Our results confirm the negative effects of crude oil spills on bird species assemblages with impacted sites having lower bird abundance and richness than the reference sites. This is expected as unlike the reference sites that still had their vegetation structure intact, the vegetation structure at impacted sites were altered. Time of exposure also reveals the temporal effect of oil spills on both the abundance and richness of bird species at impacted sites, with effects lasting about 2 years.
Oil spills might affect birds directly (Heubeck et al. 2002) or indirectly (Velando et al. 2005; Votier et al. 2005). Indirectly, crude oil spills have been found to have significant negative effects on vegetation cover, which is necessary to meet the niche requirements of bird species (Khan et al. 2013; Pezeshki et al. 2000). Sometimes, crude oil spills can result in fire outbreaks that will lead to loss of biodiversity, fertile soil and environmental degradation (Ogbeibu and Iyobosa 2013), thus leading to habitat alteration. This was the case in some of the crude oil spill sites (Ifie, Asisan and Ubaeje) where the spills had resulted in a fire outbreak and had cleared the vegetation growth. With less vegetation cover available, which has proven to be an important habitat requirement, birds are exposed to high predation risk (Marone 1991; Whittingham and Evans 2004). The reduction in soil arthropod diversity as a result of the spill could result in poor vegetation growth (Rotimi et al. 2014) as soil arthropods are necessary for enriching the soils for vegetation growth. On the other hand, reference sites in the study area have a wide range of habitat structures that offer feeding opportunities, nesting grounds and cover from predators thus allowing them to harbour relatively higher bird richness and abundance.
In addition, because many birds are top consumers, they are particularly vulnerable to habitat and trophic-level alterations arising from damage to prey communities (Bergeon Burns et al. 2014). A study by Rotimi et al. (2014) on the effects of oil spill on arthropod diversity in the Niger Delta area of Nigeria showed that oil spills reduce soil arthropod diversity. Such an effect could result in a reduced abundance of arthropod prey for foraging birds, thus limiting prey choice for the foragers and resulting in a shift in dietary choice as discovered by (Martin et al. 2020) or a displacement to alternative habitats in search of preferred prey (Rush et al. 2010). Following the 2003 Prestige oil spill in Galicia, Spain, Velando et al. (2005) discovered that the European Shag population suffered lowered reproductive performance and poorer chick condition resulting from the reduction of the availability of a highly preferred forage fish. Paterson et al. (2014) also showed that a reduction in preferred forage prey and a switch in diet can result in the decline of energy content and reproductive potential as was seen in the herring gull which eventually led to a decrease in bird abundance. Displacement to alternative habitats and lowered reproductive performance will eventually result in reduced population size.
Also, crude oil spill affects bird species directly by making it difficult for them to escape predation as well as forage efficiently (O’Hara and Morandin 2010; Troisi et al. 2016). Some of the birds like the woolly-necked stork Ciconia episcopus were observed to feed on aquatic organisms which might be due to their close proximity to water bodies. The appearance of the woolly-necked stork became largely altered as a result of its mode of feeding (Fig. 4). This alteration may affect its attraction to conspecifics, thus negatively affecting its reproductive ability. Also, fouling of bird’s feathers by crude oil may result in plumage collapse or stickiness of plumage by crude oil (Golightly et al. 2002; King et al. 2021). This eventually results in impaired flight performance, increased energy expenditure decreased body mass, increased take off speeds (King et al. 2021; Maggini et al. 2017a, b; Perez et al. 2017). Also, the bird species may ingest PAHs as they try to preen their feathers. Birds have also been reported to suffer from impairment of organs such as the liver, intestine, and nasal glands (USCG 1999; NOAA 2020) after ingesting PAHs, which could result in a high mortality rate, thereby negatively impacting bird abundance and richness. These could have caused the displacement of bird communities from contaminated sites as was seem on sites with on-going spills in our study. This corroborates the findings of Wiens (1995) who revealed that crude oil spills can affect marine birds by altering their population size and structure, reproduction and habitat occupancy.
Crude oil spills had strong initial impacts on species richness and abundance with bird richness and abundance being the lowest at the onset of the spill, but eventually increasing as time progressed (Figs. 2 and 3). Considering the time scale of the effect of oil spills on the environment, a review by King et al. (2021) revealed that the impacts are strongest and negatively affect avian communities from the onset of the spill. Consequently, at the initial stage of oil spills, the effect tends to be stronger which results in an obvious decrease in both richness and abundance of bird species at the impact sites. Several studies have also shown that there is a high rate of recovery of bird communities after a spill event (Day et al. 1995, 1997; Wiens et al. 2001). Although there is an obvious recovery of birds to the sites that have experienced oil spills (Figs. 2 and 3), Bergeon Burns et al. (2014) reports that contaminants such as PAHs take time to degrade and can persist in the environment for many years. Noteworthy, however, the time at which the effects of the oil spills completely wear off is not within the scope of this study.
Our study confirms the effect of oil spills on terrestrial habitats and by implication, terrestrial vertebrates; birds in a tropical environment. The differences in bird species richness and abundance in contaminated and uncontaminated sites clearly shows the effects of oil spills on the environment. The pollution from oil spills affect the entire ecosystem function with huge conservation implications; from soil microbes to human health (Aroh et al. 2010; Rotimi et al. 2014). Birds are suggested to reflect pollutant hazards to humans better than most other organisms (Furness 1993). Given their popularity with the general public and the trend observed in our study, it is easier to create awareness and collaborative effort in mitigating oil spillage and strategic plans for clean-ups with stakeholders. Moreso, the redistribution of birds into sites that were previously exposed to the oil spills indicates that these habitats are at least in the process of recovering and that the birds themselves find the habitat suitable for occupancy. Since habitat recovery is a prerequisite for recovery of population (Murphy et al. 1997), the trend for recovery in the study sites appears to be good. It is also an indication for the need for immediate habitat sanitation/restoration and effective management plan to curb the harmful effects of contaminants in the environment.
Conclusively, oil spills in the oil-rich Niger-Delta region of Nigeria shows negative effects on the environment. Our study clearly shows that bird species avoid areas contaminated by oil spills, an indication of the state of the health of an environment. These are also evident in the observed habitat utilization of the birds and supported by the findings in Murphy et al. (1997) who evaluated the effects of spill on habitat-use and bird communities in contaminated areas. With the paucity of data from the tropical regions of the world and few general studies that consider the effects of oil spills on terrestrial organisms, we recommend a long-term monitoring assessment of areas that are prompt to oil spills, especially the Niger-Delta region in Nigeria. From the findings of this study, we also suggest an assessment of the individual feeding guilds of the birds in a contaminated area for an effective ecological measurement to perhaps inform a better restoration and management plan.