Species Site Jan Feb Feb Mar Mar 2 Apr

Marbled godwit TCN 16.4 18.3 19.3 19.6 21.9 25.0
 TCS 6.9 33.5 8.4 10.2 11.1 12.1
 BP 10.3 8.6 16.3 16.3 17.1 21.1
Small peeps TCN 10.1 9.3 11.9 18.0 19.5 41.6
 TCS 8.8 16.7 7.6 13.8 7.6 5.7
 BP 5.6 11.4 7.1 7.1 4.8 3.1
Willet TCN 5.7 11.8 9.8 6.5 7.2 7.4
 TCS 6.4 2.3 5.6 6.4 7.6 4.5
 BP 1.5 2.4 2.8 2.8 1.8 2.2
Dowitchers TCN 0.5 1.8 2.1 0.5 2.1 2.1
 TCS 7.6 4.7 6.6 1.9 2.7 3.8
 BP 0.8 4.7 10.1 10.1 4.9 7.7
Black-bellied plover TCN 1.3 1.5 3.3 2.6 2.9 2.5
 TCS 0.6 1.3 0.9 1.6 2.0 0.8
 HP 0.5 0.8 0.6 0.6 0.6 0.0
Ruddy turnstone TCN 0.0 0.3 2.9 2.9 2.5 4.9
 TCS 0.4 1.6 0.1 2.2 0.9 0.8
 BP 0.0 0.0 0.0 0.0 0.0 0.0
American avocet TCN 0.0 0.2 0.0 0.2 0.0 0.5
 TCS 0.0 0.0 0.0 0.0 0.8 0.0
 BP 1.4 3.8 1.4 1.4 0.8 0.9
Long-billed curlew TCN 1.0 0.7 0.8 0.8 0.8 0.3
 TCS 0.4 0.3 0.4 0.3 0.5 0.5
 BP 0.5 0.5 0.6 0.6 0.5 0.2
Semipalmated plover TCN 0.0 0.0 0.3 0.8 0.0 0.0
 TCS 0.1 0.6 0.3 0.5 0.1 0.1
 BP 0.0 0.0 0.2 0.2 0.0 0.0
Greater yellowlegs TCN 0.5 0.3 0.2 0.2 0.3 0.2
 TCS 0.1 0.5 0.2 0.1 0.2 0.2
 BP 0.2 0.1 0.3 0.3 0.2 0.1
Whimbrel TCN 0.3 0.2 0.3 0.2 0.2 0.2
 TCS 0.2 0.1 0.1 0.1 0.4 0.1
 BP 0.2 0.1 0.1 0.0 0.1 0.1
Total TCN 35.8 44.4 51.1 52.4 57.4 84.6
 TCS 31.5 61.5 30.2 37.1 33.8 28.5
 BP 21.0 32.3 39.3 39.2 30.7 35.2

 12-14 29 Apr- [not equal
Species Apr 1 May Total p to]

Marbled godwit 21.3 11.1 153.0 0.007 a
 9.2 8.2 9.6 b
 11.4 1.9 103.0 ab
Small peeps 69.6 13.1 193.1 0.004 a
 8.5 6.5 75.3 ab
 0.7 0.0 5.6 b
Willet 5.9 4.1 58.4 0.004 a
 4.9 1.5 39.2 ab
 1.3 0.8 15.6 b
Dowitchers 3.1 4.9 17.2 0.030 b
 4.5 6.1 38.0 a
 1.2 0.0 39.7 ab
Black-bellied plover 3.3 2.6 20.0 <0.001 a
 0.9 1.2 9.4 ab
 0.0 0.0 3.2 b
Ruddy turnstone 4.7 2.6 20.9 0.004 a
 0.0 0.6 6.5 ab
 0.0 0.0 0.0 b
American avocet 1.3 0.0 2.1 0.016 ab
 0.0 0.0 0.8 b
 0.0 0.0 9.9 a
Long-billed curlew 0.3 0.0 4.7 0.072
 0.1 0.2 2.6
 0.1 0.1 3.2
Semipalmated plover 4.4 2.1 7.7 0.018 a
 0.1 1.4 3.1 ab
 0.0 0.0 0.4 b
Greater yellowlegs 0.0 0.0 1.6 0.866
 0.1 0.0 1.3
 0.3 0.0 1.5
Whimbrel 0.3 0.0 1.6 0.093
 0.2 0.2 1.3
 0.1 0.2 0.9
Total 114.2 40.6 480.5 0.012 a
 28.6 25.9 277.2 ab
 15.2 3.0 216.0 b

