Tritrophic effects of birds and ants on a canopy food web, tree growth, and phytochemistry

Insectivorous birds and ants co-occur in most terrestrial communities, and theory predicts that emergent properties (i.e., nonadditive effects) can determine their combined influence on arthropods and plants. In a three-year factorial experiment, I investigated whether the effects of birds on pine and its arthropods differed based on the presence of ants that were predators of most arthropods, but mutualists with tended aphid species. Birds and ants reduced the abundance of most herbivorous and carnivorous arthropods in an additive fashion, with the effects of ants being stronger than those of birds. In sharp contrast, the opposing influences of birds and ants on tended aphid species interacted strongly; ants only increased tended aphid abundance in the absence of birds, while birds only reduced their abundance in the presence of ants. This interaction was mirrored in total herbivore abundance because tended aphids dominated the herbivore community. I develop a novel lexicon to discuss the emergent properties from these effects of opposing sign (predation, mutualism). Despite having emergent effects on herbivores, birds indirectly increased pine wood and foliage growth to a similar extent whether or not ants were present, while ants had no detectable effects. Birds also indirectly increased the abundance of some pine phloem monoterpenes, but these effects differed based on the presence or absence of ants. Thus, I report on a novel yet possibly widespread indirect interaction between intraguild predators, herbivore mutualists, and plant traits (growth, secondary chemistry) mediated through a species-rich community of arthropods.     

Feeding capabilities and limitation of herbivorous molluscs: A functional group approach

The susceptibility of an alga to an herbivorous mollusc depends, in part, upon the size and toughness of the plant relative to the feeding ability of the mollusc. In this study, algae are subdivided into seven functional groups based on these and other physiological characteristics. Herbivorous prosobranchs and chitons are subdivided into four functional groups based on the structure of their feeding apparatus. Distinct patterns in the diets of these molluscs are evident when feeding data, based on these functional groups, are examined. Most herbivorous mollusc species eat algal forms that are either minute (i.e., micro- and filamentous algae) or very large and expansive (kelp-like or crustose algae). Algae of intermediate size (erect forms 1- to 10-cm tall) are eaten to a lesser extent, possibly because they are too large to be rasped from the substratum and too small for most herbivores to occupy. Herbivorous archaeogastropods (excluding limpets) and mesogastropods tend to eat filamentous and microscopic algal forms predominantly, whereas limpets and chitons feed on large, leathery and crustose algae. These dietary differences reflect functional differences in the feeding apparatus of these herbivore groups. Radulae of herbivorous mesogastropods function like rakes and can ingest larger, tougher algae than can radulae of nonlimpet archaeogastropods. The latter function more like brooms by sweeping the substratum broadly, but exerting little force. Limpets and chitons have superior excavating abilities because their radulae have: robust buccal muscles surrounding them, a reduced number of points of contact on the substratum, and minerally hardened teeth. The feeding apparatus of chitons is most versatile since it possesses features found in all herbivorous gastropod functional groups, and thus, it can sweep and excavate simultaneously. This functional group approach suggests various hypotheses concerning algal community structure, plant/herbivore and herbivore/herbivore interactions, the relative importance of structural defenses in algae, and the evolution of specialized grazers. These hypotheses are examined using data from published accounts.     

The importance of seed dispersal in the Alexandria Coastal Dunefield, South Africa

The endozoochorous dispersal of seeds by mammals and birds between distinct vegetation communities was assessed to determine the importance of these processes in coastal dune field management. Isolated pockets of thicket vegetation (bush-pockets) within a large coastal dune field provided the opportunity to study vertebrate seed dispersal and its contribution to their origin and maintenance. Mammalian and avian faeces were collected for the quantification of seeds dispersed via endozoochory. Birds and mammals showed considerable overlap, dispersing intact seeds of 17 and 29 plant species, respectively, but mammals dispersed a greater diversity and size range than birds. Extrapolation of mammalian faecal data indicates an annual input of 23 million intact seeds to the dune field. Significantly more seeds are deposited by mammals and birds in the bush-pockets than on open sand, and birds deposited greater numbers of seeds nearer the seed source. Zoochory appears to be critical for the maintenance of the bush-pocket habitats through the dispersal of climax woody plant species into the dune field. Directional dispersal by birds and mammals to the bush-pockets is considered to be responsible for the maintenance and possible origin of these bush-pockets. The high number of exotic plant propagules dispersed by both avian and mammalian zoochory highlights the importance of management of the Alexandria Coastal Dunefield (ACD) beyond the reserve boundaries. In a dynamic system such as the ACD which is within a declared nature reserve, the continued existence of the bush-pockets may depend on the maintenance, beyond the reserve boundaries, of a reservoir of not only plant material but vertebrate dispersers as well.     

