David Morley's 2021 analysis of how birds determine their geographical position
In about 1995 a pair of tiny birds, Spotted Flycatchers, started nesting in a rose trellis on the front of my house.
These little birds weighed less than 20 grams each and had flown from Southern Africa. They returned year after year only ceasing in about 2008. I thought that more than one generation must have been involved.
I started muttering about "How did they know the way" until my friends started ragging me, saying "Well, you're the scientist, you tell us !"
I started to study the matter, joining the Royal Institute of Navigation to get access to their extensive collection of published papers. I devoted about two months of evenings for several years to analysing what was known.
I had a bit of a head start because I had studied astronomy at University and knew the basics of spherical geometry and how people went about marine navigation.
At a time when most of the world's physics budget is being disposed on advanced quantum physics and on studying, for example, the nature of dark matter, the problems posed by 30 grams of bird probably seems quite trivial.
I believe in quantum mechanics because it works, but I have never really understood it. I try to restrict my thinking to more classical physics and to problem solving where data and the application of arithmetic may be thought to yield results.
I freely confess that our inability to understand the homing of pigeons and the navigational success of tiny birds has irritated me beyond reasonable measure.
The system just has to be blindingly simple.
THE ALFIC THEORY
The work reported here is in two stages, involving seemingly different mechanisms.
For terrestrial birds, migrating mostly over the land, the ALFIC system is proposed
ALFIC stands for "Approximate Longitude from Inhaled Chemicals". The inhaled chemical in this case is ozone. The MOSAIC-IAGOS project together with a growing number of other papers, has demonstrated that the tropospheric ozone follows the solar illumination level.
By troposphere, in this context, we are talking about ground level. Until the MOSAIC-ARGOS results became known, ozone was thought to somehow percolate downwards from the stratosphere.
The ozone's precursors are airborne hydocarbons. Over cities they tend to be byproducts of combustion, while over unindustrialised country the pre-cursor is often isoprene which is emitted from a variety of vegetation and most especially from conifers. So long as the bird can maintain clock synchrony for a day or two, then it can get a rough longitude by comparing the time of the ozone peak with what it has remembered or been born with.
At the time I was exploring this idea I knew that the system would not work over the deep ocean. There did not seem to be any suitable vapour or gas source and even stretching the imagination to the limit, I could not think that the bird could have a clock that would keep time for months.
This made me uneasy; I didn't believe that two groups of birds would have evolved completely different navigation tools.
Also, trying my ideas out on a few colleagues I got quite short shrift.
I therefore published the idea in a small book thinking that, if nothing else, this would give me priority.
THE DGF THEORY
As the Earth rotates past the directions of the sun and the moon the gravitational force changes. If you measure g it changes in a roughly sinusoidal fashion, the peaks being separated by a little over 24 hours. If you then calculate dg the difference between the maximum and minimum values then you can quite easily show that dg depends on latitude
If you then allow that the bird can measure F, the total magnetic field, a matter that does not seem to be in much doubt, then you have two independent coordinates. They are not orthogonal as are latitude and longitude but that does not matter; they only have to intersect
A bird flying over a land mass cannot deduce latitude by measuring dg. The terrain forces the bird to change height continually and this will obscure the detail of changing g. It is probably quite hard for a pelagic bird to make the measurements; for a terrestrial bird it seems a non-starter.
Nevertheless being able to detect changes in g at all opens the possibility of the bird having an internal clock
PIGEONS AND HOMING
I cannot convince myself that either of these methods completely accounts for the homing of pigeons.
Although there are some long distance homing flights, from the South of France to the United Kingdom, for example, in most cases the distances and time scales seem far too short to me.
I am still looking at this, but I tend to favour recent work published by Zannoni et al DOI 10.1038/s41598-020-72525-2 .