In a complete departure from my normal blogs, I thought I would post a brief review of the excellent “Vagrancy in Birds” by Alexander Lees and James Gilroy, which I have just finished reading.
The scientist in me has always been fascinated by the causes of vagrancy in birds. This excellent book has certainly enlightened me in regard to our current understanding of what causes migrating birds to head off in the wrong direction, sometimes ending up many thousands of kilometres away from where they should be.
The book is split into two main sections. The first section deals with the general subject of the cause of vagrancy in birds including the factual evidence behind the various theories. The second part of the book details the recorded vagrancy in birds by species.
The first section explains the two key requirements for successful migration, namely a compass to tell you which direction to go in and a clock to tell you when to start and stop your journey.
To understand vagrancy in birds it is therefore necessary to understand how birds navigate while on migration. It is now pretty much proven that birds use at least four methods to help them navigate all of which are reliant on the identification of north. These four known methods of identifying north are the sun, the pattern of polarised light, the stars and the Earth’s magnetic field. Evidence suggests that birds use at least two of these concurrently and possibly all four.
There is now compelling evidence that many birds can sense magnetic fields. Magnetic particles in their eyes have been shown to align with the Earth’s magnetic field to provide them with this 6th sense of the magnetic north. Sensing polarised light removes the main obstacle with using the sun to navigate, i.e cloudy days. The alignment of polarised light is strongest at 90 degrees from the sun and it is now known that most birds have an ability to sense patterns in polarised light, in fact recent experiments have shown that most humans can sense this, too!
Lees and Gilroy go into much detail as to the experimental evidence regarding these four avian compasses refencing back to many published scientific papers. For readers so minded, including myself, this section of the book was a really compelling read.
The book goes on to explain how one or more errors in reading the information from these four compasses lead to vagrancy. The most commonly discussed of these is reverse migration where the bird confuses north and south, probably as the result of a genetic mutation, and heads off at 180 degrees to the required direction. There are many maps in the book showing normal migration routes and where a 180 degree error might take a bird referenced back to actual vagrancy records. For example, this suggests an explanation as to why the Red breasted Flycatcher is an annual vagrant to the British Isles but the Collared Flycatcher is a very rare one. The maps illustrate well that reverse migration is not the whole story. In a typical year several thousand Yellow-browed Warblers are recorded in the UK, but a plot of the reverse migration line goes absolutely nowhere near the UK. Without going into too much detail here, the book goes on to examine other more complex types of compass errors, such as so called mirror image rhumb line disorientation would account for these autumnal UK records. Read the book for a very clear explanation of this!
The next section of the book deals in detail with wind drift vagrancy, i.e adverse weather condition blowing birds off course. This topic is by no means as simple as it might seem as birds have various coping methods to deal with wind direction change while migrating as explained with clear figures in the book.
The book then goes onto deal with overshoot as a cause of vagrancy, i.e the compass is read correctly but the clock telling a bird when to stop migrating is read incorrectly.
The book then moves into its second major section organised by bird family and detailing the know records of vagrancy in species.
Overall I thoroughly enjoyed the book and finished it feeling a lot more knowledgeable on the subject of bird vagrancy than when I started. As an added bonus, the book is illustrated throughout with beautiful plates of vagrant birds with text detailing the record.
In summary, I would thoroughly recommend Lees and Gilroy’s book to anyone who wants to understand more about vagrancy in birds.
Footnote – my blogs are posted with sometimes rather imaginative spelling and grammar due to my extreme dyslexia!
Very informative and a goddsummary of a complex book. Thanks
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