The highly unusual “semi-identical” Australian twins reported last week are the result of a rare event. It’s thought the brother and sister (who have identical genes from their mother but not their father) developed from an egg fertilised by two different sperm at the same moment.
In humans, it’s the sperm that determines whether an embryo is pushed along a male or female development pathway. But in birds, it’s the other way around. Eggs are the deciding factor in bird sex.
There are other fascinating aspects of bird sex that are not shared with humans. Female birds seem to have some capacity to control the sex of their chicks. And occasionally a bird that is female on one side and male on the other is produced – as in recent reports of this cardinal in the United States.
X and Y, Z and W chromosomes
So what is it about bird chromosomes that makes bird sex so different from human sex?
In humans, cells in females have two copies of a large, gene-rich chromosome called X. Male cells have one X, and a tiny Y chromosome.
Birds also have sex chromosomes, but they act in completely the opposite way. Male birds have two copies of a large, gene-rich chromosome called Z, and females have a single Z and a W chromosome. The tiny W chromosome is all that is left of an original Z, which degenerated over time, much like the human Y.
When cells in the bird ovary undergo the special kind of division (called “meiosis”) that produces eggs with just one set of chromosomes, each egg cell receives either a Z or a W.
Fertilisation with a sperm (all of which bear a Z) produces ZZ male or ZW female chicks.
Birds can control the sex of their chicks
We would expect that, during meiosis, random separation of Z and W should result in half the chicks being male and half female, but birds are tricky. Somehow the female is able to manipulate whether the Z or W chromosome gets into an egg.
Most bird species produce more males than females on average. Some birds, such as kestrels, produce different sex ratios at different times of the year and others respond to environmental conditions or the female’s body condition. For example, when times are tough for zebra finches, more females are produced. Some birds, such as the kookaburra, contrive usually to hatch a male chick first, then a female one.
Why would a bird manipulate the sex of her chicks? We think she is optimising the likelihood of her offspring mating and rearing young (so ensuring the continuation of her genes into future generations).
It makes sense for females in poor condition to hatch more female chicks, because weak male chicks are unlikely to surmount the rigours of courtship and reproduction.
How does the female do it? There is some evidence she can bias the sex ratio by controlling hormones, particularly progesterone.
How male and female birds develop
In humans, we know it’s a gene on the Y chromosome called SRY that kickstarts the development of a testis in the embryo. The embryonic testis makes testosterone, and testosterone pushes the development of male characteristics like genitals, hair and voice.
But in birds a completely different gene (called DMRT1) on the Z but not the W seems to determine sex of an embryo.
In a ZZ embryo, the two copies of DMRT1 induce a ridge of cells (the gonad precursor) to develop into a testis, which produces testosterone; a male bird develops. In a ZW female embryo, the single copy of DMRT1 permits the gonad to develop into an ovary, which makes estrogen and other related hormones; a female bird results.
This kind of sex determination is known as “gene dosage”.
It’s the difference in the number of sex genes that determines sex. Surprisingly, this mechanism is more common in vertebrates than the familiar mammalian system (in which the presence or absence of a Y chromosome bearing the SRY gene determines sex).
Unlike mammals, we never see birds with differences in Z and W chromosome number; there seems to be no bird equivalent to XO women with just a single X chromosome, and men with XXY chromosomes. It may be that such changes are lethal in birds.
Birds that are half-male, half-female
Very occasionally a bird is found with one side male, the other female. The recently sighted cardinal has red male plumage on the right, and beige (female) feathers on the left.
One famous chicken is male on the right and female on the left, with spectacular differences in plumage, comb and fatness.
The most likely origin of such rare mixed animals (called “chimaeras”) is from fusion of separate ZZ and ZW embryos, or from double fertilisation of an abnormal ZW egg.
But why is there such clear 50:50 physical demarcation in half-and-half birds? The protein produced by the sex determining gene DMRT1, as well as sex hormones, travels around the body in the blood so should affect both sides.
