Article curated by Ginny Smith
We see birds around us every day, so you might think we know all there is to know about our feathered friends - but that is far from the case! There are still many things we don't know about how birds behave, their history, and their future.
How do birds migrate without getting lost?
Many birds migrate - leaving behind cold winters where food may be scarce for sunnier, more plentiful areas. But how they know where to go, and avoid getting lost, is still not fully understood. Like early human travellers, it is thought that some birds use the sun and/or the stars to help make sure they continue in the right direction. Others use the earth's magnetic field to help them navigate with their own internal compass, although exactly how they do this is still unclear.
Some researchers believe quantum entanglement might explain birds' ability to detect the earth's magnetic field. Special cells which contain proteins with entangled electrons have been found to exist in birds' eyes. When light hits these proteins, one of the entangled electrons moves just a few nano-metres. This is enough for the two electrons to feel different forces from the earth's magnetic field. Different chemicals are produced depending on these forces, and the bird senses these, giving it a picture of the earth's magnetic field. 
Despite advances in this area, we don't understand how exactly these processes give rise to a useful navigational system in the animal, or how it maintains the delicate quantum entanglement long enough to make use of it. We also don't know how these systems evolved. Even more fundamentally, it isn't clear how birds know which direction to head for in the first place. Even on their first migration, many species know exactly where they are aiming for. This suggests an innate mechanism, but more research is needed to discover exactly how this works.
Why do birds bob their heads when walking?
You have probably noticed that some birds, such as chickens, bob their head as they walk, in a distinctive but rather comical motion! Some studies suggest this might be to do with vision, while others believe it could help their balance. But while researchers try to decide which of these is the more important reason, why exactly birds bob their heads remains unknown.
How are human behaviours affecting birds?
One thing that scientists agree on is that climate change is happening, and is something animals will have to deal with in the coming years. How different species will handle it, however, is less clear.
Over the last 50 years, scientists have been studying a population of great tits in Oxford, UK  . They found that the birds are flexible enough to adapt their behaviour to fit the changing climate. A clear indication of this is the fact that the birds now lay their eggs an average of 2 weeks earlier than they did 50 years ago- to fit in with the peak in caterpillar numbers, and ensure their young are well fed. This ability to change behaviour on shorter timescales than evolution allows means these birds could survive a change of up to 0.5C per year (better than the current predicted rise of 0.03C per year). If they didn't have this ability, it is estimated that there’s a 60% chance the birds would become extinct.
Although this is good news for Tits, and other short-lived, fast reproducing birds, it is unclear whether longer-lived species would adapt so well to a changing climate. This study shows that evolution alone is not enough for even tits to be able to survive the changes we are seeing in our climate. This means that any birds which do not have this flexibility in their behaviour could be in real trouble as the earth warms.
And there are other challenges that birds are facing thanks to human effects on the environment. One of these is the quantity of plastic in the seas. Birds have been found with remnants of plastic bags in their stomachs, which they have presumably mistaken for food. This can be hugely damaging, but there is at least the possibility of removing large pieces of plastic from the oceans to protect wildlife. Smaller pieces of plastic, known as micro-plastics, however, are impossible to move, and we don't know what damage they might be causing.
Some microplastics are generated directly, to use in face and body scrubs, toothpastes and other beauty products, while others are produced when larger pieces of plastic break down. They are too small to be seen by the human eye, and because of their small size they aren't filtered out during waste-water treatment. That means they wash down your plughole and straight out to sea, where they are an attractive food source for small marine animals.
We know that these plastics are having a negative effect on the animals that eat them, but what we don't know is what effect they can have as they are passed up the food chain, for example in birds that eat the affected marine animals. Even if the plastics themselves have broken down by the time they reach the birds, it is possible that this breakdown process could produce toxic substances that would damage the birds' health.
Cats are one of our most popular pets, with an estimated quarter of households now owning one. But compared to dogs, cats have been domesticated much more recently, so retain a lot of their wild characteristics, such as their hunting instinct. There have been few studies looking at the impact domestic cats have on wild birds and small mammals. In some regions of the world, such as Australasia, wild cat species were never present, so the introduction of pet cats has more risk of being problematic. Species they pray on are not used to avoiding them, so feral cats whose ancestors were originally brought over as pets have had a huge impact on certain native animals.
Keeping cats as pets allows the number to rise far above the maximum that would be possible for a natural population as the natural processes that normally keep predator numbers in check- access to food and territory, no longer apply. Because of this, there is concern that cats could have a destructive influence on bird numbers even in places where wild cats have existed, although how big a problem house cats are is still unknown.
This article was written by the Things We Don’t Know editorial team, with contributions from Ginny Smith, Cait Percy, and Johanna Blee.
why don’t all references have links?
 Lisa Grossman, "In the Blink of a Bird's Eye, a Model for Quantum Navigation" (2011) wired.com
 Vedder O, Bouwhuis S, Sheldon BC (2013) Quantitative Assessment of the Importance of Phenotypic Plasticity in Adaptation to Climate Change in Wild Bird Populations. PLoS Biol 11(7): e1001605. doi: 10.1371/journal.pbio.1001605
 Vedder, Oscar, Sandra Bouwhuis, and Ben C Sheldon. "Quantitative Assessment of the Importance of Phenotypic Plasticity in Adaptation to Climate Change in Wild Bird Populations" PLOS Biology 11.7 (2013): e1001605 DOI: 10.1371/journal.pbio.1001605