Population development of three sympatric dove species in African acacia savanna following a drought

In avian behavioural and spatial ecology, issues related to population density, territoriality, and philopatry are especially important. These are often interrelated. The aim of this study was to analyse these behaviours in shrikes inhabiting an acacia savanna in northern Namibia (400 ha). Population densities of one shrike and four bushshrike species were studied by means of the territory mapping method. About half of the study area had been modified by humans into cultivated fields, orchards, sport fields, and human settlements. This study was conducted in 2012/2013, 2017/2018, and 2019/2020. The most common species were the Black-backed Puffback (2.5 pairs/100 ha) and Crimson-breasted Shrike (1.9); the least common were the Southern White-crowned Shrike (0.8) and Brubru (1.0). Statistics for the Brown-crowned Tchagra indicated a density of 1.5 pairs/100 ha. While the density of the Brown-crowned Tchagra and Brubru remained stable over the years, the population densities of the Black-backed Shrike, Crimson-breasted Shrike, and Southern White-crowned Shrike showed a remarkable increase over the years 2012-2020. All bushshrike species showed a preference for patches of natural savanna vegetation. This was especially evident in the Crimson-breasted Shrike and the Brubru. However, the Southern White-crowned Shrike from the shrike family did not show this preference. The presented studies showed marked interspecific differences in territoriality and philopatry. Furthermore, even within the same species, marked temporal differences were shown in population density, probably also in philopatry. Within families, these behaviours may shape both resource competition and mate competition, while between families, these behaviours only shape resource competition.

Background: Bird species belonging to the orders of Coraciiformes, Psittaciformes and Piciformes may be regarded as good indicators of environmental quality. Since most species are fairly large and have distinctive plumage colouration, they attract human attention. Most of them are also strongly territorial and vocally conspicuous. However, not much is known about their population densities and virtually nothing about interannual population changes. Objectives: The aims of this study were to: 1) estimate population densities; 2) assess interannual variations in population densities; 3) study site fidelity; 4) find preferences (or their lack) for natural or man-modified savanna of all coraciiform, piciform and psittaciform species breeding in a mosaic of natural and man-modified savanna; and 5) compare population densities of all these species recorded in the study area with those obtained in other regions of southern Africa. Methods: Population densities of coraciiform, psittaciform and piciform species were assessed by means of the territory mapping method in an acacia savanna in the Cuvelai Drainage System, north-central Namibia. Studies were carried out in three separate years: 2012, 2017 and 2020. Results: In total, 15 species representing these orders were recorded as breeding in the study plot (400 ha). In 2020, all three parrot species, namely Meyer’s Parrot (Poicephalus meyeri), Rüppell’s Parrot (P. rueppellii), and Rosy-faced Lovebird (Agapornis roseicollis), nested in a similar density of 0.50–0.75 pairs per 100 ha. The African Hoopoe (Upupa epops) was the most abundant coraciiform species reaching a density of 3.3 pairs per 100 ha in 2017. The second most abundant coraciiform species was the Lilac-breasted Roller (Coracias caudatus), which population increased from 0.5 pairs per 100 ha in 2012 to 1.4 pairs per 100 ha in 2020. Among the other coraciiform species, only the Common Scimitar-bill (Rhinopomastus cyanomelas) reached a density higher than one pair per 100 ha, in 2017. Among piciform species the most abundant was the Acacia Pied Barbet (Tricholaema leucomelas; up to 2.5 pairs per 100 ha). Conclusion: Contrary to expectation, for most species and species groups studied, their population densities were negatively related to the precipitation. No other species of this order reached a density higher than 1 pair per 100 ha in any year. These population densities are compared with those obtained in 10 sites in Namibia and the South African Highveld. Lilac-breasted Roller, Common Scimitarbill, parrots and hornbills showed high, while barbets showed low site fidelity.

The community structure and population density of birds was studied at four sites in Acacia savanna in the lowveld of Swaziland. Birds were surveyed monthly between June 1998 and May 1999 using a standard point-count technique. A total of 128 species of birds was recorded in Acacia savanna (of these, population estimates were calculated for 107 species). Species richness was similar at the four sites, as was total bird population density (when the highly mobile finches and canaries were excluded from calculations). Bird densities increased from winter to summer and then declined in autumn. These seasonal changes in density are expected, and are due to an influx of migrants and newly-fledged offspring in late spring and summer. In contrast to species richness and population density, species composition varied between the four areas. This finding suggests that Acacia savanna is not as homogenous as commonly thought. The conservation implications of this are discussed.

