Egg success, hatching success, and nest-site selection of Brown Pelicans, Gaillard Island, Alabama, USA.
Animals are distributed non-randomly within a given habitat due to the pressures of natural selection (Southwood 1977). Non-random distribution is the result of organisms selecting particular patches within their habitats and selection is presumed to be adaptive; organisms choosing optimal habitats will be more successful than those that do not (Martin 1998). Selecting an optimal nest-site by birds can have implications for nestling fitness and overall survival.
Brown Pelicans (Pelecanus occidentalis) are now reaching population levels along the Gulf Coast that were achieved prior to the widespread use of DDT as a pesticide (Schreiber and Risebrough 1972, Wilkinson et al. 1994). This species is now a common breeder in the southeastern United States, but little has been reported on its breeding biology along the Gulf Coast (Sachs and Jodice 2009) with no reports for Alabama.
Brown Pelicans, while historically commonly observed on the coast of Alabama, had not nested in the state (Imhof 1976) until four nests were discovered on Gaillard Island in 1983 (Wilkinson et al. 1994). As many as 5,000 breeding pairs have returned to Gaillard Island each year since 2003 (Roger Clay, pers. comm.). Our objectives were to: (1) investigate nest-site selection, and (2) the implications of nest-site selection on hatching and egg success of Brown Pelicans.
Study Site.--This study occurred over two breeding seasons (2007-2008) at Gaillard Island, Alabama (30[degrees] 30' N, 88[degrees] 02' W). Gaillard Island is a man-made, dredge spoil island in Mobile Bay that is 2.6 km at its greatest width and 3.6 km at its greatest length. It is just east of Dog River and 17.7 km south of downtown Mobile (Robinson and Dindo (2009). There is a 6.1-m dirt berm perimeter completely around the island that protects it from storm surge and provides protected nesting habitats for numerous bird species. The dominant vegetation on the island is marsh elder (Iva frutescens) and cord grass (Spartina spp.). Brown Pelicans commonly nest in marsh elder on the island and many nest on the ground near vegetation. The southern end of Gaillard Island, comprising about 20% of the island, is used by Brown Pelicans for nesting, as much of the island is non-vegetated dredge spoil. Brown Pelicans typically arrive on Gaillard Island in late March and begin building nests in April. New nests can be found through June. Gaillard Island is the only known nesting site in Alabama for Brown Pelicans.
Field Methods.--Eleven sites (and quadrats) on Gaillard Island were selected in February 2007 and February 2008, prior to arrival of Brown Pelicans. Quadrats were chosen based on vegetation cover and the sites' positions on the island. Vegetation density was ranked as: 0 = no vegetation (<2% cover above 0.5 m from the ground), 1 = low vegetation density (<30% cover), 2 = moderate vegetation density (30-60% cover), and 3 = high vegetation density (>60% cover). Sites were selected for each category inside and outside of the berm. Percent vegetation cover was estimated following Sneddon (1993). Quadrats (20 [m.sup.2]) were established at each of the sites in locations that represented the diverse vegetation cover differences and position on the island.
Nests and young were counted in each marked quadrat at least four times each month starting in February 2007 and ending in September 2008. Nests on the border of each quadrat were counted if they were touching the border of that quadrat (Sutherland et al. 2004).
Statistical Methods.---Egg success is the probability of an egg surviving from date of initial laying to time of fledging (Mayfield 1961) and was calculated for each quadrat rather than each nest. Hatching success was defined as the number of young hatched per egg laid in a given quadrat (Schreiber and Risebrough 1972, Mayfield 1975), assuming that all eggs laid in a given nest have the same chance to hatch.
Each quadrat was divided into four vegetation layers: from the ground to 1 m, 1-2 m, 2-3.5 m, and 3.5 m to the top of the vegetation (Sneddon 1993). Percent cover at each layer for each quadrat was used in the statistical analysis. Seven variables were measured to examine their effects on egg and hatching success: date of arrival (Julian date), vegetation density (DV) of the layer < 1 m (ground height DV), vegetation density of the layer 1-2 m (low height DV), vegetation density of the layer 2-3.5 m (moderate height DV), vegetation density of the layer 3.5 m to the top of the vegetation (tall height DV), nest density (number of nests/[m.sup.2]), and number of nests on the ground (as a percent of total nests).
