Tag Archives: foraging

Birds’ feathers reveal their winter diet

(AOS 21 June 2017; Photo RM Jensen)
Influences outside the breeding season can matter a lot for the population health of migratory birds, but it’s tough to track what happens once species scatter across South America for the winter months. A study from The Condor: Ornithological Applications tries a new approach for determining what declining migratory grassland birds called Bobolinks eat after they head south for the winter—analyzing the carbon compounds in their plumage, which are determined by the types of plants the birds consume while growing their feathers during their winter molt.

Thanks to a quirk of photosynthesis, rice contains a different ratio of carbon isotopes than most of the native grasses in South America where Bobolinks winter. Rosalind Renfrew of the Vermont Center for Ecostudies and her colleagues took advantage of this, collecting feather samples from wintering Bobolinks in a rice-producing region and a grassland region and from breeding Bobolinks in North America. When they analyzed the feathers’ isotopes ratios, the results from South America confirmed that isotopes in Bobolinks’ feathers reflected the differences in their diets between regions with and without rice production. The samples taken in North America showed that the winter diet of most individuals was weighted more toward non-rice material, but that rice consumption was highest late in the winter, when rice is nearing harvest and the birds are preparing for their northbound migration.

Rice could be beneficial by providing the birds with needed calories as they prepare for their journey north, but it could also increase Bobolinks’ exposure to pesticides and threats from farmers who see them as pests. According to Renfrew and her colleagues, maintaining native grasslands, encouraging integrated pest management programs to reduce toxic pesticide applications, and compensating farmers for crops lost to feeding birds all would be helpful.

“The time spent coordinating the field work for this study may well have been greater than the time spent collecting the data,” says Renfrew. “It was truly a team effort, and the assistance we received from our partners was absolutely essential, especially in South America. Aves Argentinas and the Museo de Historia Natural de Noel Kempff Mercado provided priceless logistical support, and this study could not have happened without them. Some of the same partners have provided input on a Bobolink Conservation Plan that lays out actions to address threats to grassland birds in North and South America, based on results from this and other studies.”

“As Bobolink populations continue to decline, Renfrew and her colleagues use state-of-the-art isotope analysis techniques to assess the Bobolink’s diet on its South American wintering grounds,” according to John McCracken of Bird Studies Canada, an expert on grassland bird conservation who was not involved with the study. “The authors conclude that rice may have negative effects on Bobolinks, owing to its relatively low nutritional quality and from exposure to insecticides.

Birds of all feathers work together to hunt when army ants march

ruddy_woodcreeper.jpg
(SD, Drexel University 19 June 2017)
Army ants scare up a lot of food when they’re on the move, which makes following them valuable for predator birds. But instead of competing and chasing each other off from the ant “raids,” as scientists had thought, birds actually give each other a heads up when the ants are marching, according to a new Drexel University study.

For more than a decade — from 2005 until 2016 — Sean O’Donnell, PhD, a professor in Drexel’s College of Arts and Sciences, observed army ant “raids” and the birds that follow them. He hoped to find out whether birds really were aggressive toward each other during the ant marches or whether they actually cooperated to access the food (other insects and bugs) that ants rustle out of hiding.

After observing 74 swarms in Costa Rica, it seems birds are much more likely to play nice with each other.

“Overall, the results strongly supported facilitation — species help each other to exploit shared resources,” O’Donnell said of his study that was recently published in Biotropica.

In watching for the raids and the flocks that “attend” them, a key to avian cooperation may be what are termed “bivouac-checking” birds. These are birds that perch near the sites where army ants make their nests (bivouacs) and watch to see where and when the ants move. Birds that fall into that category include the ocellated antbird and the blue-diademed motmot.

The prevailing thought has been that these specialized birds liked to keep the ant colonies they watched to themselves, not allowing other species to horn in on their finds.

But a frequent high diversity of species in flocks following the ant columns showed O’Donnell that birds that didn’t specialize in tracking army ants (like the migrant species Kentucky warbler) were allowed to join and hunt.

So when bivouac-checking birds see the movement of the columns and take off, other birds take the cue. They either know birds like the ocellated antbird follow ant columns or recognize vocalizations the specialized birds make when chasing the colonies.

“Birds may use each other as a way of finding army ant raids, which are very hard to locate in the forest because they are widely spaced and the ants are mobile,” O’Donnell said. “Observations suggest some birds are attracted to other birds at raids, and birds may even follow each other when moving among raids of different ant colonies.”

However, there did seem to be some bullies.

O’Donnell noticed some pairs of species were almost never found in flocks together despite, independently, being ant-chasers. That indicated that these bird species might chase each other off as competition, or just avoid each other entirely. Pairs that seemed to be unable to be around each other included the blue-throated toucanet and the brown jay, as well as the wood thrush and the white-eared ground sparrow.

