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Nesting characteristics of the Texas seaside sparrow (Ammodramus maritimus sennetti) on Mustang Island, Texas.

Seaside sparrows (Ammodramus maritimus) are salt marsh specialists that are never found >25 km from the sea (Post and Antonio, 1981). The species is currently divided into seven extant subspecies based upon minor differences in plumage and size and is distributed along most of the coastlines of the southern and eastern United States from the delta of the Rio Grande to Maine (Post and Greenlaw, 2009, http://bna.birds.cornell.edu/bna/ species/127). Characteristics of nesting habitat have been described for birds nesting in New York and Florida, but very little is known about the placement and architecture of nests of the Texas seaside sparrow, A. m. sennetti. In the Rio Grande Delta, seaside sparrows have been found nesting in mixed marshes consisting primarily of Avicennia germinans (black mangrove; Phillips and Einem, 2003). In this study, the nests of nine A. m. sennetti nesting on the bayside of Mustang Island, Texas, were studied to determine details of their construction and placement.

Using singing male seaside sparrows as cues, we located and marked nests for later collection in a mixed mangrove marsh surrounded by a coastal grassland near Wilson's Cut, Mustang Island, Texas between 7 May and 18 June 2010. After the nesting season was over, we collected nests and measured their cup height, inside and outside diameter, and inside depth in centimeters and total weight in grams. Nests were dissected to determine the materials used.

At the sites of the nests, distance from the ground to the bottom of the nest (centimeters), total height (centimeters) of the plant containing the nest, distance (meters) to the nearest neighboring nest, the nearest body of water, the nearest road or other human-made structure, and the edge of the marsh were measured. The percentage of the nest concealed by the branches or other structures of the plant was approximated. Composition of the floral community adjacent to nesting areas was determined.

A total of nine nests were found (Fig. 1) forming two loose colonies. Although significant efforts were made to find nests in the other areas of mangrove shown on the map, these were the only nests found in the area. Most nests were found on the edges of the wetland. There was little aversion to human activity; nests 5-9 were built near a road, parking lot, and boat landing, and the nests on the edge of the northern grouping (1, 4) were very near Wilson's Cut and a significant amount of boat traffic, especially during weekends. Most nests were relatively close to water (mean distance = 33 m), although the nests in the northern group were somewhat more distant from the water, particularly Nest 3 (156 m). Nests were fairly widely spaced (mean distance = 75.2 m). All but one nest was found in a black mangrove, and the understory floral community where nests were found was dominated by Distichlis spicala (saltgrass).

Nests were generally only moderately concealed with overhead coverage averaging only ca. 50%, much less than the average 77-80% described for other subspecies (Post and Greenlaw, 2009). Only two nests exhibited the woven grass canopy, a common characteristic of nests that females of other subspecies construct by manipulating the surrounding canopy (Post and Greenlaw, 2009). Only one nest was on the western side of the plant; the others were located in the eastern portion of the structure and mostly in line with the prevailing southeasterly winds. The nests themselves did not appear to be oriented in any particular direction, possibly because they lacked the domed canopy. The nests of A. m. mirabilis (Cape Sable seaside sparrow) that were domed were typically oriented to the east (Werner and Woolfenden, 1983).

The average dimensions of nests of A. m. sennetti were generally larger than the other subspecies (Table 1). For example, the average inside diameter and heights (excluding canopy) of the nests built by A. m. mirabilis and A. m. maritimus (northern seaside sparrow) are about 20% smaller than the average inside diameter and heights of the nests constructed by A. m. sennetti measured in this study. Heights of nests of seaside sparrow range from on or only a few inches above the ground to 4.3 m in trees (Post and Greenlaw, 2009). In this study, the mean height of the nests above the ground was higher than the heights measured for most other subspecies, and at least double the mean of height of nests built by A. m. maritimus, A. m. mirabilis, and A. m. peninsular (Scott's seaside sparrow). Nest 6, which was built in marsh grasses, was only 28.7 cm above the ground while the highest nests were >60 cm above the ground. Mean height of vegetation in which nests were found in this study was 100.5 [+ or -] 36.5 cm, about the same as in Gulf Hammock, Florida, but about twice as tall as the mean height of vegetation with nests in New York.