ABSTRACTDue to frequent fire, low nutrient availability, and prolonged drought, tropical savanna is a stressful environment for the survival and growth of woody plant seedlings. To understand why forest species do not succeed in this environment while savanna species are able to persist, the effects of fire and woody cover on seedlings of these two functional groups were investigated in the Brazilian Cerrado. Seedlings were established in experimental plots under three densities of woody cover, in sites protected from fire and sites to be subjected to fire. There was a clear difference in the ability of savanna and forest species to survive fire. None of the three forest species were able to survive fire during the first two years of life, whereas eight of the nine savanna species were able to resprout following fire. The small seed size of the ninth savanna species, Miconia albicans, predisposed its seedlings to be sensitive to fire, because there was a strong positive correlation between seed size and survivorship. Savanna species were less dependent on woody cover than were forest species, which exhibited higher growth and survival under tree canopies than in open grassland. The low rates of establishment and survival of forest trees in savanna, combined with high sensitivity to fire, appear sufficient to prevent the expansion of forest into savanna under current fire regimes in the Cerrado.

Forest expansion into savannas is widespread even though fire and seasonal drought provide environmental conditions against encroachment by forest specialists. A distinct suite of species can establish under savanna trees, forming bush clumps and facilitating forest establishment. Understanding the functional traits of clump-forming species is crucial for uncovering encroachment mechanisms and devising management strategies. Bark likely plays a key role in enabling clump initiation. Fire resistance can be achieved by accumulation of outer bark thickness (OBT), height and/or stem diameter (SD), while drought resistance may be enhanced by greater inner bark thickness (IBT), associated with water and carbohydrate storage. We selected representative savanna, clump-forming and closed-canopy species (ecological categories) at two South African sites experiencing forest expansion and differing in rainfall and fire frequency. We compared OBT-SD and IBT-SD allometries across ecological categories and sites and examined whether categories separated along axes reflecting fire/drought resistance (OBT and IBT) and resource allocation strategy (density and water content, leaf size). OBT-SD scaling of clump-forming species was more similar to savanna than forest species, and savanna species at the more fire-prone savanna had steeper OBT-SD scaling, consistent with high OBT providing fire protection in early clump formation. Similar IBT-SD slope across groups was consistent with similar metabolic needs, while higher intercepts in savanna and clump-forming species indicated higher water storage. 'Cheap' low-density tissues in savanna species allow fast accumulation of SD and OBT and resistance to fire topkill. Closed-canopy species had denser tissues and thin stems and bark for a given height, while the clump-forming species were intermediate. Bark and probably other traits are key in the capacity of some species to form bush clumps. Identifying these traits and the mechanisms underlying clump formation is essential for managing encroached savannas and grasslands.

Large baobabs are regarded as key plant species in the savannah biome. In this study their role in shaping the avian community has been evaluated. The territory mapping method has been employed to quantify the avian breeding community in the town Outapi (ca. 130 ha), Northern Namibia, well-endowed with large baobabs. A total of 29 breeding bird species were recorded. The alien House Sparrow was by far the most numerous species comprising 48.4% of all breeding birds. The Blue Waxbill and African Palm Swift were also classified as dominant species, comprising together 17.4%. Granivores were by far the most numerous feeding guild, comprising 77.2% of all birds breeding, while the insectivores comprised only 11.8%. Birds nesting in/on buildings comprised 52.7%, those nesting on trees/shrubs – 42.0%. The population densities of many bird species (e.g. doves from the genus Stepropelia, bulbuls, weavers) were comparatively low. Such a situation could have been caused by the exceptionally high population density of the Pied Crow. In the study area, there were 15 breeding pairs and about 100–150 non-breeding individuals. The baobabs may provide them with a feeding resource in the form of the sweet fruits. The crows, as nest predators, feed also on the eggs and nestlings of other birds. Doves seem to be especially prone to such predation, as their nests are easily to detect and destroy. As a result, they have to breed in low density. This in turn, may release other dominant granivores in the study area, namely sparrows. They are not vulnerable to crow nest predation, as their nests are usually well-concealed under eaves and in holes of buildings.