“These antagonistic pairs were often species of very similar body size or feeding behavior,” O’Donnell explained. “Perhaps these species do compete very strongly at army ant raids.”

All in all, finding that birds largely work together to forage at army ant raids seems to demonstrate that cooperation is a better survival strategy than trying to keep food from the raids for their own species.

“Having other birds around may be an advantage because there are more eyes and ears to detect predators,” O’Donnell said. “If the raid is hard to monopolize, and food is very abundant there, then the costs of allowing other birds to attend may be low, further favoring positive species interactions.”

Sandeels and seabirds: Protecting our seas in post-Brexit waters

kittiwakers.jpg(RSPB 14 June 2017; Photo: Andy Hay)

New research led by the RSPB shows that UK seabird populations could be affected by the amount of a critical fish species caught in the North Sea by an industrial fishery, highlighting the importance of continuing to work with other countries on fisheries management after leaving the European Union.

The study suggests a link between the amount of sandeels caught by fishermen and the breeding success of kittiwakes (a small species of gull, currently red-listed in the UK), with higher intensity fishing leading to lower numbers of chicks being produced.

In the North Sea, sandeels provide a vital food source for breeding seabirds but are also the target of an industrial fishery conducted mainly by Denmark. Tracking data of individual breeding kittiwakes by RSPB scientists indicates that the most productive sandeel fishing grounds, an area known as the Dogger Bank, overlap with foraging areas of kittiwakes from eastern English colonies, raising the prospect that the fishery could adversely affect the birds’ populations.

The Dogger Bank is the largest sandbank in the North Sea, straddling the waters of the UK (about 100 miles off the Yorkshire coast), Netherlands and Germany, and supporting a high density of sandeels.

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Kestrels’ strategies for flight and hunting vary with the weather

Kestrels adapt their flight and hunting strategies to weather conditions, including solar radiation, wind speed, and air temperature, according to a study published June 7, 2017 in the open-access journal PLOS ONE by Jesús Hernández-Pliego from Estación Biológica de Doñana, Spain, and colleagues.
Theory predicts that to maximize fitness, foraging animals should gain the most energy for the least cost of time and energy. Hernández-Pliego and colleagues tested this theory in kestrels, small insectivorous falcons that return to their nest to feed their chicks after a foraging trip. Kestrels have the options of commuting via time-saving flapping flights or energy-saving soaring-gliding flights, and of capturing prey via time-saving active hovering flights or energy-saving perch-hunting.
To explore whether the birds’ behavioral decisions reflect trade-offs during the commuting and searching parts of foraging trips, the researchers tracked lesser kestrels with GPS and tri-axial accelerometers, which provide data on energy and time budgets in wild animals. The study included six kestrels (four males and two females) from two breeding colonies with in the Guadalquivir river basin of southwestern Spain.
The researchers found that kestrels’ behavioral decisions varied with weather conditions. When solar radiation increased — which strengthens thermal updrafts — commuting birds replaced flapping with soaring-gliding. When wind speed increased — which provides a stronger lift — searching birds replaced perch-hunting with hovering. But kestrels also hovered more when air temperature increased possibly as warmer weather boosts the activity of large grasshoppers, the birds’ preferred prey.
While flight and hunting strategies varied dramatically with the weather, the researchers also found that the energy expended per foraging trip remained constant. This suggests that the birds have a fixed energy budget per foraging trip and adjust their behavior accordingly when environmental conditions vary.

Picky fruit-eating birds are more flexible

(Senckenberg Research Institute and Natural History Museum 11 May 2017; Photo Matthias Dehling )

Researchers from Senckenberg and the German Centre for Integrative Biodiversity Research have found that South American birds that are seasonally specialized on particular fruit types are the most flexible in switching to different fruit types in other seasons. This flexibility in their diet is good news in view of the predicted loss of plant species under global change. The study is published in the Journal of Animal Ecology.

The Plumbeous pigeon is a picky eater. Whereas its relatives on European streets and squares feed on whatever they encounter, its South American relative almost exclusively feeds on certain fruits. Together with other birds such as toucans or the turkey-like guans it belongs to a bunch of large fruit-eating birds that are specialized on particular types of fruits. One might think that being peculiar goes hand-in-hand with being inflexible in food choice — however, it does not.

“We compared neotropic fruit-eating birds that are specialized on a small range of fruit resources in a particular season with birds that eat a large ranage of fruits. The specialists are those which are most flexible in adapting their foraging choices across seasons,” explains Irene Bender, Senckenberg Biodiversity and Climate Research Centre and German Centre for Integrative Biodiversity Research (iDiv), lead author of a new study on this topic. She adds “The flexible avian fruit eaters prefer to feed on large fruits. However, their favourite resources are not constantly available throughout the year which forces them to switch.”

