“Live Fast, Die Young” Lifestyle Reflected in Birds’ Feathers

AUK-17-176 R Terrill 2

A museum specimen ready to be photographed. Photo credit: R. Terrill

Animals’ lives tend to follow a quicker tempo as they get farther from the equator—birds at more northern latitudes mature faster, start reproducing younger, and live shorter lives, probably as a way of dealing with seasonal variation in resources. A new study from The Auk: Ornithological Advances shows for the first time that this pattern also plays out in birds’ feathers, with northern birds completing their annual molt faster to keep up with the demands of life far from the tropics.

Louisiana State University’s Ryan Terrill looked at museum specimens of four bird species with ranges that span a wide swath of latitude in both the Northern and Southern Hemispheres. Slight differences in feather growth between day and night during birds’ annual molt produce visible pairs of light-colored bars, each pair representing 24 hours’ growth. Terrill could determine the rates at which individual feathers grew by measuring their spacing. He found that for all four species, individuals collected at higher latitudes had grown their feathers faster.

Terrill sees two potential explanations for this pattern, which aren’t mutually exclusive. First, where the availability of food changes with the seasons, birds may need to molt faster so that they have the necessary resources. Second, because birds at higher latitudes tend to be more invested in producing offspring than in extending their own survival, faster production of lower-quality feathers may be an acceptable tradeoff.

“Working with museum specimens was a lot of fun,” says Terrill. “One of my favorite things about museum specimens is using them in ways that other folks might not consider, and especially using them in ways for which the original collector couldn’t have known they might be useful. It wasn’t until recently that many people considered that how feathers grow might be important for birds or realized that you could measure feather growth rates on specimens, and I hope this study will publicize yet another way that museum specimens are useful for understanding birds.”

“Most aspects of avian molt, with the exception of feather-replacement sequence, are thought to be rather flexible. The timing, location, and extent of molts appear to respond quickly to environmental constraints, even within populations of the same species occurring at different latitudes, as either permanent or winter residents,” adds the Institute for Bird Population’s Peter Pyle, an expert on bird molt patterns who was not involved with the study. “Yet molt strategies remain vastly understudied compared to other avian topics such as breeding, migration, and behavioral responses. This paper shows that a fourth component of molt, feather growth rate, also appears to vary, with equatorial populations showing slower molt intensity than those of higher latitudes. The author ties this nicely in to other studies suggesting a decelerated pace of other life history traits in less seasonal environments, perhaps as a function of slower basal metabolic rates.”

Feather growth rate increases with latitude in four species of widespread resident Neotropical birds is available at http://americanornithologypubs.org/doi/full/10.1642/AUK-17-176.1.

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.

AUTHOR BLOG: Count me in! I am available for detection at 6 AM on May 26th

Péter Sólymos

Linked paper: Evaluating time-removal models for estimating availability of boreal birds during point count surveys: Sample size requirements and model complexity by P.  Sólymos, S.M. Matsuoka, S.G. Cumming, D. Stralberg, P. Fontaine, F.K.A. Schmiegelow, S.J. Song, and E.M. Bayne, The Condor: Ornithological Applications 120:3, August 2018.

Point count survey duration rarely changes within projects but varies greatly among projects. As more and more large-scale studies are compiling and analyzing point count data from disparate sources, standardization becomes critical, because count duration greatly affects observations. The Boreal Avian Modelling (BAM, www.borealbirds.ca) project aims to further continental scale avian conservation through the integration and analysis of point count data collected across northern North America. In order to estimate population density and population size for landbird species, data integration became a real issue for us.

Two of the main sources of variation in the observed counts have nothing to do with ecological variables of interest, such as land cover and climate, but rather are considered nuisance variables. These are point count radius and point count duration. Recognizing that most studies do not follow survey protocol recommendations aimed to facilitate data integration (see e.g. Matsuoka et al. 2014), we opted to use model-based techniques to place our variable data on a common footing.

We first tackled standardizing for varying point count radii through distance sampling (Matsuoka et al. 2012) and eventually combined this with a removal model-based correction for varying point count duration. We named the method QPAD, referring to terms of our statistical notation: probability of detection (q), probability of availability (p), area surveyed (A) and population density (D) (Solymos et al. 2013). In the present paper we assessed different ways of controlling for point count duration. As the title indicates, we performed a cost-benefit analysis to make recommendations about when to use different types of the removal model.

