**4.1 Birds' distribution in** *Commiphora spp.* **compared with** *V. tortilis-* **and**  *V. robusta***-dominated vegetations**

The higher species richness, diversity, and abundance of tree-feeding birds in *V. tortilis*-dominated vegetation were due to greater tree density and underlying grass cover; which attracted birds for feeding and perching. The results are consistent with the first prediction which suggested that the greater structural complexity of the *Vachellia* trees provided more resources for birds to exploit, and thus increases species diversity [46, 47]. However, the results for *V. robusta*-dominated vegetation, which showed lower richness, abundance, and diversity than those for *Commiphora*dominated vegetation, are not consistent with this prediction. This suggests that it is not just canopy size that matters but also other factors such as the abundance of food from insects and fruits. The tannin content of leaves in *V. tortilis*-dominated vegetation is much lower than that in *V. robusta*-dominated vegetation making the former more palatable and attractive to insects and hence attracting birds [48, 49].

The higher abundance and diversity of ground-feeding birds in *Commiphora*dominated vegetation was due to the low canopy cover of *Commiphora* spp. which provided bare ground for birds to forage. It was observed that *Commiphora*-dominated vegetation changed by succession into *Vachellia* vegetation in the same area over the 1970–2000 period [8], change in geology and soil properties and a change in the herb layer species communities probably affected ground-feeding birds. The herb layer does differ between these vegetation type [50–52] in most habitats, vegetation determine the physical structure of the environment, and therefore, have a considerable influence on the distributions and interactions of bird species. Renwald [53], however, pointed out that the ecological distinction between plant communities does not uniformly correlate with differences in animal communities. Such apparent inconsistency is explained by the observation that wildlife species including birds most often select vegetation on the basis of structure rather than plant species composition [15, 54].

#### **4.2 Birds in grassland habitats compared with** *V. drepanolobium* **and** *V. seyal*

Secondly, it is predicted that higher diversity and abundance of birds in *V. drepanolobium-* and *V. seyal*-dominated vegetations than in grassland. The higher abundance of birds in *A. seyal*-dominated vegetation than those in the western grassland of the Serengeti National Park was consistent with this prediction; the differences observed were due to the trees providing greater food resources, trees providing hiding areas from predators, and provision of nesting sites compared with the grassland. However, the opposite result occurred in the northern Serengeti National Park with grassland having a higher abundance than that in *V. drepanolobium*-dominated vegetation, which was inconsistent with the hypothesis. Some of this effect was due to tree-feeding birds moving into grasslands to exploit the available resources, for example, Rufous-tailed weaver *Histurgops ruficauda* and buffalo weavers were frequently observed to forage in grasslands next to *V. drepanolobium* but not *V. seyal*dominated vegetations. The reason why the mentioned bird species avoid foraging very close to *V. seyal*-dominated vegetation needs more investigation. Mwangomo et al. [22] observed that Rufous-tailed weaver preferred to forage in grassland and avoided feeding in most *Vachellia*-dominated vegetation. This pattern of birds that nest and roost in *V. seyal* moving into grassland to feed was not observed suggesting more research to determine the reason why did not forage close to *V. seyal*-dominated vegetation. So some but not all of the results were consistent with the second hypothesis which predicted that changing habitat structure from grassland to *V. drepanolobium* or *V. seyal*-dominated vegetations may influence increases in the abundance of the birds' fauna. Thus, individual tree species provide particular resources for birds not found in grassland.

#### **4.3 The effect of grass height on the abundance of ground-feeding bird species**

The lack of difference in abundance with grass height between this vegetation was due to different groups of birds compensating for each other. For example, widowbird and whydahs were abundant in tall grass but larks, lapwing, coursers, storks, and pipits were abundant in short grass. The higher abundance of ground-feeding birds observed in shorter grass levels (0–25 cm) was contrary to the third hypothesis which predicted that bird abundance and diversity would decline with changing grass height from tall to short grass due to lower grass cover and structure. Grass structure and cover may not be the only factors determining the types of bird species using the grassland. Some species prefer tall grass while others prefer short grass and densities may not be related to the biomass and structure.

Lower grass height in the Serengeti National Park was created by herbivore grazing and anthropogenic fires, which changed the vegetation cover by exposing ground and creating short grass habitats for specialized birds such as the Red-capped Lark *Calandrella cinerea*, Crowned Lapwing *Vanellus coronatus*, African Pipit *Anthus cinnamomeus*, and Fischer's Sparrow-lark *Eremopterix leucopareia*. Although grass height was lower, habitat heterogeneity was greater than in tall grass. In addition, short grass increases the visibility of food sources for birds. Thus, the short grass community was abundant or even more than that in grass height taller than 75 cm community, one may replace the other, a replacement can be of species that prefer to forage in tall grass replaced with those forage in short grass as has been observed on the Serengeti National Park plains [55].

However, there was a higher abundance of birds on short grass areas in *Commiphora*-dominated vegetation than in *V. tortilis* and *V. robusta*-dominated vegetations. Grass height could not explain this observation, which suggests that it is the vegetation structure that is affecting ground bird abundance rather than just the height of the herb layer.

This study found that the abundance of birds did not decrease with grass height; it reached a peak at an intermediate level of grass heights interval of 10–25 cm, contrary to the initial prediction. This was especially clear in the western part of the Serengeti National Park but was also evident in the northern part of the park where bird diversity increased as grass height became shorter (≤25) due to fire and grazing by ungulates. These findings are consistent with the generally accepted pattern that ecological succession following intermediate disturbance is characterized by increases in species richness, equitability, and similarity due to an increase in patchiness of habitat [56–58]. It is generally accepted that animal species richness increases with increasing habitat complexity, given that more complex habitats offer a greater variety for potential exploitation of resources. For example, Brown [59] reported that bird species diversity increased with an increase in grass height in America. However, the present results conform more to the intermediate disturbance hypothesis developed by Connell [60] which suggests that the highest diversity of living things is maintained at intermediate scales of disturbance. Nkwabi et al. [61] reported that both bird species diversity and composition change with grass height. There have been similar results in southern Africa by Jansen et al. [62] who described that both density and species richness of francolins (*Francolinus* spp.) change with changes in grass height. Several studies have revealed that the structure of the vegetation, its complexity, and vertical arrangement are primary defining factors in bird community nesting and foraging [46, 60, 63–68].
