The Secretarybird is a large raptor of the tropical grassy biomes of sub-Saharan Africa, listed as Endangered due to global population declines and ongoing habitat loss. Despite its wide range, ecological knowledge of the species remains limited, with significant gaps in resource selection and demographic dynamics. Studying populations within relatively intact ecosystems, such as the Greater Serengeti-Mara Ecosystem (GSME), provides a robust reference for identifying the species’ ecological requirements and defining environmental thresholds necessary for long-term population viability. This doctoral thesis aimed to estimate abundance and density of the Secretarybird within Serengeti National Park (SNP) and identified key ecological factors influencing the species’ abundance and productivity. In 2023, road-based surveys were conducted along 52 replicated transects. Abundance and density estimates were derived using multiple-covariate distance sampling, while habitat selection was assessed through generalised additive mixed models (GAMMs) and generalised linear models. An average density of 1.3 Secretarybirds/10 km2 and an abundance of 1,513.3 individuals were estimated. Areas of highest density, located in the central and southern SNP, averaged 2.3 Secretarybirds/10 km2 and 622.5 individuals. The presence of high-quality grasslands with closed ground cover emerged as a key predictor of abundance, especially when covering ≥35% of the landscape. In contrast, sparser grasslands and open grassed woodlands had a negative effect when they covered more than 10 and 20% of the landscape, respectively. Secretarybirds preferred flat terrain and areas receiving 700–850 mm of annual rainfall. Breeding pairs showed stronger avoidance of more heterogeneous habitats and wooded savannas compared to single individuals. Moderate prescribed burning (25–55% of burned area) was associated with higher abundances, while excessive fire use had detrimental effects. Based on these results, the study assessed the effects of grassland fragmentation and rainfall variability on reproduction. Using nest density estimators and GAMMs, productivity was quantified across three habitat types with different grassland fragmentation levels and rainfall regimes during the 2023 and 2024 breeding seasons. Highly fragmented habitats (<50% grassland cover) exhibited the lowest nesting success (18.9%) and productivity (0.8 successful nests/100 km²), possibly due to increased exposure to edge effect. In contrast, habitats with moderate to low grassland fragmentation (50−<90% cover) supported the highest nest densities (7.6 nests/100 km²) and the greatest average productivity (2.5 successful nests/100 km²), despite low nesting success (29%). Habitats with minimal grassland fragmentation (≥90% cover) showed significantly higher nesting success (71%) but lower nest densities, resulting in intermediate productivity (1.7 successful nests/100 km²). Nest abundance was strongly linked to rainfall: in the wetter 2024 rainy season, nest density tripled compared to 2023, with a corresponding increase in productivity (2.2 vs. 0.8/100 km²), despite similar nesting success. These findings highlight the Secretarybird’s sensitivity to vegetation structure, grassland quality, and climate variability, stressing the risks posed by woody encroachment, rainfall shifts, and grass layer degradation. While the GSME remains a stronghold for the species, it also includes areas of conservation concern, particularly where grasslands are fragmented or degraded. Conservation strategies, especially in suboptimal rangelands, should focus on preserving the extent and integrity of grasslands through rotational fire and grazing regimes and limiting annual burned areas to <50%. Short-term actions should include nest protection in the most vulnerable areas. This integrated ecological framework provides a basis for long-term monitoring and management of the species in East Africa.
The Secretarybird is a large raptor of the tropical grassy biomes of sub-Saharan Africa, listed as Endangered due to global population declines and ongoing habitat loss. Despite its wide range, ecological knowledge of the species remains limited, with significant gaps in resource selection and demographic dynamics. Studying populations within relatively intact ecosystems, such as the Greater Serengeti-Mara Ecosystem (GSME), provides a robust reference for identifying the species’ ecological requirements and defining environmental thresholds necessary for long-term population viability. This doctoral thesis aimed to estimate abundance and density of the Secretarybird within Serengeti National Park (SNP) and identified key ecological factors influencing the species’ abundance and productivity. In 2023, road-based surveys were conducted along 52 replicated transects. Abundance and density estimates were derived using multiple-covariate distance sampling, while habitat selection was assessed through generalised additive mixed models (GAMMs) and generalised linear models. An average density of 1.3 Secretarybirds/10 km2 and an abundance of 1,513.3 individuals were estimated. Areas of highest density, located in the central and southern SNP, averaged 2.3 Secretarybirds/10 km2 and 622.5 individuals. The presence of high-quality grasslands with closed ground cover emerged as a key predictor of abundance, especially when covering ≥35% of the landscape. In contrast, sparser grasslands and open grassed woodlands had a negative effect when they covered more than 10 and 20% of the landscape, respectively. Secretarybirds preferred flat terrain and areas receiving 700–850 mm of annual rainfall. Breeding pairs showed stronger avoidance of more heterogeneous habitats and wooded savannas compared to single individuals. Moderate prescribed burning (25–55% of burned area) was associated with higher abundances, while excessive fire use had detrimental effects. Based on these results, the study assessed the effects of grassland fragmentation and rainfall variability on reproduction. Using nest density estimators and GAMMs, productivity was quantified across three habitat types with different grassland fragmentation levels and rainfall regimes during the 2023 and 2024 breeding seasons. Highly fragmented habitats (<50% grassland cover) exhibited the lowest nesting success (18.9%) and productivity (0.8 successful nests/100 km²), possibly due to increased exposure to edge effect. In contrast, habitats with moderate to low grassland fragmentation (50−<90% cover) supported the highest nest densities (7.6 nests/100 km²) and the greatest average productivity (2.5 successful nests/100 km²), despite low nesting success (29%). Habitats with minimal grassland fragmentation (≥90% cover) showed significantly higher nesting success (71%) but lower nest densities, resulting in intermediate productivity (1.7 successful nests/100 km²). Nest abundance was strongly linked to rainfall: in the wetter 2024 rainy season, nest density tripled compared to 2023, with a corresponding increase in productivity (2.2 vs. 0.8/100 km²), despite similar nesting success. These findings highlight the Secretarybird’s sensitivity to vegetation structure, grassland quality, and climate variability, stressing the risks posed by woody encroachment, rainfall shifts, and grass layer degradation. While the GSME remains a stronghold for the species, it also includes areas of conservation concern, particularly where grasslands are fragmented or degraded. Conservation strategies, especially in suboptimal rangelands, should focus on preserving the extent and integrity of grasslands through rotational fire and grazing regimes and limiting annual burned areas to <50%. Short-term actions should include nest protection in the most vulnerable areas. This integrated ecological framework provides a basis for long-term monitoring and management of the species in East Africa.