There must be another biological pathway, something else on sex chromosomes that fixes sex in the two sides of the body and interprets the same genetic and hormone signals differently.
What genes specify sex differences birds?
Birds may show spectacular sex differences in appearance (such as size, plumage, colour) and behaviour (such as singing). Think of the peacock’s splendid tail, much admired by drab peahens.
You might think the Z chromosome would be a good place for exorbitant male colour genes, and that the W would be a handy place for egg genes. But the W chromosome seems to have no specifically female genes.
Studies of the whole peacock genome show that the genes responsible for the spectacular tail feathers are scattered all over the genome. So they are probably regulated by male and female hormones, and only indirectly the result of sex chromosomes.
Stephen Garnett, Charles Darwin University; Alienor Chauvenet, Griffith University; April Reside, The University of Queensland; Brendan Wintle, University of Melbourne; David Lindenmayer, Australian National University; David M Watson, Charles Sturt University; Elisa Bayraktarov, The University of Queensland; Hayley Geyle, Charles Darwin University; Hugh Possingham, The University of Queensland; Ian Leiper, Charles Darwin University; James Watson, The University of Queensland; Jim Radford, La Trobe University; John Woinarski, Charles Darwin University; Les Christidis, Southern Cross University; Martine Maron, The University of Queensland; Molly K Grace, University of Oxford; Paul McDonald, University of New England, and Sarah Legge, Australian National University
Glossy Black-Cockatoos used to be common on South Australia’s Kangaroo Island until possums started eating their eggs and chicks. After volunteers helped protect nest hollows and erect safe nest boxes, the population more than doubled.
But how do you measure such success? How do you compare cockatoo nest protection with any other investment in conservation?
Unfortunately, we have few ways to compare and track the different efforts many people may be making to help conserve our natural treasures.
That’s why a group of us from a dozen Australian universities along with scientists and private researchers around the world have created metrics of progress for both our understanding of how to manage threats of different intensity, and how well that management has been implemented. We also provide guidance on what still needs doing before a threat no longer needs active management.
For the first time, we looked at every threatened bird in Australia to see how well – or not – they are managed. Hopefully, we can use this to avoid compounding our disastrous recent track record of extinctions in Australia.
The state of Australian birds
What we did differently was collect the same data across different species, which meant we could compare conservation efforts across all bids.
When we applied these metrics to Australia’s 238 threatened bird species, the results were both encouraging and daunting. The good news is that we understand how to reduce the impact of about 52% of the threats – although of course that means we know little about how to deal with the other 48%.
But the situation is decidedly worse when we consider how effectively we are putting that research into practice. Only 43% of threats are being managed in any way at all – and just a third of the worst threats – and we are achieving good outcomes for just 20%.
But at least we now know where we are. We can celebrate what we have accomplished, appreciate how much needs doing, and direct our efforts where they will have the greatest benefit.
The threats to our birds
Introduced mammals, particularly cats, have been (and continue to be) a significant threat to Australian birds. Although we have successfully eradicated feral animals on many islands, saving many species, they remain a grave threat on the mainland.
The effect of climate change is becoming the top priority threat for the future. About half of all threatened birds are likely to be affected by increases in drought, fire, heat or sea level. Given the policy prevarication at a global level, targeted research is essential if birds are to be helped to cope.
By looking at multiple species, we can also identify what helps successful conservation. Monitoring, for instance, has a big impact on threat alleviation – better monitored species receive more attention.
There is also – unsurprisingly – a strong connection between knowledge of how to manage a threat and successful application of that knowledge. Often policy people want instant action, but our work suggests that action before knowledge will squander money.
Where to from here?
So what can we use this analysis for? One use is helping species close to extinction.
Using the same approach for multiple species groups, it is apparent that, while birds and mammals are in a parlous state, the most threatened fish are far worse off. We can also identify some clear priorities for action.
