Can seed-eating birds exert top-down effects on grasses of the Monte desert?

Effects of livestock grazing on flocks of seed-eating birds in the central Monte desert, Argentina

Animal populations often decline due to habitat disturbance, but the initial response of organisms to human-induced environmental change is usually behavioral. Intra- and inter-specific interactions can restrict or facilitate access to resources, resulting in changes to individual fitness, and resource depletion may affect the frequency and strength of interactions. In birds, it is often assumed that feeding in groups increases foraging efficiency. We assessed how the reduction of seed resources provoked by cattle grazing affected different properties of seed-eating bird flocks in woodlands having the same structural characteristics but differing in seed abundance. Under lower availability of grass seeds (i.e., under grazing), flocks were smaller and less rich and birds showed a lower flocking propensity. This pattern could be explained by three non-exclusive hypotheses. Food reduction caused by grazing (i) decreases the number of seed-eating birds and concomitantly generates smaller flocks; (ii) reduces the density of nuclear species, decreasing the group cohesion in large flocks; (iii) makes large flocks less attractive by increasing individual competence for food. Our results provide evidence that cattle grazing affect the interactions of seed-eating birds and suggest the importance of understanding flocking behavior to bring about management actions.

Are all patches worth exploring? Foraging desert birds do not rely on environmental indicators of seed abundance at small scales

Background

Consumers should show strong spatial preferences when foraging in environments where food availability is highly heterogeneous and predictable. Postdispersal granivores face this scenario in most arid areas, where soil seed bank abundance and composition associates persistently with vegetation structure at small scales (decimetres to metres). Those environmental features should be exploited as useful pre-harvest information, at least to avoid patches predicted to be poor. However, we did not find the expected spatial association in the algarrobal of the central Monte desert by observing foraging seed-eating birds, a field technique influenced by how much they exploit visited patches. In this work we tested if the first stage of foraging by granivorous birds (patch visit, encounter or exploration) is positively associated with environmental indicators of patch quality by recording the removal of single seeds from 300 scattered experimental devices during seasonal trials. Spatial selectivity was analysed by comparing the structural characteristics of used vs. available microhabitats, and evaluated against bottom-up and top-down hypotheses based on our previous knowledge on local seed bank abundance, composition and dynamics. Their foraging activity was also explored for spatial autocorrelation and environmental correlates at bigger scales.

Results

Postdispersal granivorous birds were less selective in their use of foraging space than expected if microhabitat appearance were providing them relevant information to guide their search for profitable foraging patches. No microhabitat type, as defined by their vegetation structure and soil cover, remained safe from bird exploration. Analyses at bigger temporal and spatial scales proved more important to describe heterogeneity in seed removal.

Conclusions

Closeness to tall trees, probably related to bird territoriality and reproduction or to their perception of predation risk, seemed to determine a first level of habitat selection, constraining explorable space. Then, microhabitat openness (rather than seed abundance) exerted some positive influence on which patches were more frequently visited among those accessible. Selective patterns by birds at small scales were closer to our predictions of a top-down spatial effect, with seed consumption creating or strengthening (and not responding to) the spatial pattern and dynamics of the seed bank.

Electronic supplementary material

The online version of this article (10.1186/s12898-019-0242-z) contains supplementary material, which is available to authorized users.

Aliens will provide: avian responses to a new temporal resource offered by ornithocorous exotic shrubs

Frugivorous birds are able to track spatiotemporal changes in fruit availability. Food resource fluctuations, characteristic of seasonal environments, can be affected by the naturalization of exotic ornithocorous plants. In the mountain forest of central Argentina, invasive shrubs of the genus Pyracantha provide a new temporal resource that modifies fluctuations of natural resource availability because the invasives fructify in autumn-winter (largely uncoupled with the fruiting of native species). The contrasting patterns of resource fluctuation between non-invaded and invaded areas throughout the year provide a good study system to test predictions of the fruit-tracking hypothesis, and to understand the relationship between food resources offered by fleshy fruited invasives and abundances of avian trophic guilds. By means of point counts conducted during five time periods at invaded and non-invaded sites we found that the presence of Pyracantha, and time periods, significantly affected frugivorous bird abundance, which in autumn-winter was greater in invaded sites and in spring-summer similar between invaded and non-invaded sites. On the other hand, granivores and insectivores did not show a significant relationship with the presence of Pyracantha. Abundances of the most common seed disperser were significantly affected by the interaction between time period and presence of Pyracantha. These results indicate that the abundances of birds that legitimately disperse Pyracantha seeds are temporally and spatially associated with fruit abundance provided by this exotic plant. This underscores fruit availability as an important ecological factor affecting frugivorous bird abundance, and suggests that Pyracantha seed dispersers are capable of detecting changes in the availability of its fruit, likely contributing to the effectiveness of its dispersal.