Movement of foraging Tundra Swans explained by spatial pattern in cryptic food densities

We tested whether Tundra Swans use information on the spatial distribution of cryptic food items (below ground Sago pondweed tubers) to shape their movement paths. In a continuous environment, swans create their own food patches by digging craters, which they exploit in several feeding bouts. Series of short (<1 m) intra-patch movements alternate with longer inter-patch movements (>1 m). Tuber biomass densities showed a positive spatial auto-correlation at a short distance (<3 m), but not at a larger distance (3-8 m). Based on the spatial pattern of the food distribution (which is assumed to be pre-harvest information for the swan) and the energy costs and benefits for different food densities at various distances, we calculated the optimal length of an inter-patch movement. A swan that moves to the patch with the highest gain rate was predicted to move to the adjacent patch (at 1 m) if the food density in the current patch had been high (>25 g/m2) and to a more distant patch (at 7-8 m) if the food density in the current patch had been low (<25 g/m2). This prediction was tested by measuring the response of swans to manipulated tuber densities. In accordance with our predictions, swans moved a long distance (>3 m) from a low-density patch and a short distance (<3 m) from a high-density patch. The quantitative agreement between prediction and observation was greater for swans feeding in pairs than for solitary swans. The result of this movement strategy is that swans visit high-density patches at a higher frequency than on offer and, consequently, achieve a 38% higher long-term gain rate. Swans also take advantage of spatial variance in food abundance by regulating the time in patches, staying longer and consuming more food from rich than from poor patches. We can conclude that the shape of the foraging path is a reflection of the spatial pattern in the distribution of tuber densities and can be understood from an optimal foraging perspective.     

Avian Diversity and Functional Insectivory on North-Central Florida Farmlands

Abstract:  We studied the potential for native birds to control insect pests on farms. We assessed habitat factors correlated with diversity, distribution, and insect-foraging activity of native birds on farms in north-central Florida and then characterized common bird species that consumed insect biomass in crops as "functional insectivores" (birds most likely to contribute to pest control). Analyses of point-count survey data and foraging observations collected over 2 years on paired organic and conventional farm sites indicated that (1) farms supported most (82–96%) land birds known to breed in the region; (2) bird species richness and abundance varied significantly with matrix habitat and field border type (but not with year or farm management type); (3) the highest bird abundances were associated with mixed crop plantings, field borders, and adjacent matrix composed of forest and hedge; and (4) abundances of 10 species identified as functional insectivores were primarily influenced by crop type (mixed crops attracted significantly more insect foragers into fields than monocrops). We documented birds eating pest insects in crops and did not observe substantive crop damage by birds during growing-season observations. We advocate use of the term functional insectivore to emphasize the potential positive role of avian insectivory on farms during the growing season.     

Dominance and the dynamics of phenotype-limited distribution in common cranes

We studied the behavior of 13 radiotagged cranes dispersing from a communal roost over days when they changed their main daily foraging area between consecutive days during two winter seasons. Individuals went to a new foraging zone when on the previous day their morning food intake had fallen below their mean morning food intake measured over the whole winter. Food intake on the day before a change in foraging area was positively correlated with dominance rank. Dominant cranes changed to new zones with higher numbers of birds and food density, while subordinate cranes went to new zones with lower numbers of birds. As a result, all birds increased their food intake over that of the previous day. Dominant cranes remained more faithful to their most preferred foraging zone, where they spent 69% of the mornings, while subordinate birds were more mobile, switching among zones frequently. Dominant birds left the roost later than subordinate birds on the days they changed to a new zone, which could be used to track the main departing flows. The results suggest that the dynamics that led to a truncated phenotype-limited distribution were determined by social dominance and food abundance, with dominant cranes shifting to a new zone to maintain their high intake levels and subordinates changing more frequently whenever their daily intake did not reach the minimum metabolic requirements. Received: 16 December 1996 / Accepted after revision: 22 February 1997     

Why do plants ‘talk’?