Finally, we must acknowledge this work emerged not from a government research grant, but from a non-government organisation (NGO). BirdLife Australia needed an overview of the country’s performance with threatened birds and was able to draw on the volunteered skills of biologists and mathematicians from around the country, and then the world.
Indeed, one of the future projects will be using the new assessment tool to see just how much of the conservation action around the country is being driven by volunteers, from the many people who contributed their knowledge and skills to this paper through to those keeping glossy black-cockatoo chicks safe on Kangaroo Island.
Stephen Garnett, Professor of Conservation and Sustainable Livelihoods, Charles Darwin University; Alienor Chauvenet, Lecturer, Griffith University; April Reside, Researcher, Centre for Biodiversity and Conservation Science, The University of Queensland; Brendan Wintle, Professor Conservation Ecology, University of Melbourne; David Lindenmayer, Professor, The Fenner School of Environment and Society, Australian National University; David M Watson, Professor in Ecology, Charles Sturt University; Elisa Bayraktarov, Postdoctoral Research Fellow in Conservation Biology, The University of Queensland; Hayley Geyle, Research Assistant, Charles Darwin University; Hugh Possingham, Professor, The University of Queensland; Ian Leiper, Geospatial Scientist, Charles Darwin University; James Watson, Professor, The University of Queensland; Jim Radford, Principal Research Fellow, Research Centre for Future Landscapes, La Trobe University; John Woinarski, Professor (conservation biology), Charles Darwin University; Les Christidis, Professor, Southern Cross University; Martine Maron, ARC Future Fellow and Associate Professor of Environmental Management, The University of Queensland; Molly K Grace, Postdoctoral Fellow in Zoology, University of Oxford; Paul McDonald, Associate professor, University of New England, and Sarah Legge, Associate Professor, Australian National University
Plastic pollution has the potential to cause the worst damage to seabirds in the seas around Aotearoa New Zealand, where many of them come to feed and breed.
Aotearoa boasts the greatest diversity of seabirds in the world. Of the 360 global seabird species, 86 breed here and 37 are endemic, which means they breed nowhere else.
Some 90% of New Zealand’s seabirds are threatened with extinction. They (and many other marine species) are under pressure from pollution, climate change, and overexploitation of marine resources. Plastic pollution could be the final nail in the coffin for many seabirds that are already struggling for survival.
Plastic – not so fantastic
Every week, another grotesque story illustrates the impact of plastic in the environment. A whale was recently found with 80 plastic bags in its stomach – it died, of course.
One-third of marine turtles have died or become ill due to plastic ingestion in Aotearoa New Zealand.
A 2015 study suggested that 99% of seabirds would be ingesting plastic by 2050. The authors also predicted that seabirds in our backyard, the Tasman Sea (Te Tai o Rēhua) would be the hardest hit, because of the high densities of seabirds foraging in the region, and the overlap with plastic. This not that surprising, given that the earliest observations of Aotearoa’s seabirds ingesting plastic go back to 1958.
Sentinels of ocean plastic pollution
Seabirds are particularly vulnerable to ingesting plastics because most species feed at or near the ocean surface. They forage along eddies and oceanic convergence zones – the same areas where marine plastics accumulate. The impacts of plastic on seabirds and other marine wildlife include death by entanglement. Ingested plastic can inhibit a bird’s feeding capacity, leading to starvation or internal ulcers, and eventually death.
Flesh-footed shearwater populations in Aotearoa may have declined up to 50% to around 12,000 pairs since the 1980s, and have gone extinct at some of their Hauraki Gulf breeding sites. These declines continue in spite of predator eradication and an end to harvesting on many of the islands where they breed.
Autopsies of birds caught in fisheries in Aotearoa’s waters show flesh-footed and sooty shearwaters are more likely to contain plastic fragments than other species. Plastic fragments found in New Zealand flesh-footed shearwater colonies showed a linear relationship between the number of nest burrows and plastic fragments, indicating that plastic ingestion may be a driver in their population decline.