Molecular analysis of stomach contents reveals important grass seeds in the winter diet of Baird's and Grasshopper sparrows, two declining grassland bird species

We analyzed the diet of Baird's Sparrow (Ammodramus bairdii) and Grasshopper Sparrow (A. savannarum) in three different sites and sampling periods across the Chihuahuan Desert in northern Mexico. DNA from seeds in regurgitated stomach contents was sequenced using NGS technology and identified with a barcoding approach using the P6 loop of the trnL intron as genetic marker. During each sampling period, we collected random soil samples to estimate seed availability in the soil seed bank. Due to the variability and size of the genetic marker, the resolution was limited to a family level resolution for taxonomic classification of seeds, but in several cases a genus level was achieved. Diets contained a high diversity of seeds but were dominated by a limited number of genera/families. Seeds from Panicoideae (from the genera Panicum, Setaria, Eriochloa, Botriochloa, and Hackelochloa) contributed for the largest part to the diets (53 ± 19%), followed by Bouteloua (10 ± 12%). Depending on the site and sampling period, other important seeds in the diets were Eragrostideae, Pleuraphis, Asteraceae, Verbena, and Amaranthus. The most abundant seeds were not always preferred. Aristida and Chloris were common in the soil seed bank but these seeds were avoided by both bird species. Baird's and Grasshopper sparrows did not differ in seed preferences. This work highlights the importance of range management practices that favor seed production of Panicoideae and Bouteloua grasses to enhance winter habitat use and survival of Baird's and Grasshopper sparrows in the Chihuahuan Desert.

Context-dependent foraging by seed-eating birds does not necessarily mean low ecological predictability

Flexibility of foraging behaviour affects our capacity to predict ecological outputs such as population responses to habitat change. Some birds forage following rules of absolute value of the food item (i.e., absolute valuation). Their realized diet is strongly correlated with the profitability of the food item and it is predictable. Consumers, however, do not always follow absolute rules. Opportunistic foragers adjust food consumption based on the availability of the food item. Their diet is still predictable but more elusive. Relativistic or context-dependent foragers change the ranks of food preferences depending on the presence of alternative food options in the choice set. Predicting their contingent diet is particularly difficult. We tested if the context of seed availability affects foraging decisions of three seed-eating bird species (the Rufous-collared Sparrow (Zonotrichia capensis (Statius Muller, 1776)), the Many-colored Chaco Finch (Saltatricula multicolor (Burmeister, 1860)), and the Common Diuca Finch (Diuca diuca (Molina, 1782))) using choice experiments aimed at detecting if seed preferences for two types of target seeds changed according to context. Birds showed very similar rankings of preferences for target seeds; however, preferences for attractive food items were not fixed but often increased in less valuable contexts. Although results imply some degree of context-dependent behaviour, predictability of bird diet was preserved because the ranking of preferences remained mostly unchanged between contexts (and among bird species), and the higher consumption of target grass seeds in a less attractive context was widely expected from the intrinsic properties of the seeds.