A large number of herbaceous and woody plants from tropical woodland, savanna, and monsoon forest were analysed to determine the impact of environmental factors (nutrient and water availability, fire) and biological factors (microbial associations, systematics) on plant delta(15)N values. Foliar delta(15)N values of herbaceous and woody species were not related to growth form or phenology, but a strong relationship existed between mycorrhizal status and plant delta(15)N. In woodland and savanna, woody species with ectomycorrhizal (ECM) associations and putative N(2)-fixing species with ECM/arbuscular (AM) associations had lowest foliar delta(15)N values (1.0-0.6 per thousand ), AM species had mostly intermediate delta(15)N values (average +0.6 per thousand ), while non-mycorrhizal Proteaceae had highest delta(15)N values (+2.9 to +4.1 per thousand ). Similar differences in foliar delta(15)N were observed between AM (average 0.1 and 0.2 per thousand ) and non-mycorrhizal (average +0.8 and +0.3 per thousand ) herbaceous species in woodland and savanna. Leguminous savanna species had significantly higher leaf N contents (1.8-2.5% N) than non-fixing species (0.9-1.2% N) indicating substantial N acquisition via N(2) fixation. Monsoon forest species had similar leaf N contents (average 2.4% N) and positive delta(15)N values (+0.9 to +2.4 per thousand ). Soil nitrification and plant NO(3)(-) use was substantially higher in monsoon forest than in woodland or savanna. In the studied communities, higher soil N content and nitrification rates were associated with more positive soil delta(15)N and plant delta(15)N. In support of this notion, Ficus, a high NO(3)(-) using taxa associated with NO(3)(-) rich sites in the savanna, had the highest delta(15)N values of all AM species in the savanna. delta(15)N of xylem sap was examined as a tool for studying plant delta(15)N relations. delta(15)N of xylem sap varied seasonally and between differently aged Acacia and other savanna species. Plants from annually burnt savanna had significantly higher delta(15)N values compared to plants from less frequently burnt savanna, suggesting that foliar (15)N natural abundance could be used as marker for assessing historic fire regimes. Australian woodland and savanna species had low leaf delta(15)N and N content compared to species from equivalent African communities indicating that Australian biota are the more N depauperate. The largest differences in leaf delta(15)N occurred between the dominant ECM Australian and African savanna (miombo) species, which were depleted and enriched in (15)N, respectively. While the depleted delta(15)N of Australian ECM species are similar to those of previous reports on ECM species in natural plant communities, the (15)N-enriched delta(15)N of African ECM species represent an anomaly.

Summary The cultivation and dissemination of alien ornamental plants increases their potential to invade. More specifically, species with bird‐dispersed seeds can potentially infiltrate natural nucleation processes in savannas. To test (i) whether invasion depends on facilitation by host trees, (ii) whether propagule pressure determines invasion probability, and (iii) whether alien host plants are better facilitators of alien fleshy‐fruited species than indigenous species, we mapped the distribution of alien fleshy‐fruited species planted inside a military base, and compared this with the distribution of alien and native fleshy‐fruited species established in the surrounding natural vegetation. Abundance and diversity of fleshy‐fruited plant species was much greater beneath tree canopies than in open grassland and, although some native fleshy‐fruited plants were found both beneath host trees and in the open, alien fleshy‐fruited plants were found only beneath trees. Abundance of fleshy‐fruited alien species in the natural savanna was positively correlated with the number of individuals of those species planted in the grounds of the military base, while the species richness of alien fleshy‐fruited taxa decreased with distance from the military base, supporting the notion that propagule pressure is a fundamental driver of invasions. There were more fleshy‐fruited species beneath native Acacia tortilis than beneath alien Prosopis sp. trees of the equivalent size. Although there were significant differences in native plant assemblages beneath these hosts, the proportion of alien to native fleshy‐fruited species did not differ with host. Synthesis. Birds facilitate invasion of a semi‐arid African savanna by alien fleshy‐fruited plants, and this process does not require disturbance. Instead, propagule pressure and a few simple biological observations define the probability that a plant will invade, with alien species planted in gardens being a major source of propagules. Some invading species have the potential to transform this savanna by overtopping native trees, leading to ecosystem‐level impacts. Likewise, the invasion of the open savanna by alien host trees (such as Prosopis sp.) may change the diversity, abundance and species composition of the fleshy‐fruited understorey. These results illustrate the complex interplay between propagule pressure, facilitation, and a range of other factors in biological invasions.

A recurrent behavioral trait model to study adaptation to urban environments is the flight initiation distance (FID), measured as the distance at which animals flee from an approaching threat. It has previously been shown that urban birds display shorter FID than their non-urban (rural) counterparts. However, discerning whether this is the result of habituation to human presence and frequentation, or of ecological factors related to the size of the city (considered as “systemic habituation”), has not yet been addressed. In this study, we analyzed House Sparrow (Passer domesticus) FIDs in a network of 26 small towns and villages within the same region in northeastern Spain. Our aim was to relate FID to human population density and settlement size. If the habituation to human presence hypothesis was supported, we should expect FIDs to decrease with the density of the human population across the human settlements, since this type of habituation is related to the rate of human exposure and this is proportional to human density. However, if the systemic habituation hypothesis was supported, FIDs should instead relate to the size of the human settlements, as the abundance of predators, similarly to other ecological variables, is often proportional to the size of towns. Results showed House Sparrows to be bolder in larger human settlements, but not necessarily the ones with a higher density of human population. This supports the idea that the fact that urban birds display shorter FIDs than their rural counterparts is the result of systemic ecological factors rather than the results of a simple habituation to humans.