Habitat selection and reproduction of a savannah avian predator: environmental factors driving the abundance and productivity of the Secretarybird Sagittarius serpentarius in the Serengeti National Park
ROMANI, FEDERICO
2025-12-15
Abstract
The Secretarybird is a large raptor of the tropical grassy biomes of sub-Saharan Africa, listed as Endangered due to global population declines and ongoing habitat loss. Despite its wide range, ecological knowledge of the species remains limited, with significant gaps in resource selection and demographic dynamics. Studying populations within relatively intact ecosystems, such as the Greater Serengeti-Mara Ecosystem (GSME), provides a robust reference for identifying the species’ ecological requirements and defining environmental thresholds necessary for long-term population viability. This doctoral thesis aimed to estimate abundance and density of the Secretarybird within Serengeti National Park (SNP) and identified key ecological factors influencing the species’ abundance and productivity. In 2023, road-based surveys were conducted along 52 replicated transects. Abundance and density estimates were derived using multiple-covariate distance sampling, while habitat selection was assessed through generalised additive mixed models (GAMMs) and generalised linear models. An average density of 1.3 Secretarybirds/10 km2 and an abundance of 1,513.3 individuals were estimated. Areas of highest density, located in the central and southern SNP, averaged 2.3 Secretarybirds/10 km2 and 622.5 individuals. The presence of high-quality grasslands with closed ground cover emerged as a key predictor of abundance, especially when covering ≥35% of the landscape. In contrast, sparser grasslands and open grassed woodlands had a negative effect when they covered more than 10 and 20% of the landscape, respectively. Secretarybirds preferred flat terrain and areas receiving 700–850 mm of annual rainfall. Breeding pairs showed stronger avoidance of more heterogeneous habitats and wooded savannas compared to single individuals. Moderate prescribed burning (25–55% of burned area) was associated with higher abundances, while excessive fire use had detrimental effects. Based on these results, the study assessed the effects of grassland fragmentation and rainfall variability on reproduction. Using nest density estimators and GAMMs, productivity was quantified across three habitat types with different grassland fragmentation levels and rainfall regimes during the 2023 and 2024 breeding seasons. Highly fragmented habitats (<50% grassland cover) exhibited the lowest nesting success (18.9%) and productivity (0.8 successful nests/100 km²), possibly due to increased exposure to edge effect. In contrast, habitats with moderate to low grassland fragmentation (50−<90% cover) supported the highest nest densities (7.6 nests/100 km²) and the greatest average productivity (2.5 successful nests/100 km²), despite low nesting success (29%). Habitats with minimal grassland fragmentation (≥90% cover) showed significantly higher nesting success (71%) but lower nest densities, resulting in intermediate productivity (1.7 successful nests/100 km²). Nest abundance was strongly linked to rainfall: in the wetter 2024 rainy season, nest density tripled compared to 2023, with a corresponding increase in productivity (2.2 vs. 0.8/100 km²), despite similar nesting success. These findings highlight the Secretarybird’s sensitivity to vegetation structure, grassland quality, and climate variability, stressing the risks posed by woody encroachment, rainfall shifts, and grass layer degradation. While the GSME remains a stronghold for the species, it also includes areas of conservation concern, particularly where grasslands are fragmented or degraded. Conservation strategies, especially in suboptimal rangelands, should focus on preserving the extent and integrity of grasslands through rotational fire and grazing regimes and limiting annual burned areas to <50%. Short-term actions should include nest protection in the most vulnerable areas. This integrated ecological framework provides a basis for long-term monitoring and management of the species in East Africa.| File | Dimensione | Formato | |
|---|---|---|---|
|
PhD_Tesi_Federico_Romani.pdf
accesso aperto
Descrizione: Habitat selection and reproduction of a savannah avian predator: environmental factors driving the abundance and productivity of the Secretarybird Sagittarius serpentarius in the Serengeti National Park
Tipologia:
Tesi di dottorato
Dimensione
10.38 MB
Formato
Adobe PDF
|
10.38 MB | Adobe PDF | Visualizza/Apri |
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
