Avian Consumption Research Articles

Cumulative effects of piscivorous colonial waterbirds on juvenile salmonids: A multi predator-prey species evaluation.

We investigated the cumulative effects of predation by piscivorous colonial waterbirds on the survival of multiple salmonid (Oncorhynchus spp.) populations listed under the U.S. Endangered Species Act (ESA) and determined what proportion of all sources of fish mortality (1 –survival) were due to birds in the Columbia River basin, USA. Anadromous juvenile salmonids (smolts) were exposed to predation by Caspian terns (Hydroprogne caspia), double-crested cormorants (Nannopterum auritum), California gulls (Larus californicus), and ring-billed gulls (L. delawarensis), birds known to consume both live and dead fish. Avian consumption and survival probabilities (proportion of available fish consumed or alive) were estimated for steelhead trout (O. mykiss), yearling Chinook salmon (O. tshawytscha), sub-yearling Chinook salmon, and sockeye salmon (O. nerka) during out-migration from the lower Snake River to the Pacific Ocean during an 11-year study period (2008–2018). Results indicated that probabilities of avian consumption varied greatly across salmonid populations, bird species, colony location, river reach, and year. Cumulative consumption probabilities (consumption by birds from all colonies combined) were consistently the highest for steelhead, with annual estimates ranging from 0.22 (95% credible interval = 0.20–0.26) to 0.51 (0.43–0.60) of available smolts. The cumulative effects of avian consumption were significantly lower for yearling and sub-yearling Chinook salmon, with consumption probabilities ranging annually from 0.04 (0.02–0.07) to 0.10 (0.07–0.15) and from 0.06 (0.3–0.09) to 0.15 (0.10–0.23), respectively. Avian consumption probabilities for sockeye salmon smolts was generally higher than for Chinook salmon smolts, but lower than for steelhead smolts, ranging annually from 0.08 (0.03–0.22) to 0.25 (0.14–0.44). Although annual consumption probabilities for birds from certain colonies were more than 0.20 of available smolts, probabilities from other colonies were less than 0.01 of available smolts, indicating that not all colonies of birds posed a substantial risk to smolt mortality. Consumption probabilities were lowest for small colonies and for colonies located a considerable distance from the Snake and Columbia rivers. Total mortality attributed to avian consumption was relatively small for Chinook salmon (less than 10%) but was the single greatest source of mortality for steelhead (greater than 50%) in all years evaluated. Results suggest that the potential benefits to salmonid populations of managing birds to reduce smolt mortality would vary widely depending on the salmonid population, the species of bird, and the size and location of the breeding colony.

Fruit and seed traits of native and invasive plant species in Hawai‘i: implications for seed dispersal by non-native birds

For alien invasive plant species dependent on frugivores for seed dispersal, traits that influence consumption can be important determinants of invasion and spread. However, trait comparisons between native and invasive species have documented mixed results. In Hawai‘i, one of the most invaded systems in the world, nearly all frugivory, and thus seed dispersal, is dependent on non-native birds. Moreover, the majority of dispersal events also involve invasive plants, suggesting the potential for an “invasional meltdown”. Here we compare fruit and seed traits between native and invasive plants and evaluate how those traits influence consumption by non-native avian frugivores. Although we found that most traits examined were similar between native and invasive fruiting species, invasive species tended to have higher fruit protein content, longer fruiting duration, were less likely to have orange fruits, and have less variable seed length. Longer fruiting duration and smaller seed size were important predictors for avian frugivore consumption. In combination, these results suggest that traits that increase probability of encounter (fruiting duration) and ability to be consumed (seed size) are more important for dispersal by frugivores than traits associated with fruit preferences and, thus, are driving the spread of invasive species and limiting dispersal of some native species. Further, we document an apparent seed size threshold for avian consumption (~ 7.0 mm), supporting previous work suggesting that large seeded species, particularly native plant species adapted for extinct large-bodied frugivores, are likely dispersal limited based on gape size limitation of the current non-native frugivore community.