[FIGURE 1 OMITTED]

The nests examined in this study were composed primarily of woven grasses (saltgrass, smooth cordgrass, Spartina alterniflora) and sedges (mostly marsh fibry, Fimbristylis castanea), with a thick layer of matting, a hardened base of inseparable, densely packed material. Bent (1968) noted that A. m. pelonata (Smyrna seaside sparrow, currently incorporated into MacGillivray's seaside sparrow, A. m. macgillivraii) constructed a nest of dead marsh grass with adhering sticky mud. The layer of matting found in the nests we examined did not appear to be adhered with mud. Shoalgrass (Halodule wrightii), found in three nests in this study, has only been recorded from nests of A. m. peninsulae (Bent, 1968). The nests of Texas sparrows lacked the lining of finer grass blades noted for other subspecies (Post and Greenlaw, 2009). Materials such as fishing line and plastic that were found in five of the nests we examined have not been noted in the nests of other subspecies.

Seaside sparrows nest in grasses, trees, and forbs and under tidal debris (Post and Greenlaw, 2009). In the Rio Grande valley, Texas seaside sparrows were found nesting in black mangroves, saltmeadow cordgrass (Spartina spartinae), and saltwort (Batis maritima; Phillips and Einem, 2003). In our study, sparrows used black mangrove almost exclusively; only one nest was not found within a mangrove tree. Because they were located in trees, they tended to be higher off the ground than nests in other locations and in other types of vegetation, particularly grasses. Post and Greenlaw (2009) suggest that canopies over the nests of other subspecies were made by manipulating overhanging vegetation. However, the canopies on the two nests in this study were made by weaving grasses in a similar fashion as the rest of the nest. The general lack of canopies, and, indeed, the lower overall concealment of the nests examined in this study suggests that the trees offer better protection than do grasses. When A. m. macgillivraii and A. m. fisheri (Louisiana seaside sparrow) nested in mangroves, neither constructed canopied nests (Bent, 1968). Taller mangroves may provide sufficient concealment or protection of nests rendering the canopy unnecessary. The two nests in this study that were canopied were located nearer the ground than other nests.

Overall, the primary type of material from which nests were constructed, dried grass, was the same as for other subspecies. Nests were larger and were much heavier than those of either A. m. mirabilis or A. m. marithmus. Perhaps their placement further from the ground and in line with the southeasterly winds required somewhat stouter construction. The overall consistency in the architecture of nests among the subspecies is remarkable given their wide geographic range and the variability of sites and materials that each encounters. The sparrows nesting at Wilson's Cut also seemed undeterred by the human activity that occurred there. Despite the large expanses of mangrove that were distant from the areas of activity, five of the nine nests were located adjacent to the area where the most human disturbance occurred. Although we have no data on nesting success, it seems unusual that the birds would choose to nest in that area when other areas were available. However, these nests contained large amounts of marsh fimbry, and the grassland source of marsh fimbry was nearest this area. The location of most nests near the periphery of the mangrove wetlands also suggests that proximity to building materials for nests, grasses, may have a great deal of influence on where sparrows choose to nest. The nests that were deepest into the interior of the mangroves were composed primarily saltgrass, which was the only species of grass that occurred within the mangrove community itself. As distance from Wilson's Cut increased, availability of marsh grasses decreased. The sparrows likely require a suitable source of grasses in sufficient supply very close to where they build their nests.

The range of A. m. sennetti, is very restricted, Aransas Bay (just north of Corpus Christi, Texas) to the mouth of the Rio Grande (Post and Greenlaw, 2009). This area of the coast of Texas is not characterized by grassy marshes, due in large part to the aridity of the climate and the lack of freshwater inflow. Marsh vegetation consists, for the most part, of succulent halophytes such as Saliroi nia. grasses such as Distichlis spicala or Monanthochloe littoralis, black mangroves, and, at most, a bayward fringe of Spartina alterniflora, or vegetation is mostly absent, replaced by wind-tidal flats covered with mats of cyanobacteria, especially in Laguna Madre. Other subspecies of seaside sparrows nest primarily in grassy vegetation in the high marsh, especially taller species such as Spartina bakeri (sand cordgrass) or Juncus roemerianus (black needlerush; Post and Greenlaw, 2009), vegetation which is limited or unavailable in southern Texas. Since the late 1980s, after a series of freezes reduced populations of black mangrove in southern Texas, mild winters have resulted in a significant expansion of black mangrove, especially into areas where there was no or little previous marsh habitat. Breeding seaside sparrows need nesting sites that are above spring tides and, in breeding and nonbreeding seasons, unvegetated openings where they can forage on insects and small crustaceans on the surface of the mud or the base of vegetation (Post and Greenlaw, 2009). Within the range of A. m. sennetti, black mangroves represent essentially the only suitable vegetation that is above spring tides where a nest can be built, and where suitable materials (grasses) for construction of nests may be found nearby, that also provide the open-mud foraging areas. Populations of Texas seaside sparrow appear to be healthy, but understanding resources needed for nesting is important for conservation. A chief concern is preservation of marshes and plants the species uses in construction of nests in conjunction with the mangrove habitats that provide vegetative structure that is high enough for nests to be safe from spring tides.