: Hunting in tropical forests is typically most intense near human settlements, and this creates gradients of decreasing animal densities toward those settlements. Within the context of this spatial pattern, we evaluated the status of game in the hunting grounds of an indigenous community in eastern Ecuador. We constructed a spatially explicit model of hunter‐prey interactions that mimicked the hunting in the village and included realistic animal‐dispersal rules. We compared predictions from the model with distributions of animal harvest rates and catch per unit effort of 12 game species. Six species were overharvested in part or all of the area, and two other species were probably being overharvested, although high dispersal rates complicated the interpretation. We then compared our method with methods that have been used previously. We argue that because our method provides information about the spatial extent of overharvesting, it could be particularly useful in informing decision makers about where to establish no‐take areas and could therefore aid in improving the sustainability of hunting in tropical forests.

Avian response to prolonged drought in drylands: The case of two dove species in Brazil's semi-arid region

Two main hypotheses predominate in the literature on mistletoe-host specificity: (1) mistletoes are only likely to specialize on plant species on which they are frequently deposited; and (2) compatibility between mistletoes and plant species is a prerequisite for mistletoe-host parasitism. I explored these hypotheses by studying the seed deposition patterns and mistletoe-host compatibility in populations of three congeneric and sympatric mistletoe species of the genus Psittacanthus (P. biternatus, P. eucalyptifolius and P. plagiophyllus - Loranthaceae). I recorded the presence or absence of these mistletoe species in 15 tree species in a savanna patch in Amazonia. Among the five tree species that I found to be potential hosts (at least one tree individual infected), I also recorded if they had at least one mistletoe seed of any species attached to their branches. Finally, I planted seeds of all mistletoe species on the same individual trees in various hosts and non-host species and recorded seed survivorship and seedling establishment within 7 (P. plagiophyllus) to 12 months (P. biternatus and P. eucalyptifolius) after planting. There was no overlap among trees used as hosts by the three Psittacanthus species. Th e most specialized mistletoe species occurred in different host tree species with low relative abundance at the study site (Psittacanthus eucalyptifolius on Vatairea macrocarpa (Benth.) Ducke, and P. plagiophyllus on Anacardium occidentale L.). Mistletoe-host compatibility, and not seed deposition patterns, was the factor most likely to explain patterns of host use by Psittacanthus species at this study site.

A comparative analysis of mopane woodland structure under different utilization pressure was done in Omusati region, northern Namibia. Heights, basal area, biomass and densities of trees, shrubs and stumps were compared between a game park, densely-populated (central) and sparsely-populated (western) areas. Heavy utilization significantly reduced tree basal area, biomass and plant densities in central Omusati, leading to an unsustainable situation. Woodland recovery from previous disturbance in the game park has been slow. Contrary to expectation, there were fewer dead stumps in central area because of continual harvesting due to firewood scarcity. Height structure differed significantly with more than expected numbers of plants in the ≤ 1 m, 1.1 to 2 m and less than expected plants in the 3.1 to 4 m and 4.1 to 5 m height classes in central Omusati. More than expected numbers of trees were recorded in 4.1 to 5 m height class in western area and in 2.1 to 3 m and 3.1 to 4 m height classes in Game Park. This indicates over-harvesting of medium-sized trees in central Omusati. Mopane regenerates profusely through coppicing. However, no relationship existed between stump size and number of shoots due to continual harvesting of shoots, especially in central area. Proper coppice management and pollarding strategies should be implemented to improve the situation. Key words: Colophospermum mopane, coppicing, Omusati, woodland structure, woodland utilization.

Both cuculiform and piciform species can be used as bioindicators and, to a lesser extent, as umbrella species. A territory mapping method was employed in 2015 to assess their population densities in a riparian forest on the Zambezi River near Katima Mulilo, NENamibia. The forest, c. 280 ha in surface area, was partly transformed by human settlement and croplands. A total of 9 cuculiform and 6 piciform species were recorded. Population densities (male’s territories per 100 ha) were assessed as follow: Diederick, Klaas’s and Jacobin Cuckoo — each one with 1.1, African Emerald and African Black Cuckoo — each one with 0.7, Red-chested Cuckoo — 0.4; Senegal Coucal — 4.3, White-browed and Coppery-tailed Coucals — each one with 0.7; Bearded Woodpecker — 1.8, Golden-tailed Woodpecker — 1.1, Cardinal Woodpecker — 0.4; Black-collared Barbet — 5.7, Crested Barbet — 0.7, and Yellow-fronted Tinkerbird — 1.8. Both cuculiform and woodpecker species avoided built-up areas in the man-modified riparian forest, while all barbet species appeared to prefer them. Chrysococcyx cuckoos are not territorial in respect to Cuculus/Clamator cuckoos. The latter ones are, however, territorial in respect to other species from the same genera. Coucals display territorialism in regard to members of other species of the genus Centropus.

Large decline of birds in Sahelian rangelands due to loss of woody cover and soil seed bank