Inefficiency of anthraquinone-based avian repellents when applied to sunflower: the importance of crop vegetative and floral characteristics in field applications.

Blackbirds (Icteridae) cause significant damage to sunflower (Helianthus annuus L.) prompting the need for effective management tools. Anthraquinone-based repellents can reduce feeding by > 80% in laboratory settings, but require birds to learn the negative association through repellent ingestion. We evaluated an anthraquinone-based repellent applied directly to mature sunflower plants for its ability to reduce bird damage. We used captive male red-winged blackbirds (Agelaius phoeniceus) to evaluate efficacy of two anthraquinone-based formulations in varying concentrations and applied in a manner attainable by sunflower producers. We also assessed field application methods for repellent coverage and anthraquinone residues when using ground-rigs equipped with drop-nozzles situated below the crop canopy. The repellents failed to reduce feeding and birds did not exhibit a preference between untreated and treated sunflowers at concentrations 2.7× the suggested application rate (i.e. 9.35 L ha-1 of repellent). In the absence of disk flowers, which obstruct repellent from reaching the achenes, the repellents failed to reduce consumption. Anthraquinone concentrations in field applications were considerably less than those in the laboratory experiments and did not reduce bird damage. Efficacy is difficult to achieve in the field due to application issues where growth patterns and floral components of sunflower limit residues on achenes, thus contact with foraging birds. Although field residues could be improved by increasing anthraquinone concentrations in tank mixtures and decreasing droplet size, repellents optimized for loose achenes are inefficient in reducing avian consumption of sunflower when applied to intact plants in a manner representative of commercial agriculture. © 2020 Society of Chemical Industry.

Pacific herring (Clupea pallasii) consumption by marine birds during winter in Prince William Sound, Alaska

AbstractFollowing the 1989 MV Exxon Valdez oil spill (EVOS) and subsequent herring population collapse in Alaska's Prince William Sound (PWS), the Pacific herring (Clupea pallasii) fishery was closed. In the 25 yr since EVOS, herring and several herring‐dependent marine bird species have failed to reach pre‐spill population levels. One hypothesis is that intense predation pressure may be inhibiting herring recovery. To inform herring modeling efforts, this study estimated marine bird consumption of juvenile and adult herring in PWS for 10 winters over an 18‐yr period (1989–90 through 2006–2007). Total estimated herring consumption by wintering marine birds averaged 2409 ± 950 t, indicating that avian consumption represents a substantial and inter‐annually variable source of herring mortality. Common murre (Uria aalge) consumed the greatest portion (16–80%) of herring in all years among marine bird species. Juvenile herring biomass consumed annually by common murre was greater than murre consumption of adult herring biomass. Time lag analyses showed that marine bird consumption of adult herring is negatively correlated with the amount of herring spawn observed in subsequent years, but such effects were not observed more than 2 yr. Our models indicate that during years in which herring recruitment is low or bird populations are particularly large, marine birds can consume up to 10% of the annual adult herring biomass. Our results highlight the importance of herring to wintering PWS birds. We propose that future management of herring stocks seeks to reduce negative impacts on marine birds that prey on herring.

Avian food selection with application to pesticide risk assessment: Are dead and desiccated insects a desirable food source?

Past evaluations of pesticide exposure have been conducted with substantial uncertainty regarding avian consumption of contaminated food items. One question is whether birds consume invertebrates that are killed by a chemical application and that may present an increasing chemical concentration as they desiccate. We addressed the research question in two phases. First, a laboratory study was conducted in which wild-caught birds were individually offered three food choices, i.e., live, fresh-dead, and desiccated insect larvae. Second, these same food choices plus live, fresh-dead, and desiccated crickets were presented in study plots in two agricultural crops, i.e., a cornfield and an orchard. The experimental food items were monitored with videography equipment to determine their fate and to compare laboratory and field results. Laboratory results showed that birds have a strong preference for live and fresh-dead prey over desiccated prey, with live prey taken before fresh-dead prey in most trials. The field study revealed a similar preference for live prey over desiccated prey, with preference for fresh-dead prey intermediate to the two other types.