Feeding and digestive responses to fatty acid intake in two South American passerines with different food habits

Specific fatty acids (FA) such as unsaturated (UFA) and saturated (SFA) fatty acids contained in foods are key factors in the nutritional ecology of birds. By means of a field and experimental approach, we evaluated the effect of diet on the activity of three esterases involved in FA hydrolysis; carboxylesterase (CE: 4-NPA-CE and a-NA-CE) and butyrylcholinesterase, in two South American passerines: the omnivorous rufous-collared sparrow (Zonotrichia capensis) and the granivorous common diuca-finch (Diuca diuca). The activity of the three esterases was measured in the intestines of freshly caught individuals over two distinct seasons and also after a chronic intake of a UFA-rich or SFA-rich diet in the laboratory. In turn, we assessed the feeding responses of the birds choosing amongst diets contrasting in the kind of specific FA (UFA- vs. SFA-treated diets). During summer, field CE activities (4-NPA-CE and a-NA-CE) in the small intestine were higher in the rufous-collared sparrow (25.3 ± 3.3 and 81.4 ± 10.8 µmol min(-1) g tissue(-1), respectively) than in the common diuca-finch (10.0 ± 3.0 and 33.9 ± 13.1 µmol min(-1) g tissue(-1), respectively). Two hour feeding trial test indicated that both species exhibited a clear preference for UFA-treated diets. On average, the rufous-collared sparrow consumed 0.46 g 2 h(-1) of UFA-rich diets and 0.12 g 2 h(-1) of SFA-rich diets. In turn, the consumption pattern of the common diuca-finch averaged 0.73 and 0.16 g 2 h(-1) for UFA-rich and SFA-rich diets, respectively. After a month of dietary acclimation to UFA-rich and SFA-rich diets, both species maintained body mass irrespective of the dietary regime. Additionally, the intestinal 4-NPA-CE activity exhibited by birds fed on a UFA-rich or SFA-rich diet was higher in the rufous-collared sparrow (39.0 ± 5.3 and 44.2 ± 7.3 µmol min(-1) g tissue(-1), respectively) than in the common diuca-finch (13.3 ± 1.9 and 11.2 ± 1.4 µmol min(-1) g tissue(-1), respectively). Finally, the intestinal a-NA-CE activity exhibited by the rufous-collared sparrow was about two times higher when consuming an UFA-rich diet. Our results suggest that the rufus-collared sparrow exhibits a greater capacity for intestinal FA hydrolysis, which would allow it to better deal with fats from different sources.

Adaptive divergence in seed color camouflage in contrasting soil environments

Although adaptive plant population divergence across contrasting soil conditions is often driven by abiotic soil factors, natural enemies may also contribute. Cryptic matching to the native soil color is a form of defensive camouflage that seeds can use to avoid detection by seed predators. The legume Acmispon wrangelianus occurs across a variety of gray-green serpentine soils and brown nonserpentine soils. Quantitative digital image analysis of seed and soil colors was used to test whether genetically based seed color is a closer match to the color of the native soil than to the color of other nearby soils. Lineages bear seeds that more closely match the color of their native serpentine or nonserpentine soil type than the opposing soil type. Further, even within a soil type, lineages bear seeds with a closer color match to the soil at their native site than to other sites. The striking concordance between seed and native soil color suggests that natural selection for locally camouflaged seed color morphs, probably driven by seed predators, may maintain adaptive divergence in pigmentation, despite the opportunity for migration between soil environments.

Tolerance to dietary phenolics and diet breadth in three seed-eating birds: implications for Graminivory

The ability to cope with plant secondary compounds (PSCs) has profound implications for an animal's behavior. In the present study, we assessed the tolerance to dietary phenolics in three seed-eating birds: Zonotrichia capensis, Saltatricula multicolor, and Diuca diuca, which differ in their diet breadth. Seeds in their habitat have distinct chemical composition: grass seeds have less PSCs, specifically, less total phenolics than forb seeds. Based on the detoxification limitation hypothesis and using published data of the natural history of these birds in the central Monte desert, we postulate that predominantly and exclusively graminivorous birds such as D. diuca and S. multicolor, respectively, are less tolerant due, in part, to a lower detoxification capacity than those with greater diet breadth, Z. capensis. To achieve this goal, we measured the food intake of diets varying in their concentration of tannic acid (TA). Indicators of tolerance were body mass change, food, TA and water intake, and glucuronic acid output throughout the experiment. Zonotrichia capensis performed better along the tolerance experiment: it maintained body mass from 0 to 4% TA diet, food and TA intake were higher than the other two species at the end of the experiment, and glucuronic acid output by Z. capensis was greater than D. diuca and S. multicolor from 2% TA diet until the end of the experiment. Our results suggest that Z. capensis is the most tolerant species and this physiological trait may explain their greater diet breadth.