The researchers’ surprising observations shed light on the question how species might respond to resource fluctuations that are not naturally determined. “This flexibility of consumer species may be an important, but so far widely neglected mechanism that could stabilize consumer-resource relationships, for instance in response to human disturbance and environmental change. Being flexible might mean that specialized foragers are able to adapt to future changes in resource availability.” says Dr. Matthias Schleuning, Senckenberg Biodiversity and Climate Research Centre.

The study was based on observational records by a team around Irene Bender and Matthias Dehling of the fruit choices of birds in the Manú Biosphere Reserve on the western slopes of the Andes in Peru. The team also measured traits of the plants which provided the fruits, such as plant height and fruit size. The flexibility of bird species was defined as their ability to switch seasonally among plant species with different traits; their specialization was measured by comparing their fruit choices to those of other bird species in the community.

‘Happy wife, happy life’ meaningful for birds, too

‘Happy wife, happy life’ meaningful for birds, too (Victoria University; 19 April 2017)

Research from Victoria University of Wellington has shown for the first time that wild male birds read their partner’s behaviour to appropriately cater to her food desires.

Dr Rachael Shaw, a postdoctoral research fellow in Victoria’s School of Biological Sciences, conducted a study on a group of North Island robins based at Zealandia.

The research investigated whether male robins could give their mate the type of food that she was most likely to want during reproduction.

“Robins are a monogamous, food-sharing species, so were ideal for this experiment. The experimental procedure has only previously been used in the laboratory on Eurasian jays,” she says.

“We found male robins appropriately catered to their mates’ desire, even when the female’s behaviour was the only cue available to guide their choices.

“This suggests that females can signal their current desires to their mates, enabling males to respond to that.”

Dr Shaw says the finding raises the possibility that other species might be capable of doing the same.

“In many species food sharing by the male is vital to help the female offset the energetic costs of reproduction, such as egg laying and incubation. The male’s ability to give his mate what she wants could in fact be an important factor in determining the success of a pair, as well as influencing whether they stay together. These are really exciting avenues for future research.”

The experiment first involved establishing female robins’ eating habits. “I fed the females either meal worms or wax worms, and then gave them the choice between these two types of insect larvae. I found that after the females had eaten one type of insect, they would prefer to eat the other type when given the choice. This means that the female’s desire for a particular food is affected by what she has previously eaten.”

Based on this, Dr Shaw then tested if the male would also be able to choose the type of insect his mate was most likely to want (the one she had not just eaten).

“Regardless of whether or not he had seen what his mate ate first, the male still made the appropriate choices. This suggests that the female is likely to be displaying her current desire in her behaviour, and that the male is using these cues to identify the food that she wants.”

The research, co-authored by Victoria’s Associate Professor Kevin Burns and Professor Nicola Clayton from the University of Cambridge in the United Kingdom, was recently published in the journal Scientific Reports.

The field work and data collection was carried out by Dr Shaw with help from Victoria student Regan MacKinlay. Dr Shaw’s research is supported by a Rutherford Foundation postdoctoral fellowship and a Marsden Fast Start grant from the Royal Society of New Zealand.

Odd Things Birds Eat

american robin.JPG

(Laura Erickson; 14 April 2017)

In the past few weeks, I’ve received a couple of letters from listeners about their backyard birds eating unusual items.

Craig Magnuson of Forks, Washington, wrote that he’s seen Evening Grosbeaks, during several different summers, eating cold campfire charcoal shortly after dawn at the Lake Kachess campground in the Wenatchee National Forest. He noted that someone else posted photos of Evening Grosbeaks eating campfire charcoal at a different location about 20 miles east of where he’s seen it.

True finches, such as Evening and Pine Grosbeak, crossbills, siskins, goldfinches, and Purple Finch, eat mostly seeds. This diet has two problems: birds don’t have teeth, so can’t chew seeds to mash them up, and seeds are missing many minerals and other nutrients that birds need. By eating various forms of grit, birds solve both problems. The grit remains in the first chamber of their stomach—the gizzard—for a while, helping the muscular walls to pulverize the food. And little by little, the grit itself gets pulverized, and the minerals within are picked up by the body, too.

According to the Cornell Lab of Ornithology’s Birds of North America, Evening Grosbeaks are known to eat snow, salt, and mineral-rich soil, and are drawn to areas where salt or calcium carbonate are artificially added to soil. A few studies have shown that they eat coal and coke ashes, and a scientist named Orr, in a 1951 paper titled “Observations on the birds of northeastern Idaho” in The Proceedings of the California Academy of Science, wrote that they regularly take charcoal from campfires.

Back in the 1980s and early 90s, when Evening Grosbeaks were abundant in my own neighborhood…..

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