Vertebrate predation on Holm Oak, Quercus ilex, acorns in a fragmented habitat: effects on seedling recruitment

Rodent and avian consumption of Holm Oak, Quercus ilex, acorns were examined in central Spain during two high-production seasons, 1990–1991 and 1993–1994, in 1 and 3 large stands and in 6 and 10 small stands (0.2–12 ha) respectively, to study the effects of vertebrate predation on seedling recruitment. Previous data indicate that wood mouse abundance is much higher in small stands. Tree size, frequency of acorn-bearing oaks and acorn abundance were similar in the two sizes of forest stands. Overall acorn consumption was 33.7 times higher in the small stands compared to that in the large stands in 1991, and 3.1 times larger in 1994. Consumption of acorns by mice in the small stands was 49 and 95 times greater than that by birds in the first and second year, respectively. Seedling recruitment was significantly higher in the large stands in both study seasons. Regeneration by ramets was roughly similar in small and large stands in both seasons. Thus, predation by mice was apparently responsible for the failure of sexual reproduction in the small stands. The results suggest that vegetative regeneration might be the prevalent reproductive method of Holm Oaks in small forest tracts.

Grit Color Selection by House Sparrows and Northern Bobwhites

Granule color may be an important factor influencing avian exposure to granular pesticides when they are mistakenly picked up as grit, and is clone of the most easily altered characteristics of pesticide granules. We studied colored grit use by house sparrows (Passer domesticus) and northern bobwhites (Colinus virginianus) by offering captive birds a grit mixture consisting of equal amounts of 8 colors (red, brown, yellow, green, blue, black, white, clear), either on a light-brown or a dark-brown soil background. After 7 days, gizzards of both species consistently contained more yellow and white particles and fewer black and blue particles than expected by chance (sparrows: P < 0.025; bobwhites: P < 0.001). Soil background color was not an important influence on grit color selection by either species. To examine the influence of food color on grit color selection, we repeated the experiments (on dark soil only), using birds maintained on food dyed to match 3 of the 8 grit colors (red, yellow, blue). Grit color use again was nonrandom (P < 0.001). Overall, house sparrows preferred brown, yellow, and white grit, and northern bobwhites preferred yellow and green house sparrows preferred brown, yellow, and white grit, and northern bobwhites preferred yellow and green grit. Black again received little use by both species. Food color did not influence grit color selection by house sparrows, but was associated with small but statistically significant differences in the use of black and clear grit by northern bobwhites. Our results suggest that the colors black andd blue should be tested further if the goal is to design pesticide granules in colors unattractive to birds. On the other hand, if the goal is to deter avian consumption of pesticide-treated granules by applying a mixture of unattractively-colores, treated particles and attractively-colored, untreated ones, then yellow (and perhaps white and green) should receive further study as colors likely to attract birds.

Avian consumption of blank pesticide granules applied at planting to Iowa cornfields

AbstractConsumption of pesticide granules by free‐ranging birds was quantified by applying blank (untreated silica granules to Iowa cornfields at rates varying from 3.9 to 17.3 kg/ha and examining the gizzards of birds collected from these fields for the presence of granules. One or more granules were found in the gizzards of 39% of 256 birds collected. Counts of gizzard granules averaged 0.6 to 0.9 for the three avian groups collected (horned larks, blackbirds, and sparrows). More than 95% of the specimens contained ≤3 granules; the maximum count was 6. On the basis of granule voiding experiments, we estimate that gizzard examinations detected 24% of the granules consumed per day. Median daily consumption for birds in general was estimated to be 2.4 granules, with 95% of individuals estimated to consume ≤18 granules. For the species with the highest consumption rate, the savannah sparrow (Passerculus sandwichensis), median consumption was 11 granules per day, with 95% of individuals estimated to consume ≤23 granules per day. Granule availability on the soil surface was highly correlated with the application rate. Counts within the application band immediately after application averaged 15 granules/ft2 for every 1 kg/ha applied, or about 6% of the granules applied. Our results suggest that when granular pesticides are applied to cropland, many birds may ingest granules, but the numbers ingested will generally be low.

