How Does Agriculture Affect Vulnerable Insect-Eating Birds?

CONDOR-18-16 C Michelson

Tree Swallow with prey. Photo credit: C. Michelson

Aerial insectivores—birds that hunt for insect prey on the wing—are declining across North America as agricultural intensification leads to diminishing insect abundance and diversity in many areas. A new study from The Condor: Ornithological Applications looks at how Tree Swallows’ diets are affected by agriculture and finds that while birds living in cropland can still find their preferred prey, they may be working harder to get it.

The University of Saskatchewan’s Chantel Michelson, Robert Clark, and Christy Morrissey monitored Tree Swallow nest boxes at agricultural and grassland sites in 2012 and 2013, collecting blood samples from the birds to determine what they were eating via isotope ratios in their tissues. Tree Swallows usually prefer aquatic insects, which they capture in the air after they emerge from wetlands to complete their life cycles. The researchers suspected that birds living in crop-dominated areas would be forced to shift to eating more terrestrial insects, due to the effects of insecticide use and other agricultural practices on wetland habitat.

Instead, they found that swallows were eating more aquatic than terrestrial insects at all sites, and in 2012 it was actually the grassland birds whose diet contained a higher proportion of terrestrial insects. The results suggest that wetland habitat may provide a buffer against the negative effects of agriculture. However, birds living in cropland weighed less on average than their grassland-dwelling counterparts—a sign that they may be struggling.

“We set up this study to see if insectivorous swallows would be disadvantaged in agricultural croplands by shifting their normally aquatic diet to terrestrial insects to compensate for lower food availability. We were surprised that the birds did not generally do this,” says Morrissey. “Adult swallows in particular were heavily reliant on aquatic prey regardless of land use type. At the grassland dominated site, in fact, they fed their nestlings a wider variety of prey from both aquatic and terrestrial origin. Diet did not seem to influence body condition, but birds in cropland sites were lighter on average which may signal they are working harder in croplands to obtain their preferred aquatic prey. This work shows how important wetlands are for maintaining birds in agricultural landscapes and these are important reservoirs for conserving biodiversity in an otherwise heavily altered landscape.”

“Grasslands are one of the most imperiled ecosystems on the planet because their rich soils are ideal for agriculture. Pesticides and fertilizers are applied in ever-increasing quantities, which has serious implications for organisms that live there,” adds Acadia University’s Dave Shutler, an expert on Tree Swallow ecology who was not involved with the study. “This study compared the diets of Tree Swallows in natural grasslands and croplands, each of which had roughly similar wetland densities. Although diet composition was similar in both areas, it appears that diet quality was better in the natural grasslands, because birds there were heavier and in better condition than those in the cropland.”

Agricultural land cover does not affect diet of Tree Swallows in wetland dominated habitats is available at http://www.bioone.org/doi/full/10.1650/CONDOR-18-16.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology, published by the American Ornithological Society. For the past two years, The Condor has had the number one impact factor among 27 ornithology journals.

Woodpeckers and Development Coexist in Seattle

CONDOR-17-171 J Tomasevic

Pileated Woodpeckers peek from a tree cavity. Photo credit: J. Tomasevic

The two largest woodpeckers in North America, the Imperial Woodpecker and Ivory-billed Woodpecker, are believed to have gone extinct during the twentieth century. Can their surviving cousin, the Pileated Woodpecker, persist when standing dead trees and other crucial resources are lost to urbanization? A new study published by The Condor: Ornithological Applications tracked birds in suburban Seattle and found that as long as tree cover remains above a certain threshold, Pileated Woodpeckers and housing developments can coexist.

The University of Washington’s Jorge Tomasevic (now at the Universidad Austral de Chile) and John Marzluff trapped and radio-tagged 16 Pileated Woodpeckers at 9 sites with varying degrees of urbanization in suburban Seattle. Tracking each bird for about a year, they found that Pileated Woodpeckers used not only forested areas such as parks, but also lightly and moderately urbanized areas where some trees had been retained, taking advantage of resources such as backyard birdfeeders.

These results show that retaining at least 20% forest cover, including standing dead trees, over large suburban areas may help sustain Pileated Woodpeckers and perhaps even other species tied to them. Despite potential risks from threats such as feral cats and collisions with windows, the researchers believe that cities can play an important role in the conservation of biodiversity.