Toxic plastic soup
In Australia, up to 100% of flesh-footed shearwater fledglings contained plastic, the highest reported for any marine vertebrate. Fledglings with high levels of ingested plastic exhibited reduced body condition and increased contaminant loads.
The chemical structure of plastics means that they act as toxin sponges, attracting harmful contaminants from the surrounding seawater, including persistent organic pollutants and heavy metals. When an animal ingests plastic, there is the potential for those toxic chemicals to leach into its tissues.
Chemicals such as PCBs and flame retardants that are added to plastics during manufacture have been found in seabird tissue around the Pacific. High concentrations of toxic chemicals can retard growth, reduce reproductive fitness and, ultimately, kill.
Sooty shearwater (tītī) chicks, which are harvested and consumed by Māori in Aotearoa, have a high potential for ingesting plastic, given evidence of plastic ingestion in shearwaters from Australia and anecdotal evidence from harvesters on Stewart Island (Rakiura). The closely related short-tailed shearwater, which breeds in Australia, has also been show to consume plastic. In one study, 96% of chicks contained plastics in their stomachs and chemical loads in their tissue.
Ocean health and human health
Few, if any, studies have specifically looked at contaminant loads derived from plastics in any species of seabird in Aotearoa. However, Elizabeth Bell from Wildlife Management International is now collecting samples of preen glands, fat and liver tissue for analysis of toxic chemicals in bycatch birds found with plastic inside them. This research is crucial to understanding the implications of the transfer of toxins to people from harvested species that ingest plastic.
Seabirds are the sentinels of ocean health. They tell us what we can’t always see about the health of the oceans and its resources that we rely on.
Plastics are sold to us on the perceived benefits of strength, durability and inexpensive production. These qualities are now choking our oceans.
In a few decades, we have produced an estimated 8.3 billion tonnes. The expedited pace of production has not been met with adequate waste management and recycling capacity to deal with it all. As a result, an estimated 8 million tonnes of plastic pollute the environment each year.
Global production of plastics is doubling every 11 years. It is predicted to be an order of magnitude greater than current production levels by 2040. The time is ripe for the initiation of an international agreement to lessen plastic pollution in the world’s oceans and save our seabirds and marine wildlife.
Many Australians feed wild birds in their gardens – yet the practice is discouraged by many bird groups and governments. That’s in stark contrast to what’s encouraged in other countries, so what should we be doing?
It’s an issue I studied in detail for my new book The Birds At My Table: Why We Feed Wild Birds and Why it Matters, out this month.
But first, let’s look at what happened when a sudden cold snap gripped parts of the Northern Hemisphere recently. This provides a clear example of a positive relationship between birds and humans, and how bird feeding can work.
When the “Beast from the East” rolled through Great Britain a few weeks ago it brought both dismay and delight to those housebound people peering out at their gardens smothered in metres of snow.
Birds – sometimes of species almost never seen in towns – were everywhere. Twitter (no pun intended) was filled with images of desperate animals.
Feed the birds
For the millions of people who provide food for wild birds in their gardens, this became a time for action. Social media was filled with pleas for people to venture through the drifts to refill their feeders: the birds need you NOW!
What struck me immediately about this desperate situation were the similarities to the UK’s infamous Great Blizzard of 1890-91. Despite the prevailing Victorian attitudes of “waste not, want not”, the severity of the conditions and the plight of the suffering birds lead to the first widespread examples of public bird feeding.
Spurred on by a multitude of items in the newspapers of the day, people were implored to search their kitchens for anything that the starving birds might eat.
A letter to the London Daily News from “Johnnie Thrush” suggested a mix of stale bread, water, oatmeal or barley meal and a few handfuls of hempseed.
This mixture made into a thick stiff paste which we can all sup with our bills, and the smallfry – those perky tits, chaffinches, sparrows etc., which abound everywhere, are equally delighted with the crumbs.
This appears to have been a pivotal moment: thereafter, feeding wild birds – a practice that would normally have been regarded as simply wasteful – became acceptable, widespread and even a sign of moral expression.