Seabirds as consumers in the southern Benguela region

Estimates of consumption of fish by breeding seabirds in the southern Benguela region converge on 50–60 × 103 tons of food per year. Total annual avian consumption of food is approximately 15,5 × 104 tons, 74 per cent being taken by species which feed on pelagic prey and most of the rest by species which frequently feed on offal at demersal trawlers. Avian consumption is similar in magnitude to that of Cape fur seals and to the catch of the commercial fishery in the southern Benguela region, but far less than consumption by snoek, squid, and, probably, sharks and jellyfish.

Avian Consumption of Guaiacum sanctum Fruit in the Arid Interior of Guatemala

Data are provided on consumption of the fruit of the Guayac6n (Guaiacum sanctum, Zygophyllaceae) by 19 avian species of 17 genera in 8 families in the arid interior of Guatemala. G. sanctum is an abundant evergreen tree of the dry deciduous scrub forest of the Motagua River valley in the Departments of Zacapa and El Progreso. Arilloids were exposed in conspicuous displays during 2 distinct fruiting periods, each lasting about 2 mo. During a fruiting period, individual trees bore ripe fruit continuously for as long as 6 wk. Diverse harvesting and handling procedures of Guayacan arilloids were employed by avian consumers, both intraand interspecifically; these are characterized and harvesting procedures are related to tree morphology. Arils and/or arilloids were consumed and/or transported by adult birds to nestlings and fledglings. Thirteen species transported seeds away from G. sanctum parent trees and did not drop seeds beneath these; two other species transported a portion of the seeds they handled away from parent trees.

Interactions between birds and fruit in a temperate woodland.

The phenology of fruit trees and avian consumption of fruit were examined in Wytham wood, Oxford in 1979-1980. Ripe fruit was available to and fed upon by birds from late August until early May. Observations made on a daily transect indicated that most of the fruit was eaten by tits and thrushes, but the two families differed in the seasonal consumption and species of fruit taken. Tits took fruits of Elder, Bramble, White Bryony, Honeysuckle, Black Bryony, and Woody Nightshade at the beginning of the season only, while thrushes consumed fruits of Elder, Bramble, Hawthorn, Sloe, Rose, and Ivy mainly in the middle and end of the season. Members of these two families also visited different habitat types following the consumption of fruit, probably effecting different patterns of dispersal. There was no correlation between feeding preferences and the abundance or profitabilities (as defined by Pyke et al. 1977) of fruits with respect to total nitrogen, total proteins, total carbohydrates, total fats, or kilocalories. It is concluded that other factors such as palatability or content of other nutrients may be important in determining the feeding preferences of different species of frugivorous birds.

The Role of Pin Cherry (Prunus pensylvanica L.) in the Maintenance of Stability in Northern Hardwood Ecosystems

The design of the life cycle of pin cherry (Prunus pensylvanica L.), a successional species common on disturbed sites throughout much of the northern hardwood and boreal forest ecosystems, assures that its occurrence is integrated into the pattern of disturbance in the climax ecosystem. The combination of buried seed strategy and the mobility offered through avian consumption of fruits ensures reasonably large populations of buried, viable seeds in the soils of forests well after the disappearance of pin cherry from a particular site. Soil sampling data indicate that sufficient numbers of viable pin cherry seeds reside in the soils of second—growth forests in central New Hampshire to account for the dense stands frequently observed after cutting or burning. Further, germination of these buried seeds is apparently triggered by some factor(s) associated with formation of a large gap. By age 25 or 30, when pin cherry individuals are dying rapidly, sufficient numbers of seeds have been produced and disseminated in a dormant condition for the cycle to renew itself with the subsequent occurrence of major disturbance. In high density stands pin cherry grows rapidly, with early attainment of canopy closure (high leaf area index), and rapid attainment of high values of net annual production and nutrient accumulation.