“You’d think that such large bird would be easy to find, especially when carrying a transmitter, but they did a very good job hiding,” says Tomasevic. “It was also very challenging to work in populated areas. I have so many anecdotes, good and bad, about dealing with people and people dealing with me doing my work. Some people were very friendly, but some were a little nervous with me walking around the neighborhood. I tried to look as official as possible, with University of Washington logos on my jacket, and I created a website for the project and printed some business cards. It was a great opportunity to do outreach, and I’m still friends with some of the neighborhood residents.”

“As suburban sprawl becomes more and more ubiquitous, it’s imperative that we consider which specific yard features can be promoted to share our neighborhoods with wildlife,” according to the University of Delaware’s Desiree Narango, an expert on avian urban ecology. “This paper is a nice example showing that even a mature forest specialist can use and navigate the suburban landscape if we provide the resources they need: large trees and some retained wooded areas.”

Use of suburban landscapes by the Pileated Woodpecker (Dryocopus pileatus) is available at http://www.bioone.org/doi/full/10.1650/CONDOR-17-171.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology, published by the American Ornithological Society. For the past two years, The Condor has had the number one impact factor among 27 ornithology journals.

AUTHOR BLOG: The real story behind murres’ pear-shaped eggs

Tim Birkhead

Linked paper: The pyriform egg of the Common Murre (Uria aalge) is more stable on sloping surfaces by T.R. Birkhead, J.E. Thompson, and R. Montgomerie, The Auk: Ornithological Advances 135:4, October 2018.

murre eggFor the past six years, Jamie Thompson, Bob Montgomerie, and I have tried to understand why murres produce a pear-shaped (pyriform) egg.

It started one evening in 2012 when I watched a well-known TV presenter take a murre’s egg from a tray of birds’ eggs in a museum. “The reason it is this shape,” he said, “is so that if it is knocked, it will spin on its axis rather than rolling off the cliff ledge.” He demonstrated this by spinning the egg.

I was appalled. That idea was nonsense and had been dismissed over a century earlier. Yes, if you take an empty eggshell you can indeed lie it on its side and spin it like a top on its side. But a murre egg full of yolk, albumen, and a developing embryo will not spin like that without undue force.

Having offered to send the presenter the papers pointing out why the spinning-like-a-top idea was wrong, I had a sudden crisis of confidence, and decided I had better re-read those papers myself.

I soon realized that the more widely accepted view — that a pyriform egg rolls in an arc and thereby minimizes the risk that it will fall off the breeding ledge — was not very convincing either. The rolling-in-an-arc idea gained support initially by some experiments in the 1960s using model eggs (made from plaster of Paris). But it was later found that model eggs simply do not roll like real eggs. Subsequent experiments with real murre eggs provided no convincing evidence for the rolling-in-an-arc idea, either.

What’s more, incubating murres invariably orient their egg with its blunt end directed up the slope, in towards the cliff, so that if the egg does roll, it will roll out to the edge. If the purpose of the pyriform egg was to prevent it from rolling off the ledge, then it would more sensible for the parent to orientate the egg the other way.

We decided to re-investigate, thinking explicitly about the selection pressures that might influence the shape of a murre’s egg.

We had two ideas. First, murres are poor flyers that breed at high density. As a result, crash landings onto incubating birds are common, so perhaps a pyriform shape confers greater strength and resilience against impacts. That proved to be a difficult idea to test.

Our second idea rested on the observation that murre ledges are filthy with excrement. Perhaps the pyriform shape enables an egg to keep its blunt end clean such that the pores for air exchange do not become blocked. We found that the density of pores on the blunt end of the egg was relatively high and, if you look at the distribution of dirt on murre eggs, most of it is on the pointed end. These results are consistent with the dirt hypothesis. However, it wasn’t clear whether avoiding dirt or avoiding damage from impacts were sufficiently strong selection pressures to have produced the shape.

Then, while climbing on murre ledges in 2017, I had a sudden thought. Perhaps the pyriform shape allows a murre’s egg to rest stably on the sloping ledges that murres often breed on. I had fresh murre eggs and Razorbill eggs (which are much less pointed and more elliptical in shape) to hand, and I tried placing them on a 30o rock slope. The murre egg rested there immediately, the Razorbill egg rolled off (into my hand, of course), and, indeed, there was no way I could position the Razorbill egg stably on that slope.