Today in the UK the feeding of wild birds in private gardens is a gigantic industry, and not just in cold weather conditions. Millions of people provide enormous amounts of bird food, mostly seed, all of which is consumed.
It is almost certainly the most popular form of interaction between humans and wild animals, and is actively promoted by organisations including the Royal Society for the Protection of Birds and the Humane Society in the United States.
The message is clear: if you care about birds, feed them!
Feeding Down Under?
In Australia, the social landscape could hardly be more different. The message – if you dare to ask – has long been emphatically, although still informally, “Don’t!” No jurisdictions have actually enacted anti-feeding legislation, but many have come close.
The abundance of (but thoroughly ignored) Do Not Feed The Birds signs now common in parks is part of this approach. But I would argue this is a very different matter to bird feeding in domestic gardens.
Those in the Northern Hemisphere who are interested in feeding birds can obtain endless and detailed advice on every conceivable aspect of the practice, and can buy a bewildering array of foods and feeders.
The contrast to Australia is stark and intriguing. Although there are plenty of bird feed products available with the label “Wild”, these are mainly mixes for cage birds. In terms of advice on feeding wild birds, however, this is almost all negative.
For example, BirdLife Australia says a “constant supply of ‘artificial’ food can be unhealthy for birds” and recommends that people opt instead for creating a “bird habitat through planting and providing water”.
Despite the ubiquity of the anti-feeding message that almost everyone in Australia is aware of, the participation rate here is virtually identical to that of countries where the practice is promoted and encouraged.
Around a third to over half of all households in this country regularly feed birds at their homes. That’s millions of people, most of whom care deeply about whether they are doing the right thing but who have nowhere to get advice or directions on best practice.
The only information available is a long list of the alarming things that can result from feeding birds, such as this advice against feeding lorikeets.
These were indeed disturbing and included (to take just the top few): dependency (the birds may become reliant on the food we provide); disease (feeders can spread disease); and nutrition (the food provided is often of poor quality).
If these concerns are valid, everyone needed to be aware of them and adjust – or stop – their seemingly trivial pastime accordingly.
Finding, distilling and understanding the research on which these issue were presumably based resulted in my new book.
It was a process that profoundly altered my perceptions and made me even more determined to encourage a meaningful discussion about bird feeding, here and around the world.
It’s a complex picture (as usual) but to address the key issues raised earlier: there is no evidence that birds become dependent on the food we provide (except in extreme conditions such as severe snow or drought).
Reassuringly, most birds visit feeders for a passing snack and the majority of their daily diet is still natural.
How to feed the birds
So if we want to feed the birds in your garden then there are a few very simple rules you should follow to make sure you feed them the correct way.
It’s a snack, not a meal. You don’t need to provide too much food. The birds only need a little; they will (and should) get most of their diet the natural way.
Keep it clean. Your bird feeder is a plate as well as a table, so clean it thoroughly every day.
Simple is best. Avoid anything processed (including mince or bread) or which contains salt or sugar. Seeds or commercial pet food is best.
Mix it up. Change the menu, and even the timing. They don’t need our food but it’s nice when they visit.
Remember that the feeders are really for us, rather than the birds. They don’t need them but they don’t seem to mind.
Have you ever got on a flight and the person next to you started sneezing? With 37 million scheduled flights transporting people around the world each year, you might think that the viruses and other germs carried by travellers would be getting a free ride to new pastures, infecting people as they go.
Yet pathogenic microbes are surprisingly bad at expanding their range by hitching rides on planes. Microbes find it difficult to thrive when taken out of their ecological comfort zone; Bali might just be a tad too hot for a Tasmanian parasite to handle.
But humans aren’t the only species to go global with their parasites. Billions of animals have been flying, swimming and running around the globe every year on their seasonal migrations, long before the age of the aeroplane. The question is, are they picking up new pathogens on their journeys? And if they are, are they transporting them across the world?