Summary Plant defence can be induced by herbivory. This is true for both direct and indirect plant defence. Induced direct defence has been the most studied of the two. However, in most cases induced direct defence does not appear to be a water-tight defence option. In contrast, induced indirect defence through the production of herbivore-induced carnivore attractants can be a decisive factor in the extermination of herbivore populations. In this paper the main characteristics of induced attraction of carnivores by plants are reviewed. This includes the similarities and dissimilarities among tritrophic systems. There are two main patterns of induced carnivore attraction. (1) Through the emission of the same bouquet as that emitted by mechanically damaged plants, but in larger quantities and for a longer period of time after damage. (2) Through emission of large amounts of new volatiles that are synthesizedde novo in response to herbivore feeding andnot in response to mechanical wounding.Herbivore populations may be decimated by carnivores. Therefore it should be realized that herbivoreinduced carnivore attractants are essential in an important step in carnivore foraging,i.e. long-distance herbivore location. Once herbivores have started feeding on a plant and direct defence is not effective, induced indirect defence may be decisive for plant survival. Therefore, it is concluded that indirect defence is an essential aspect of induced plant defence directed at herbivorous arthropods.     

Foraging in a tidally structured environment by Red Knots (Calidris canutus): ideal, but not free

Besides the "normal" challenge of obtaining adequate intake rates in a patchy and dangerous world, shorebirds foraging in intertidal habitats face additional environmental hurdles. The tide forces them to commute between a roosting site and feeding grounds, twice a day. Moreover, because intertidal food patches are not all available at the same time, shorebirds should follow itineraries along the best patches available at a given time. Finally, shorebirds need additional energy stores in order to survive unpredictable periods of bad weather, during which food patches are covered by extreme tides. In order to model such tide-specific decisions, we applied stochastic dynamic programming in a spatially explicit context. Two assumptions were varied, leading to four models. First, birds had either perfect (ideal) or no (non-ideal) information about the intake rate at each site. Second, traveling between sites was either for free or incurred time and energy costs (non-free). Predictions were generated for three aspects of foraging: area use, foraging routines, and energy stores. In general, non-ideal foragers should feed most intensely and should maintain low energy stores. If traveling for such birds is free, they should feed at a random site; otherwise, they should feed close to their roost. Ideal foragers should concentrate their feeding around low tide (especially when free) and should maintain larger energy stores (especially when non-free). If traveling for such birds is free, they should feed at the site offering the highest intake rate; otherwise, they should trade off travel costs and intake rate. Models were parameterized for Red Knots (Calidris canutus) living in the Dutch Wadden Sea in late summer, an area for which detailed, spatially explicit data on prey densities and tidal heights are available. Observations of radio-marked knots (area use) and unmarked knots (foraging routines, energy stores) showed the closest match with the ideal/non-free model. We conclude that knots make state-dependent decisions by trading off starvation against foraging-associated risks, including predation. Presumably, knots share public information about resource quality that enables them to behave in a more or less ideal manner. We suggest that our modeling approach may be applicable in other systems where resources fluctuate in space and time.     

The disruption of an ant-aphid mutualism increases the effects of birds on pine herbivores

Predators affect herbivores directly and indirectly, by consumptive and nonconsumptive effects, and the combined influence of multiple predators is shaped by interactions among predators. I documented the individual and combined effects of birds (chickadees, nuthatches, warblers) and ants (Formica podzolica) on arthropods residing in pine (Pinus ponderosa) canopies in a factorial field experiment. Birds and ants removed herbivores but simultaneously benefited them by removing predatory arthropods. Birds and ants had net negative and positive effects, respectively, on the abundance of herbivore prey, supporting the notion that vertebrate predators have stronger negative effects on herbivores than do arthropod predators. Aphids (ant-tended and untended species) constituted three-quarters of herbivore biomass. The effect of birds on ant-tended aphids was twice that on untended aphid species or tended aphid species without ants. This was not due to there being more ant-tended aphids for birds to prey on; tended and untended aphid species were in similar abundances in the absence of birds. Instead, the effects of birds were strengthened by attributes of the mutualism that rendered tended aphids susceptible to predation. These dynamics led to nonadditive effects of birds and ants: birds only reduced tended aphid species and total herbivore abundances on trees with ants, while ants only increased tended aphid species and total herbivore abundances in the absence of birds. Consequently, top predators in this system only influenced total herbivore abundance when they disrupted an ant-aphid mutualism.     