My colleague Jamie was climbing with me, so I called him over, said “Watch this!”, and demonstrated again. Same result. Then, together with Bob Montgomerie, we devised a series of tests to establish just how stable murre and Razorbill eggs are across a range of egg shapes on slopes of different steepness. We quantified egg shape using a new approach (Biggins et al. 2018). We then conducted two experiments, one using a moving slope and the other using three static slopes at 20o, 30 o and 40 o. We tested to see at what angle each egg would begin to roll on the moving slope and how successful we were at stably positioning each egg on the static slopes.

The results are clear. The more pyriform the egg, the more stable and less likely to roll out of place it is. Our results are NOT about how an egg will roll when it becomes unstable, but about whether it begins to roll in the first place, either when knocked or during changeovers. Our results indicate that the stability of a pyriform egg also makes it easier and safer for murres to manipulate (with their beak, wings and feet) their eggs during incubation and changeovers.

STABILITYinfographicV4

 

Links:

I started studying Common Murres (common guillemots in the UK) Uria aalge in 1972, on Skomer Island, off the coast of Wales, UK.  I have kept that study — whose main thrust is population monitoring — going ever since: www.justgiving.com/guillemotsskomer

The video describing our murre egg study is here:  https://youtu.be/e-189LIYa0Y

Tim Birkhead academic website: https://www.sheffield.ac.uk/aps/staff-and-students/acadstaff/birkhead

 

Other relevant papers:

Biggins, J. D., Thompson, J. E. & Birkhead, T. R. 2018. Accurately quantifying the shape of birds’ eggs. Ecology and Evolution 8: in press.

Birkhead, T. R. 2017. Vulgar errors — the point of a Guillemot’s egg. British Birds 110: 456-467.

Birkhead, T.R., Thompson, & J. E., Biggins, J. D. 2017. The point of a guillemor’s egg. Ibis 159: 255-265.

Birkhead, T. R., Thompson, J. E. & Biggins, J. D. 2017.  Egg shape in the common guillemot Uria aalge and Brunnich’s guillemot U. lomvia: not a rolling matter? Journal of Ornithology 158: 679-685.

Birkhead, T.R., Thompson, J. E., Biggins, J. D. & Montgomerie, R. 2018.  The evolution of egg shape in birds: selection during the incubation period. Ibis, in press.

Arctic Seabird Populations Respond to Climate Change

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Alaska’s Black-legged Kittiwakes are among the seabird species impacted by climate change. Photo credit: Marc Weber, USFWS

Seabirds such as gulls can be key indicators of environmental change as their populations respond to shifts in their ocean habitat over time. A new study from The Auk: Ornithological Advances investigates how several species have responded to changing environmental conditions in the Arctic over the last four decades. The authors find that a warming ocean is directly and indirectly affecting seabird populations in Alaska.

The University of Idaho’s Holly Goyert (now at the University of Massachusetts) and her colleagues used mathematical models to explore relationships between large, long-term datasets covering climate fluctuation, zooplankton abundance and distribution, and populations of several seabird species in the waters off Alaska from 1974 to 2014. They found that declines in populations of an Arctic gull called the Black-legged Kittiwake are tied to deteriorating zooplankton productivity, while their cousins the Red-legged Kittiwakes, also declining, are more sensitive to warming ocean surface temperatures. Not every seabird is in trouble, though—Common and Thick-billed Murres, relatives of puffins, have proved resilient to changing conditions and may even be benefitting.

This study is the first attempt to explain how climate and habitat variability affect seabird population dynamics across such a large scale. “Our hope is that these results will be used in a proactive approach to seabird conservation, and that measures will be taken to prevent populations from declining to small sizes. For example, although Black-legged Kittiwakes are one of the more abundant gulls in the world, their populations are undergoing significant declines, which calls their global status into question,” says Goyert. “Our paper suggests that the deterioration of food web resources such as krill, which is related to warming oceans, has contributed to these declines.”

“Mass seabird deaths and breeding failures in recent years have the scientific community puzzled, and both appear to be climate-related,” according to Melanie Smith, Audubon Alaska’s Director of Conservation Science, who was not involved in the study. “This study is an important step in clarifying the effects of changing climate on seabird population dynamics across Alaska. We can use what we’ve learned here to design detailed monitoring and to better anticipate population declines, improving managers’ ability to protect vulnerable species.”