Pearson's correlation was used to examine if year or position relative to the berm had an effect on hatching or egg success. A principal component (PC) analysis was performed using the above variables; principal components were selected for and used in regression analyses with egg success and hatching success. Quadrats one and four could not be used in this analysis as no nesting occurred in either. Pearson's correlation was performed between each variable and each principal component to examine any relationships. A stepwise regression routine was performed to select a model that best fit the relationship between date of arrival and vegetation variables.
We observed 384 Brown Pelican nests and 852 eggs over two breeding seasons on Gaillard Island. The largest clutch size observed was three, and this was a common clutch size. Brown Pelicans nested primarily on the ground and in marsh elder; however, two nests were in the invasive Chinese tallow tree (Triadica sebifera). No nests were found at a height >2 m. Study year had no effect on egg (P = 0.144) or hatching success (P = 0.176). Principal component I (PC I) and principal component II (PC II) were chosen based on eigenvalues (Table 1).
Egg success ranged from 0.046 to 0.659 (Table 2) with a mean of 0.507 and had a significant and negative relationship with PC I (P = 0.006, [R.sup.2] Adj. = 69.0%, [beta] estimate = -0.086 PC I). Hatching success ranged from 0 to 0.695 young hatched per laid egg (Table 2) with a mean of 0.539 and was significantly related to PC I (P = 0.002) and PC II (P = 0.027) ([R.sup.2] Adj.= 79.5%, [beta] estimate = -0.091 PC I, -0.068 PC II). The relationship between hatching success and PC I was also negative.
Date of arrival ranged from no arrival (no nesting in the quadrat) to Julian day 161 (9 Jun) and the mean day of arrival was Julian day 125 (4 May) for those quadrats that had an arrival date (Table 2). The best regression routine selected two variables for the regression model: low height DV and the interaction between the first two vegetation layers (ground height DV x low height DV). Arrival date was significantly related to low height DV (P = 0.004) and the interaction between the two vegetation layers (P = 0.015).
PC II was a function of the vegetation layers in which Brown Pelicans were nesting and PC I was a function of those factors that did not directly involve the nesting vegetation layers (Table 3). Both egg and hatching success increased with earlier arrival dates, lower vegetation density in the layer 2-3.5 m above ground, higher nest density, and more nests on the ground. Hatching success increased as the vegetation density in the layer < 1 m above ground and vegetation density in the layer 1-2 m above the ground decreased. Arrival date was positively related to vegetation density between 1 and 2 m above the ground. It was negatively related to the interaction term of the first two vegetation layers (ground height DV x low height DV). The vegetation density between 1 and 2 m above the ground increased as arrival date became later and the interaction between the first two vegetation layers decreased, indicating those arriving first chose sites with little to no vegetation on the ground but some vegetation above the ground. It is not surprising that year had no effect on egg or hatching success. The study was conducted for 2 years which is not sufficiently long to draw conclusions regarding year effects on Brown Pelican nesting ecology.