多齿马先蒿与刺齿马先蒿花部形态、昆虫访花频率及种子产量比较研究

若尔盖高原的马先蒿属植物花部形态多样化,为探讨花冠差异是否与不同的繁殖对策相联系,在该区四川红原县分别对短管无喙的多齿马先蒿(Pedicularis polyodonta)和长管具喙的刺齿马先蒿(P.armata)的花部形态、昆虫访花频率和种子产出进行了比较研究.多齿马先蒿的花冠呈"鸟头"状,多个花的集合与鸟群相似,因此特别检测了这种拟态是否具有规避鸟类和蝗虫或吸引蜂类传粉的功能.研究发现,与刺齿马先蒿相比,多齿马先蒿的花冠较小、数量较少,但花蜜含量较高,因此蜂类访问频率较高,结实率高.两种马先蒿的单花种子产量没有显著差异;多齿马先蒿单个个体的种子数量较多,但种子较小.研究表明,为提高繁殖成功率,多齿马先蒿倾向于依赖花质量(如花蜜含量所指示)和种子数量,而刺齿马先蒿倾向于依赖花数量.蝗虫对两种马先蒿的危害没有显著差异,说明多齿马先蒿的"鸟头"拟态不具有规避蝗虫的功能;在具有"鸟头"花冠的多齿马先蒿的样地,鸟类的拜访频率较刺齿马先蒿显著较少,但蜂类的访问频率显著较高,说明"鸟头"拟态可能与规避鸟类、保护传粉者有关,其拟态功能需要进一步实验验证.图2表1参37     

Males are faster foragers than females: intersexual differences of foraging behaviour in the Apennine chamois

Availability of food resources and individual characteristics can influence foraging behaviour, which can differ between males and females, leading to different patterns of food/habitat selection. In dimorphic species, females are usually more selective in food choice, show greater bite rates and spend more time foraging than males. We evaluated sexual differences in foraging behaviour in Apennine chamois Rupicapra pyrenaica ornata, during the warm season, before the rut. Both sexes selected nutritious vegetation patches and spent a comparable amount of time feeding. However, males had a significantly greater feeding intensity (bite rate) and a lower search effort for feeding (step rate), as well as they spent more time lying down than females. Females selected foraging sites closer to refuge areas than males. In chamois, sexual size dimorphism is seasonal, being negligible in winter–spring, but increasing to 30–40 % in autumn. Our results suggest that males enhance their energy and mass gain by increasing their food intake rate during the warm season, to face the costs of the mating season (November). Conversely, females seem to prioritize a fine-scale selection of vegetation and the protection of offspring. A great food intake rate of males in the warm season could have developed as a behavioural adaptation leading herbivores to the evolutionary transition from year-round monomorphism to permanent dimorphism, through seasonal dimorphism.     

Migratory fuelling in blackcaps (Sylvia atricapilla) under perceived risk of predation

It has been argued that the body mass levels achieved by birds are determined by the trade-off between risks of starvation and predation. Birds have also been found to reduce body mass in response to an increased predation risk. During migration, the need of extra fuel for flights is obvious and crucial. In this study, migratory blackcaps (Sylvia atricapilla) were subject to an experimental stopover situation where the predation risk was manipulated by exposure to a stuffed predator. Blackcaps that perceived an imminent risk of predation increased their food intake and fuel deposition rate during the first period of stopover compared with a control group. The pattern of night activity indicates that birds that were exposed to the predator also chose to leave earlier than birds in the control group. Since there was no cover present at the stopover site, birds might have perceived the risk of predation as higher regardless of whether they were foraging or not. Under such circumstances it has been predicted that birds should increase their foraging activity. The findings in this study clearly indicate that birds are able to adjust their stopover behaviour to perceived predation risk. Received: 8 January 1997 / Accepted after revision: 11 April 1997     