Effects of climate change and environmental variability on the carrying capacity of Alaskan seabird populations is available at http://www.bioone.org/doi/full/10.1642/AUK-18-37.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology published by the American Ornithological Society. The Auk commenced publication in 1884 and in 2009 was honored as one of the 100 most influential journals of biology and medicine over the past 100 years. The Auk has the #1 average Journal Impact Factor for the past 5 years for ornithology journals.

Where Do Crows Go in Winter?

AUK-18-23 M Jones

A crow with a satellite transmitter. Photo credit: M. Jones

“Partial migration”—where some individuals within a population migrate and some don’t—is common among birds and is speculated to be a step on the evolutionary path to complete, long-distance migration, but scientists know very little about how it actually works. A new study from The Auk: Ornithological Advances tracks where American Crows go during the winter and shows that while individuals are consistent in whether they migrate or stay put, partial migration might give them enough flexibility to adapt to changing environmental conditions.

Hamilton College’s Andrea Townsend and her colleagues captured crows in large winter flocks in Utica, New York, and Davis, California, fitting them with satellite transmitters to track their movements and collecting blood and feather samples. Their data show that 73% of western crows and 86% of eastern crows migrated at least some distance to breed, with an average journey of around 500 kilometers. Birds returned faithfully to the same breeding territory each year, and whether or not individuals migrated was consistent from one year to the next—they didn’t switch strategies depending on environmental conditions. However, they were flexible in where they spent the winter.

This information can serve as an important baseline for tracking how crows’ migratory behavior is affected by factors including climate change and urbanization. Urban “heat islands,” as well as general warming trends, could lead more birds to shorten their migration and spend the winter closer to their breeding territory. “If you live in a place, usually a city, with a huge winter flock of crows, you are seeing migratory birds that came south for the winter as well as your local, year-round crows,” says Townsend. “Personally, I find the sight of an 8000-crow roost exhilarating, but if they or their feces are driving you crazy, you can at least take comfort in knowing that most of them will disappear in early March.”

“It is surprising how much remains unknown about the seasonal movements of most partial migrant species, and this is especially true for variability among populations,” adds the Smithsonian Migratory Bird Center’s Emily Cohen, an expert on migration patterns who was not involved with the study. “This kind of information about populations-specific annual movements is not trivial to collect, but is fundamental to understanding most aspects of the evolution and ecology of species.”

Where do winter crows go? Characterizing partial migration of American Crows with satellite telemetry, stable isotopes, and molecular markers is available at http://www.bioone.org/doi/full/10.1642/AUK-18-23.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology published by the American Ornithological Society. The Auk commenced publication in 1884 and in 2009 was honored as one of the 100 most influential journals of biology and medicine over the past 100 years. The Auk has the #1 average Journal Impact Factor for the past 5 years for ornithology journals.

Mapping Endangered Red Knots’ Remote Breeding Habitat

CONDOR-17-247_REKN_Mark Peck

A nesting Red Knot. Photo credit: M. Peck

The rufa subspecies of Red Knot travels from its breeding grounds in the Canadian Arctic to its winter habitat in South America and back each year, an incredible 15,000 kilometers each way. Its numbers have fallen precipitously in recent decades, and with such a broad range, determining what’s behind the shorebird’s decline is a huge challenge. A new study from The Condor: Ornithological Applications examines Red Knot nesting habitat preferences across the Arctic and determines that while there is plenty of breeding habitat to support today’s population, climate change could pose a threat in the future.

Rutgers University’s Richard Lathrop, Conserve Wildlife Foundation of New Jersey’s Larry Niles, and their colleagues attached radio tags to 365 knots captured while migrating through Delaware Bay in 1999–2006. To learn where and in what sort of habitat the tagged birds nested, they then conducted surveys via small airplane across the south and central Canadian Arctic, a vast study area spanning from Victoria Island in the west to Baffin Island in the east. They also carried out detailed ground-based surveys on Nunavut’s Southampton Island. Across both scales, knots preferred to nest in sparsely vegetated areas on sedimentary bedrock, near but not directly adjacent to the coast or interior wetlands, to which they commute in order to forage.

The study shows that there are more than 74,000 square kilometers of suitable rufa Red Knot habitat across their Central Arctic breeding range—enough space for at least that many breeding pairs, assuming one square kilometer of territory per nest. Since the entire population currently numbers only around 42,000 individuals, it’s unlikely that a lack of available breeding habitat is contributing to the species’ decline; knots’ problems must lie elsewhere. However, suitable breeding conditions for a number of Arctic shorebirds, including knots, are predicted to shift and decline in coming decades due to climate change. An understanding of species–habitat relationships will be crucial for present and future conservation efforts.