The authors thank F. Diaz and R. March for assistance in the field and R. Lehman, R. O'Brien, and C. Bush for help identifying plants. The comments of an anonymous reviewer greatly improved the manuscript.

LITERATURE CITED

BENT, A. C. 1968. Life histories of North American cardinals, grosbeaks, buntings, towhees, finches, sparrows and allies: Order Passeriformes, Family Fringillidae--genera Pipilo (in part) through Spizella. United States National Museum Bulletin 237(2):819-868.

PHILLIPS, S. M., AND G. E. EINEM. 2003. Seaside sparrows, Ammodramus maritimus, breeding in the Rio Grande Delta, southern Texas. Southwestern Naturalist 48:465-467.

POST, W., AND F. B. ANTONIO. 1981. Breeding and rearing of seaside sparrows in captivity. International Zoo Yearbook 21:123-128.

POST, W., AND J. S. GREENLAW. 2009. Seaside Sparrow (Ammodramus maritimus). No. 127 in The birds of North America (A. Poole, editor). Cornell Lab of Ornithology, Ithaca, New York.

WERNER, H. W., AND G. E. WOOLFENDEN. 1983. The Cape Sable sparrow: its habitat, habits, and history. Pages 55-75 in The seaside sparrow, its biology and management (T. L. Quay, J. B. Funderburg Jr., D. S. Lee, E. F. Potter, and C. S. Robbins, editors). Occasional Papers of the North Carolina Biological Survey 1983-1985:1-174.

Submitted 10 June 2011. Accepted 3 February 2014.

Associate Editor was Marlis R. Douglas.

RYAN R. UBIAS, KIM WITHERS, * AND GRAHAM HICKMAN

Center for Coastal Studies, Texas A&M University-Corpus Christi, 6300 Ocean Drive, Corpus Christi, TX 78412

* Correspondent: kim.withers@tamucc.edu
TABLE 1--Measurements (mean [+ or -] 1 SD) of nests constructed by
the Texas seaside sparrow (Ammodramus maritimus sennetti) at Wilson's
Cut, Mustang Island, Texas, with measurements (range in parentheses)
available for nests of other subspecies of A. maritimus.

Measurement             A. m.          A. m.            A. m.
                       sennetti        fisher         maritimus
                                        (a)              (a)

Nest height       45.6 [+ or -] 15.1   122      14.2 [+ or -] 5.8 (c)
  (bottom of
  cup to
  ground, cm)
Cup height (cm)   7.9 [+ or -] 1.5              6.9 (5.1-8.9)
Cup depth (cm)    6.0 [+ or -] 1.6     3.2      3.5 (2.5-3.8)
Cup inside        7.4 [+ or -] 1.3     5.7      6.9 (5.1-6.4)
  diameter (cm)
Cup outside       10.9 [+ or -] 1.2             9.9 (7.6-11.4)
  diameter (cm)
Nest weight (g)   24.6 [+ or -] 5.9
Number of nests   9                    1        8

Measurement          A. m.            A. m.              A. m.
                   nigrescens       Mirabilis         peninsulae
                      (a)              (b)                (b)

Nest height       7.6-50.8 (d)   18 (6-37)         27.7 [+ or -] 8.2
  (bottom of
  cup to
  ground, cm)
Cup height (cm)   10.2           7 (4-11)
Cup depth (cm)    6.4
Cup inside                       6 (4-7)
  diameter (cm)
Cup outside       12.7           10 (8-13)
  diameter (cm)
Nest weight (g)                  16.2 (8.2-24.3)
Number of nests   1              15                47

Measurement           A. m.
                  macgillivraii
                       (a)

Nest height       20-91
  (bottom of
  cup to
  ground, cm)
Cup height (cm)
Cup depth (cm)
Cup inside
  diameter (cm)
Cup outside
  diameter (cm)
Nest weight (g)
Number of nests   Unknown

(a) Source of data is Bent (1968).

(b) Source of data is Post and Greenlaw (2009).

(c) Nest height from Post and Greenlaw (2009); n = 94.

(d) The account in Bent (1968) provided a range of heights, but
the dimensions of only one nest were measured.
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Author:Ubias, Ryan R.; Withers, Kim; Hickman, Graham
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1U7TX
Date:Dec 1, 2013
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