Effects of forest fragmentation on avian breeding activity

Biodiversity declines and ecosystem decay follow forest fragmentation; initially, abundant species may become rare or be extirpated. Underlying mechanisms behind delayed extirpation of certain species following forest fragmentation are unknown. Species declines may be attributed to an inadequate number of breeding adults required to replace the population or decreased juvenile survival rate due to reduced recruitment or increased nest predation pressures. We used 10 years of avian banding data, 5 years before and 4 years after fragment isolation, from the Biological Dynamics of Forest Fragments Project, carried out near Manaus, Brazil, to investigate the breeding activity hypothesis that there is less breeding activity and fewer young after relative to before fragment isolation. We compared the capture rates of active breeding and young birds in 3 forest types (primary forest, fragment before isolation, and fragment after isolation) and the proportion of active breeding and young birds with all birds in each unique fragment type before and after isolation. We grouped all bird species by diet (insectivore or frugivore) and nesting strategy (open cup, cavity, or enclosed) to allow further comparisons among forest types. We found support for the breeding activity hypothesis in insectivorous and frugivorous birds (effect sizes 0.45 and 0.53, respectively) and in birds with open-cup and enclosed nesting strategies (effect sizes 0.56 and 0.44, respectively) such that on average there were more breeding birds in fragments before isolation relative to after isolation. A larger proportion of birds in the community were actively breeding before fragment isolation (72%) than after fragment isolation (11%). Unexpectedly, there was no significant decrease in the number of young birds after fragment isolation, although sample sizes for young were small (n = 43). This may have been due to sustained immigration of young birds to fragments after isolation. Together, our results provide some of the strongest evidence to date that avian breeding activity decreases in response to fragment isolation, which could be a fundamental mechanism contributing to ecosystem decay.     

The distribution of herbivorous fishes on the Great Barrier Reef

The composition and functionality of ecologically important herbivorous fish assemblages were examined throughout much of Australia’s Great Barrier Reef (GBR). Diversity and abundance of surgeonfishes (Acanthuridae), parrotfishes (Labridae) and rabbitfishes (Siganidae) were strongly associated with position on the continental shelf, whilst effects of latitude were weaker and inconsistent. Species distributions varied considerably amongst taxonomic groups; parrotfishes were mostly widespread whilst distributions of surgeonfishes were often restricted. Most inshore environments supported depauperate herbivore assemblages dominated by different taxa and functional groups compared with assemblages in offshore environments. There were also strong cross-shelf transitions in the main taxa performing each functional role. Overall, this study show that the functional contributions of herbivorous fish assemblages to important ecosystem processes and the contributing taxa vary considerably amongst different GBR environments. Additionally, the two most numerically dominant herbivores actively select detritus, not algae, supporting increasing evidence for the importance of detritus in coral reef ecology.     

Birds and bats reduce insect biomass and leaf damage in tropical forest restoration sites

Both birds and bats are important insect predators in tropical systems. However, the relative influence of birds and bats on insect populations and their indirect effects on leaf damage have not previously been investigated in tropical forest restoration sites. Leaf damage by herbivorous insects can negatively affect the growth and survival of tropical plants and thus can influence the success of tropical forest restoration efforts. We used an exclosure experiment to examine the top-down effects of birds and bats on insects and leaf damage in a large-scale forest restoration experiment. Given the potential influence of tree planting design on bird and bat abundances, we also investigated planting design effects on bird and bat insectivory and leaf damage. The experiment included two planting treatment plots: islands, where trees were planted in patches, and plantations, where trees were planted in rows to create continuous cover. In both types of plots, insect biomass was highest on tree branches where both birds and bats were excluded from foraging and lowest on branches without exclosures where both birds and bats were present. In the island plots, birds and bats had approximately equal impacts on insect populations, while in plantations bats appeared to have a slightly stronger effect on insects than did birds. In plantations, the levels of leaf damage were higher on branches where birds and bats were excluded than on branches where both had access. In island plots, no significant differences in leaf damage were found between exclosure treatments although potential patterns were in the same direction as in the plantations. Our results suggest that both birds and bats play important roles as top predators in restoration systems by reducing herbivorous insects and their damage to planted trees. Tropical restoration projects should include efforts to attract and provide suitable habitat for birds and bats, given their demonstrated ecological importance.     