“It took many person-hours over many years of intensive field surveys to find the several dozen occupied Red Knot nests that we located and documented. The area we aerially searched for radio-tagged birds spanned over 1700 km in length, the same distance as from Georgia to Maine,” says Niles.

Lathrop adds, “Using the power of satellite remote sensing, data mining analysis, and geospatial modeling, we were able to extrapolate from the field and radio-telemetry surveys to map potential nesting habitat for these birds across wide expanses of the Canadian Arctic. These same tools and techniques could be applied to advance our knowledge about other Arctic-nesting shorebirds.”

“Implementing effective conservation actions for long-distance migrant birds often involves the systematic elimination of insignificant factors to identify important biotic and abiotic contributors to population dynamics,” according to Brad Andres, National Coordinator for the U.S. Fish and Wildlife Service’s Shorebird Conservation Plan, who was not involved in the study. “For the first time, Lathrop and his colleagues have described environmental conditions conducive for nesting by rufa Red Knots and provide evidence that breeding ground habitats are likely not limiting the recovery of the population. Their work also furnishes a baseline to evaluate the ability of a changing Arctic climate to provide breeding habitat for this shorebird of high conservation concern.”

Mapping and modeling the breeding habitat of the Red Knot, Calidris canutus, at landscape and regional scales is available at http://www.bioone.org/doi/full/10.1650/CONDOR-17-247.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology, published by the American Ornithological Society. For the past two years, The Condor has had the number one impact factor among 27 ornithology journals.

AUTHOR BLOG: Newly discovered crossbill species numbers few

Craig W. Benkman

Linked paper: Habitat associations and abundance of a range-restricted specialist, the Cassia Crossbill (Loxia sinesciuris) by N.J. Behl and C.W. Benkman, The Condor: Ornithological Applications 120:3, August 2018.

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A female Cassia Crossbill.

Based on the size and structure of the lodgepole pine cones and the abundance of crossbills in the South Hills and Albion Mountains, Idaho, that I observed on the way to a joint AOU and COS meeting in Boise in 1996, I told several colleagues at the meeting that I might have discovered a new form of crossbill. Although they were skeptical, over the years my students and I have found that this crossbill is engaged in a coevolutionary arms race with the pine, favoring an increase in seed defenses directed at the crossbills. This has caused the crossbill to diverge and speciate into what we now call the Cassia Crossbill (Loxia sinesciuris).

Restricted to the lodgepole pine atop two small ranges on the northeast edge of the Great Basin Desert, this bird was of clear conservation concern, especially given the forecasts of lodgepole pine disappearing from the region late in this century. This became all the more troubling as we watched the Cassia Crossbill population plummet by over 80% between 2003 and 2011. The decline was related to an increase in hot summer days (>32°C or 90°F; 8 days in 2003, 3 in 2005, 4 in 2006, and 4 in 2007) that caused many of the normally closed cones of lodgepole pine to open and shed their seeds, much like they would if there was a stand-replacing fire. Such seeds are lost to Cassia Crossbills, which rely on seeds in the older, closed cones as they weather and gradually become available throughout the year. Fortunately, hot summer days have been few since 2007, allowing the crossbill population to rebound. However, given its restricted range, apparent small population size, and vulnerability to higher temperatures, we needed an estimate of their global population size and habitat preferences to inform and guide us. Nate Behl did just that work for his master’s thesis, and it appears in the article “Habitat associations and abundance of a range-restricted specialist, the Cassia Crossbill (Loxia sinesciuris)” in The Condor: Ornithological Applications.

Cassia Crossbills occupy about 70 km2 of lodgepole pine forest and number only ~5,800 birds. Thus, at the population nadir in 2011 there were probably about 1,500 Cassia Crossbills. That is worrisome, especially given the forecasts for more extreme temperatures later in this century. Nate also found that Cassia Crossbills occur more commonly in larger, mature stands of lodgepole pine on the cooler north-facing slopes where large numbers of seeds can accumulate in closed cones. This makes sense, but the outlook for the continued accumulation of seeds in closed cones in the canopy is bleak. More hot summer days are projected, along with increasing fire frequency, preventing pine from reaching the ages most productive for the Cassia Crossbill.