Table 2. Percent of individuals of each of five shorebirds feeding in
four feeding substrates on mudflats in Estero de Punta Banda, Baja
California, Mexico, January-April 2000. M = mud, WL = water line, SP =
shallow ponds, and DW = deep water. Within species and sites, values
with the same letter were not statistically different, according to a
tukey test ([alpha] [greater tha or equal to] 0.05)

 Feeding substrate

Species Site M WL SP DW

Marbled godwit TCN 15.8 (b) 45.7 (a) 20.5 (b) 18.0 (b)
 TCS 13.4 (b) 52.8 (a) 18.4 (b) 15.4 (b)
 BP 10.8 (c) 61.8 (a) 17.5 (b) 9.9 (c)
Small peeps TCN 32.5 (b) 46.8 (a) 17.7 (c) 0.9 (d)
 TCS 42.4 (a) 25.7 (b) 31.4 (b) 0.5 (c)
 BP 10.3 (b) 80.9 (a) 8.8 (b) 0.1 (b)
Semipalmated plover TCN 34.0 (a) 30.2 (a) 24.3 (a) 11.6 (b)
 TCS 32.3 (a) 26.6 (a) 26.4 (a) 8.7 (b)
 BP 36.3 (a,b) 41.7 (a) 16.7 (b,c) 5.3 (c)
Dowitchers TCN 0.2 (b) 89.7 (a) 9.4 (b) 0.7 (b)
 TCS 5.4 (c) 78.5 (a) 14.1 (b) 1.9 (c)
 BP 6.3 (b) 65.4 (a) 7.1 (b) 1.3 (b)
Black-bellied plover TCN 94.7 (a) 1.6 (b) 3.7 (b) 0.0 (b)
 TCS 94.1 (a) 2.3 (b) 3.6 (b) 0.0 (b)
 BP 100 (a) 0 (b) 0 (b) 0 (b)

Table 3. Total density (individuals/0.1 [m.sup.2]) and relative density
(in parentheses) of benthic invertebrates in mudflat sites in Estero de
Punta Banda, Baja California, Mexico, in January and March Totals with
the same letter, 2000. Totals with the same letter, within a sampling
date are not statistically different, under a parametric Tukey test
([alpha] [greater than or equal to] .05).

Group TCN TCS BP

January 2000:
 Polychaeta 331 (84.2%) 194 (78.2%) 84 (88.4%)
 Crustacea 27 (6.9%) 37 (14.9%) 0 (0.0%)
 Gastropoda 27 (6.9%) 12 (4.8%) 5 (5.3%)
 Bivalvia 8 (2.0%) 5 (2.0%) 6 (6.3%)
 Total 393 (a) 248 (a) 95 (b)
March 2000:
 Polychaeta 270 (94.4%) 333 (91.7%) 151 (92.1%)
 Crustacea 8 (2.8%) 16 (4.4%) 0 (0.0%)
 Gastropoda 5 (1.7%) 5 (1.4%) 7 (4.3%)
 Bivalvia 3 (1.0%) 9 (2.5%) 6 (3.7%)

 Total 286 (a,b) 383 (a) 164 (b)


Acknowledgments

Marina Mondragon, Man Carmen Necoechea, Dora Waumann, Olga Flores, and Jorge Dominguez assisted during field and laboratory work. Elena Solana, Horacio de la Cueva, and Vicente Ferreira adviced us on data analysis. Eduardo Palacios, Anamaria Escofet, Nils Warnock, and one unknown colleague greatly assisted with editorial comments. Jose Maria Domiguez and Francisco Javier Ponce prepared figure 1. The Consejo Nacional de Ciencia y Tecnologia supported the first author with a graduate scholarship. To all of them, our appreciation.

Accepted for publication 31 January 2002.

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