The more successful Brown Pelicans on Gaillard Island arrived earliest and selected sites with less dense vegetation within 1 m of the ground. This provided more room to nest on the ground, but with some cover above the ground. Lower vegetation densities were selected first and nests in those areas were more successful. This could be related to size of Brown Pelicans. They must incubate eggs, sit on the nest when not incubating, and land on the nest or take flight from the nest (Shields 2002). Dense vegetation does not allow some or all of these actions. Clumsy landings of Brown Pelicans can crush eggs or knock them from nests (Maxwell and Kale 1977). These problems were minimized by nesting in less dense vegetation. Vegetation in which nesting occurred ceases to be a factor when nests were on the ground. Egg and hatching success increased significantly in those quadrats with a high percentage of nests on the ground. Another benefit to those nesting on the ground is predator avoidance. Nestling Brown Pelicans from ground nests are able to leave the nest and return earlier than those nesting in vegetation above the ground - as early as 3 weeks after hatching (Shields 2002). These nestlings could leave the nest to avoid predation (from avian predators) at a much earlier age than those from nests above the ground. Selecting for ground nests, but with vegetation above ground level provides thermoregulatory benefits (Grant 1982) and cover from avian predators. Gaillard Island is free of mammalian and reptilian predators. The only predators of Brown Pelican eggs and young are gulls and wading birds. Ground nests under the cover of vegetation hides eggs and chicks from predators hunting from the air. Another factor that may affect nest-site selection and productivity is age of the nesting birds. We collected no data on age, but Blus and Keahey (1978) have shown that older Brown Pelicans have the greatest nest success. A longer study would be needed to show these effects.
Brown Pelicans are now flourishing along the Gulf Coast of Alabama. However, with development of coastal areas, the ephemeral nature of spoil islands, natural disasters, and man-made disasters in the Gulf of Mexico, their future is uncertain. This study provides insight into nest preference and the relative success rates of microhabitats at nest sites. This study also provides important baseline data for future studies on the breeding ecology of Brown Pelicans in the Gulf of Mexico.
Funding for this project was provided by the State Lands Division of the Alabama Department of Conservation and Natural Resources, the NOAA Office of Ocean and Coastal Resource Management (# NA05NOS4191091), and NOAA's Northern Gulf Institute (DISL-01 Education and Outreach).
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Orin J. Robinson (1,2,3) and John J. Dindo (1)
(1) Dauphin Island Sea Laboratory, 101 Bienville Boulevard, Dauphin Island, AL 36528, USA.
(2) Current address: Department of Ecology, Evolution, and Natural Resources, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901, USA.
(3) Corresponding author; e-mail:firstname.lastname@example.org
TABLE 1. Eigenvalues, proportion of variance explained, and cumulative variance of principal components for Brown Pelicans nesting on Gaillard Island, Alabama. PC I PC II Eigenvalue 3.439 2.132 Proportion of variance 0.491 0.305 Cumulative variance 0.491 0.796 TABLE 2. Egg success (ES), hatching success (HS), Julian date of arrival (JD), percentage of nests on the ground (NoG), and vegetation cover in each layer (GH < 1 m, LH = 1-2 m, MH = 2-3.5 m, TH > 3.5 m) for each quadrat for Brown Pelicans nesting on Gaillard Island, Alabama. Quadrat ES HS JD NoG GH LH MH TH 1 N/A N/A N/A N/A 0.80 0.20 0.12 0.90 2 0.393 0.500 148 0.500 0.45 0.40 0.12 0.00 3 0.452 0.537 145 0.114 0.70 0.35 0.15 0.40 4 0.046 0 161 0 1.00 0.98 0.70 0.22 5 N/A N/A N/A N/A 0.90 0.02 0.00 0.00 6 0.535 0.677 113 0.979 0.84 0.57 0.01 0.00 7 0.562 0.635 113 1.000 0.90 0.73 0.02 0.00 8 0.639 0.576 108 0.834 0.97 0.70 0.15 0.02 9 0.659 0.695 117 0.510 0.87 0.47 0.35 0.09 10 0.649 0.588 112 0.620 0.70 0.40 0.12 0.30 11 0.626 0.646 112 0.711 0.95 0.60 0.11 0.01 TABLE 3. Coefficients for Pearson's correlation analysis between principal components and variables for Brown Pelicans nesting on Gaillard Island, Alabama. PC I PC II Julian date of arrival 0.929 ** -0.039 Ground height DV -0.206 0.916 ** Low height DV 0.107 0.937 ** Moderate height DV 0.764 * 0.565 Tall height DV 0.628 -0.239 Nest density -0.788 * 0.195 Nests on ground -0.961 ** 0.003 * P < 0.02 ** P [less than or equal to] 0.001