Nest site fidelity in Socotra Cormorants has not been studied [
16]. Although the documented colonies throughout its range are used repeatedly [
16], it has also been observed that Socotra Cormorants may use completely different areas within the same colony in subsequent years [
14,
16]. For example, in 2011, Muzaffar and colleagues [
14] reported nesting of Socotra Cormorants in areas with plantations on Siniya Island. This area had never been used in the past and was putatively being used by the birds in response to disturbance. In 2012, the Socotra Cormorants returned to nesting in areas on Siniya Island that had been historically used by the species for nesting [
14]. In this study, we found that Socotra Cormorant nesting areas (2011 nest site, 2012 nest site, and control site) could be discriminated based on percent cover, droppings and other (
P ≤ 0.001). Furthermore, ten out of the thirteen elements in soil could be used either independently or in combination with cover variables to distinguish these nest sites and the control site. The control site also had greater overlap with the 2012 nest site, suggesting overall similarity between sites. Thus, vegetation and soil characteristics were collectively important in distinguishing areas selected for nesting. Notably limited amount of vegetation (percent cover ranged between 7.8% and 20% in all three sites) had strong discriminating ability whereas other attributes of vegetation did not show clear patterns (data not shown). This is consistent with breeding site selection in many seabird species [
6]. Depending on species, percent cover tends to have positive and negative effects on nesting seabirds [
6]. Kelp Gulls (
Larus dominicanus) may preferentially select nesting areas with bare ground to allow them a clear view of approaching aerial predators [
23]. In contrast, Common Terns (
Sterna hirundo) may select areas with greater cover to allow protection from predators or weather [
24]. In our study, the contribution of Function 1 to the model from the discriminant analysis was estimated to be 91.1%, while Function 2 contributed only 8.9% to the discriminant model (
P ≤ 0.05). Muzaffar and colleagues [
14,
18] reported higher egg volumes and hatching success under shaded areas, suggesting that plant cover benefited Socotra Cormorants on Siniya Island. Thus, our finding suggests that percent cover is an important discriminant of nest location and could indicate better microhabitat quality.
Microhabitat quality may be strongly influenced by the topography, nutrient input, wind action and nesting density in seabird colonies, along with the underlying geology of the area [
3,
4,
5,
6,
11,
25,
26]. In this study, the elements Al, Mg, Mn, Cu, P, Ca, Na, Cr, V and Fe had strong loadings with Functions 1 and 2 and were significant in discriminating areas based on soil variables alone. Furthermore, Fe, K, Mg, P and Ca all occurred at much higher concentrations in all three areas compared with areas without seabirds. This suggested stronger input of these elements associated with breeding Socotra Cormorants. However, natural loading of certain cations and heavy metals may be high and variable in the soils of the UAE. For example, in the coastal areas of the UAE the first 10 m of soil in dominated by CaCO_3, along with high levels of Cr and Fe arising from naturally occurring mixed metal oxides of Fe and Cr [
25,
26]. Few studies have examined nutrients (including trace elements) in relation to seabird nesting activities [
1,
2,
5,
11]. Rajakaruna and colleagues [
11] showed that enhanced Ca, Mg, Zn, and Pb in the soil resulting from seabird breeding activities could be traced in the leaves although patterns of uptake by
Xanthoria parietina varied. Higher Ca concentration is expected since seabird guano typically has high Ca [
3]. However, this variation could lead to altered species composition of plants although the extent of such variation is unclear from the current study. Phosphorus and nitrogen, important limiting factors for plants, were much higher in the 2012 nesting area compared with either the 2011 nesting area or the control sites. Phosphorous has longer residence time in soils [
8,
9,
27]. Historical data extrapolated from nutrient content in sediment cores illustrate that P can be used as markers of long-term population change of seabirds. For example, abundance of P in soil has been attributed to changes in penguin population size over a 3000-year time frame [
10]. It has been showed in [
27] that contributions of phosphorus from guano from Westland Petrels (
Procellaria westlandica) on mainland New Zealand had residence times ranging in 11–41 years for total P, which could be directly associated with nesting activities. Thus we suggest that significantly higher P in the 2012 nesting areas indicates that these areas have been in longer use by Socotra Cormorants compared with the 2011 areas. This is consistent with anecdotal information on the species nesting pattern on Siniya Island that suggests that the 2011 nesting areas were never used in the past (Ministry of Environment and Water, personal communication). We did not measure nitrogen but studies comparing desert islands in Gulf of California showed that islands used by seabirds had higher
,
and total N derived from seabird guano [
5]. Furthermore, these differences also resulted in lower diversity but higher productivity of vegetation on the islands used by nesting birds. We recognize that measurement of nitrogen could be key to further evaluating the impact of seabird guano on soil chemistry and vegetation of Siniya Island and we intend to include this in future work. Although Cd, Cr and K were removed from some of the analyses due to low influence on the overall models, the concentration of Cd and Cr was much higher than baseline levels [
11]. This could indicate enhanced levels of Cd and Cr in the marine environment of the Arabian Gulf and their subsequent transport through the feces of cormorants into their terrestrial breeding habitat. Alyazouri and colleagues [
26] conducted a study of soils ranging from coastal to inland areas of some of the northern Emirates. They found consistently high levels of Cr, Fe and other metals in all their study sites, with significantly higher values in areas near the Ajman Industrial Zone. This area is located close to the UAE coastline and is about 20 km from Siniya Island. Although both Cr and Fe occur naturally in high levels in UAE soils [
25,
26], these metals may be mobilized further near contaminated areas [
26], which may then leach into the coastal marine environment. Ecological concern associated with increased mobilization of pollutants of human origin needs to be further studied in relation to seabird islands.
Physical disturbance by nesting activities is also a leading cause of breeding habitat degradation documented in many species of cormorants [
3]. All published records of Socotra Cormorants suggest that they breed in open, non-vegetated or poorly vegetated areas [
16]. Siniya Island seems to be the exception and they have been observed to nest in between scrub vegetation and under the shade of planted trees [
14]. Although disturbance caused by breeding activities seems to be important in Socotra Cormorants, the extent to which physical disturbance contributes towards degradation of the habitat remains unclear.
In summary, overall allochthonous input of elements from the marine environment seems to be high in areas used by nesting Socotra Cormorants. Further studies are required to examine how these inputs translate into changes in vegetation cover, alteration or degradation of nesting areas and ultimately, reproductive success of breeding Socotra Cormorants. Future works will focus on examining nutrients, soil pH and other measures regarding impacts of disturbance to better understand linkages between nutrient input and their impact on habitat quality considering both flora and fauna.