We evaluated a conventional removal model and a finite mixture removal model, with and without covariates, for 152 bird species. We found that the probabilities of predicted availability under conventional and finite mixture models were very similar with respect to the range of probability values and the shape of the response curves to predictor variables. However, finite mixture models were better supported for the large majority of species. We also found overwhelming support for time-varying models irrespective of the parametrization.

So what is a finite mixture model? In our case, it splits the population of birds present at a location into frequent and infrequent singers. In previous parametrizations, researchers assumed that the singing rate of the infrequent group varies with date and time, whereas frequent singers remain frequent singers independent of time of year or time of day. We compared this to an alternate parametrization that assumes that individuals change behaviour over time and switch from frequent to infrequent singing behaviour—so it is the proportion of the two groups that varies. We found that the latter assumption was favoured.

Finite mixture models provide some really nice insight into how singing behaviour changes over time and, due to having more parameters, they provide a better fit and thus minimize bias in population size estimates. But all this improvement comes with a price: Sample size requirements (or more precisely, the number of detections required) are really high. To have all the benefits with reduced variance, one needs about 1000 non-zero observations to fit finite mixture models—20 times more than needed to reliably fit conventional removal models. This is much higher than previously suggested minimum sample sizes.

All of our findings indicate that lengthening the count duration from 3 minutes to 5-10 minutes is an important consideration when designing field surveys to increase the accuracy and precision of population estimates. Well-informed survey design, combined with various forms of removal sampling, is useful in accounting for availability bias in point counts, thereby improving population estimates and allowing for better integration of disparate studies at larger spatial scales. To this end, we provide our removal model estimates as part of the QPAD R package and the R functions required to fit all the above outlined removal models as part of the detect R package. We at the BAM project and our collaborators are already utilizing the removal model estimates to correct for availability bias in our continental and regional projects to inform better management and conservation of bird populations. Read more about these projects in our annual report.

A Better Way to Count Boreal Birds

CONDOR-18-32 C Kolaczan

Common Yellowthroats are among the birds for which new statistical models could provide better population estimates. Photo credit: C. Kolaczan

Knowing approximately how many individuals of a certain species are out there is important for bird conservation efforts, but raw data from bird surveys tends to underestimate bird abundance. The researchers behind a new paper from The Condor: Ornithological Applications tested a new statistical method to adjust for this and confirmed several mathematical tweaks that can produce better population estimates for species of conservation concern.

The University of Alberta’s Péter Sólymos and his colleagues tested a type of mathematical model called a “removal model” using bird count data for 152 species from the Boreal Avian Modelling Project, or BAM, which covers a vast area of Canada, Alaska, and the northeastern U.S. They found that incorporating variation in birds’ singing behavior improved models’ accuracy—how likely birds are to sing changes over the course of the day and the year, affecting how easy they are to detect. Extending the length of individual bird counts from three or five minutes to ten minutes was also beneficial.

“The chances of spotting something—a coin on the pavement, a bear in the mountains, or the apricot jam in the freezer—increases with effort,” explains Sólymos. “The more we walk, travel, look, or listen, the more things we find, but there is also a tradeoff between the number of places one can do a search and the length of the searches. Such decisions drive how long field biologists conduct bird counting at a given place. In our study, we looked at how the duration of counting influenced finding different bird species at different times of the day and the year. We also wanted to find the best way of how to standardize for different count durations and how to use these findings to provide better estimates of bird population sizes.”

This is more than just a math problem, however—accurate estimates of population size can be crucial for effective bird conservation, and many of the boreal bird species this study looked at are experiencing severe declines. This new approach offers a way to combine data from surveys that weren’t standardized in their design. “Population size of different species is one of the key metrics that affects their conservation importance, but estimating population size is a very challenging task that involves numerous assumptions,” says Sólymos. “Besides the ability to hopefully provide more accurate population size estimates, our modeling approach and findings can also help in timing of bird surveys to maximize the number of species detected.”

“While the authors of this study present the results of a rigorous comparison of modeling techniques to achieve better estimates of bird abundance from point counts, they also provide clear and simple recommendations on how and when to apply their findings to any study that expects to use time-interval point counts,” adds Jeff Wells, Science and Policy Director of the Boreal Songbird Initiative, who was not involved in the research. “This is a rich contribution not only to avian research methodology, but in the long run, also to bird conservation.”

Evaluating time-removal models for estimating availability of boreal birds during point count surveys: Sample size requirements and model complexity is available at http://www.bioone.org/doi/full/10.1650/CONDOR-18-32.1.

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.

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

7G1A5208

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.