Strike-induced chemosensory searching is absent in the cordylid lizard,Cordylus cordylus,a scleroglossan ambush forager

Summary Strike-induced chemosensory searching (SICS) was not detected experimentally in the cordylid lizard,Cordylus cordylus. Both components of SICS, a post-strike elevation in tongue-flick rate (PETF) and searching movements for attacked and released prey, were absent. The findings are consistent with previous data showing that PETF and/or SICS are lacking in all lizard families yet studied that forage primarily by ambush, but are present in actively foraging scleroglossan families and the herbivorous iguanian family Iguanidae. It is suggested that foraging behavior is a primary determinant of the presence or absence of SICS in lizards. Nevertheless, in most families in the two major clades, Iguania and Scleroglossa, the plesiomorphic foraging mode is retained. The findings agree with the prediction that SICS is absent in families lacking lingually mediated prey chemical discrimination (PCD), presumably due to selection against movement by ambush foragers that avoid being detected by either prey or predators because they remain motionless. Although PETF and SICS were absent, labial-licking and lingual movements similar to those observed after swallowing increased after biting prey, suggesting that the functions of these lingual movements may have been related to grooming. Locomotory movements did not increase following biting and appeared to represent avoidance of the experimenter.     

Trophic segregation between sexes in the Black Skimmer revealed through the analysis of stable isotopes

The degree of individual or gender variation when exploiting food resources is an important aspect in the study of foraging ecology within a population. Previous information on non-breeding skimmers obtained through conventional methodologies suggested sex-related differences in prey species. In this study, stable isotope techniques were used to investigate the intraspecific segregation in diet and foraging habits of the Black Skimmer (Rynchops niger intercedens) at Mar Chiquita Coastal Lagoon (37°40′S, 57°22′W), Argentina. These results were compared with contemporary data on the trophic composition obtained by conventional methodologies. Blood samples were taken from birds captured with mist-nets during their non-breeding season. The isotopic signatures of skimmers showed a diet mainly composed of marine prey with some degree of estuarine fish intake. When comparing diet between sexes, males showed enrichment in ¹⁵N compared to females, while no differences were observed in ¹³C. The use of mixing models revealed differences in the relative composition of prey in the diet of male and female skimmers. This study highlights stable isotope analysis as a valuable tool to test inter-individual differences and sexual segregation in trophic ecology of Black Skimmers as compared to conventional methodologies. The results show a trophic segregation in the Black Skimmer during the non-breeding season that can be explained by differences in prey species and larger prey sizes of male skimmers. Our findings have significant implications for conservation since any environmental change occurring at wintering areas might have profound effects on several avian life-history traits, and could be different for males and females due to trophic segregation.     

Persistent bimodal activity patterns in wild and captive black-tailed godwit <Emphasis Type="Italic">Limosa limosa</Emphasis> under different environmental conditions

There is a large literature dealing with daily foraging routines of wild birds during the non-breeding season. While different laboratory studies have showed that some bird activity patterns are a persistent property of the circadian system, most of field studies preclude the potential role of an endogenous circadian rhythm in controlling bird’s foraging routines. In this study we compared the patterns of diurnal foraging activity and intake rates of migrating black-tailed godwits, Limosa limosa (radio-tagged and non-tagged individuals) at two stopover sites (habitats) with different environmental characteristics, aiming at identifying proximate factors of bird activity routines. To gain insights into the role of food availability in control of such foraging routines, we also estimated foraging activity patterns in captive godwits subjected to constant food availability. Captive and wild black-tailed godwits showed a persistent bimodal activity pattern through daylight period. Food availability had a significant effect on the intake rates, but had a subtler effect on foraging and intake rate rhythms. Temperature and wind speed (combined in a weather index) showed non-significant effects on both rhythms. Although we could not discard a role for natural diurnal changes in light intensity, an important timing cue, our findings support the idea that an endogenous circadian rhythm could be an important proximate factor regulating foraging activity and food items taken per unit time of wild black-tailed godwits during migration.     

Birds defend oil palms from herbivorous insects.

Top-down forces are expected to be important in regulating herbivore populations in most agricultural systems where primary productivity is high and species diversity is low. Under such conditions, trophic cascades are predicted to occur when predator populations are reduced or removed. Studies on how predator removal indirectly affects herbivory rates in agricultural systems are lacking. Through a bird-exclosure experiment, I test the hypothesis that insectivorous birds indirectly defend oil palms (Elaeis guineensis) from herbivorous insects. Results show that bird exclusion significantly increased herbivory damage to oil palms, and that the size of this exclusion effect increased with bird density, although the latter result was not statistically significant. These findings suggest that insectivorous birds deliver a natural pest control service for oil palm agriculture, which is important not only for the direct benefits it delivers for human welfare, but also in strengthening the economic justifications for conserving the remaining natural habitats and biodiversity in agricultural landscapes.