Migratory animals are the usual suspects for disease spread
With the rate of zoonotic diseases (pathogens that jump from animals to humans) on the rise, migratory animals have been under increasing suspicion of aiding the spread of devastating diseases such as bird flu, Lyme disease, and even Ebola.
These suspicions are bad for migrating animals, because they are often killed in large numbers when considered a disease threat. They are also bad for humans, because blaming animals may obscure other important factors in disease spread, such as animal trade. So what’s going on?
Despite the logical link between animal migration and the spread of their pathogens, there is in fact surprisingly little direct evidence that migrants frequently spread pathogens long distances.
This is because migratory animals are notoriously hard for scientists to track. Their movements make them difficult to test for infections over the vast areas that they occupy.
But other theories exist that explain the lack of direct evidence for migrants spreading pathogens. One is that, unlike humans who just have to jump on a plane, migratory animals must work exceptionally hard to travel. Flying from Australia to Siberia is no easy feat for a tiny migratory bird, nor is swimming between the poles for giant whales. Human athletes are less likely to finish a race if battling infections, and likewise, migrant animals may have to be at the peak of health if they are to survive such gruelling journeys. Sick travellers may succumb to infection before they, or their parasitic hitchhikers, reach their final destination.
Put simply, if a sick animal can’t migrate, then neither can its parasites.
On the other hand, migrants have been doing this for millennia. It is possible they have adapted to such challenges, keeping pace in the evolutionary arms race against pathogens and able to migrate even while infected. In this case, pathogens may be more successful at spreading around the world on the backs of their hosts. But which theory does the evidence support?
Sick animals can still spread disease
To try and get to the bottom of this question, we identified as many studies testing this hypothesis as we could, extracted their data, and combined them to look for any overarching patterns.
We found that infected migrants across species definitely felt the cost of being sick: they tended to be in poorer condition, didn’t travel as far, migrated later, and had lower chances of survival. However, infection affected these traits differently. Movement was hit hardest by infection, but survival was only weakly impacted. Infected migrants may not die as they migrate, but perhaps they restrict long-distance movements to save energy.
So pathogens seem to pose some costs on their migratory hosts, which would reduce the chances of migrants spreading pathogens, but perhaps not enough of a cost to eliminate the risk completely.
But an important piece of the puzzle is still missing. In humans, travelling increases our risk of getting ill because we come into contact with new germs that our immune system has never encountered before. Are migrants also more susceptible to unfamiliar microbes as they travel to new locations, or have they adapted to this as well?
Guts of migrants resistant to microbial invasion
To investigate the susceptibility of migrants, we went in a different direction and decided to look at the gut bacteria of migratory shorebirds – grey, unassuming birds that forage on beaches or near water, and that undergo some of the longest and fastest migrations in the animal kingdom.
Most animals have hundreds of bacterial species living in their guts, which help break down nutrients and fight off potential pathogens. Every new microbe you ingest can only colonise your gut if the environmental conditions are to its liking, and competition with current residents isn’t too high. In some cases, it may thrive so much it becomes an infection.
We found the migratory shorebirds we studied were exceptionally good at resisting invasion from ingested microbes, even after flying thousands of kilometres and putting their gut under extreme physiological strain. Birds that had just returned from migration (during which they stopped in many places in China, Japan, and South East Asia), didn’t carry any more species of bacteria than those that had stayed around the same location for a year.
Although these results need to be tested in other migratory species, our research suggests that, like human air traffic, pathogens might not get such an easy ride on their migratory hosts as we might assume. There is no doubt that migrants are involved in pathogen dispersal to some degree, but there is increasing evidence that we shouldn’t jump the gun when it comes to blaming migrants.
Alice Risely, PhD candidate in Ecology, Deakin University; Bethany J Hoye, Lecturer in Animal Ecology, University of Wollongong, and Marcel Klaassen, Alfred Deakin Professor and Chair in Ecology, Deakin University