Exploration and exploitation of foraging patches by desert sparrows: environmental indicators and local evaluation of spatially correlated costs and benefits

Abstract

Conventional evolutionary and behavioral reasoning expects foragers to show strong spatial preferences in environments with heterogeneous resource distribution. Moreover, consumers should benefit from exploiting the information embedded in environmental features that indicate resource abundance. In desert soils seed abundance associates strong and reliably with vegetation and litter cover at small spatial scales. However, other spatially correlated factors (substrate complexity, temperature, predation risk) may affect foraging costs, benefits and decisions by ground-feeding granivores. We used a sequence of three semi-controlled field experiments of binary spatial choice within a portable aviary to identify the main cause of foraging microhabitat selection by the most abundant postdispersal granivorous bird in the central Monte desert (Argentina). In the first experiment we placed the aviary at field to offer pairs of adjacent microhabitats of unmodified, naturally-contrasting substrates and environmental conditions to single, untrained rufous-collared sparrows Zonotrichia capensis. Birds selected covered microhabitats in winter and summer, ruling out substrate complexity or thermoregulation as main single causes of patch selection. The other two experiments dissociated seed abundance, tree cover and litter to reveal their effects on patch selection. The results indicate that 1) sparrows do not restrict microhabitat exploration relying on environmental indicators, 2) distance to tree cover influences the order of patch exploration, probably in association with apprehension or risk-assessment behavior, and 3) patch exploitation is determined by short-term local estimation of seed abundance. The integration of these with previous results obtained under variable degrees of realism and experimental control allows for a better explanation of the spatial component of postdispersal granivory and its consequences on plants. The unconstrained selective foraging strategy of these sparrows would allow them to detect sporadic or ephemeral rich patches with structural characteristics indicating 'low-quality', should promote the spatial homogenization of the palatable seed bank, and would favor indirect interactions between plants.

Effects of Personality Traits on the Food-Scratching Behaviour and Food Intake of Japanese Quail (Coturnix japonica)

“…Searching strategy within a food patch is of great significance for ground-feeding birds that often use their claws to scratch grounds for buried food [ 5 , 6 ]. The scratching behaviour is correlated with food acquisition, and the mode, posture, and frequency of scratching behaviour may vary among and within species, with important implications for animal foraging success [ 7 ].…”

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

“…Foraging far from tall trees may involve higher predation risk, as interpreted in classic studies with granivorous birds (e.g., [89, 103–106]), and also change seasonally with varying perception, evaluation or actual risk of predation [106–108]. There was some experimental [23] and observational support (as in other multi-layered habitats, most local birds flee to tall trees as refuges when disturbed; pers. obs. )…”

“…Instead, patch value may not be a function solely of seed abundance if there are correlated factors that reduce their profitability, such as seed detectability and foraging efficiency in different substrates [59, 71–73], or involve important costs to trade-off as thermoregulation or predation risk. Milesi and Marone [23] tested for patch selection in field aviaries under varying combinations of these factors and in spite of some influence of them all, seed abundance was still the main predictor of patch exploitation by individual birds. Desert granivores must cope with a dynamic extreme system with high spatio-temporal variation while foraging for an anyway abundant and dispersed small-sized prey.…”

“…However, in an additional summer trial with the same experimental design but tracking seed removal in grids J and V throughout the day, we observed that most seed removal, (again skewed towards areas around trees in the more heterogeneous grid, V), occurred during the morning and afternoon and it was not related to shade (detailed results in [60]). Milesi and Marone [23] also showed that extreme temperatures may pose an ultimate limit on foraging but can not be invoked as the single main reason for spatially selective foraging patterns by individual birds.…”

“…Though total seed bank is always more abundant under woody vegetation, grass seeds tends to homogenize among microhabitats with time after primary dispersal [22]. Local seed-eating birds, as expected, prefer to exploit more profitable patches once assessed [23, 59], so this top-down effect on the seed bank calls for granivorous birds that do not rely on environmental cues of seed abundance to only attack richer patches but, instead, preferentially explore open, bare-soil microhabitats (at least during autumn–winter).…”