Vagrancy in Birds. Lees, Alexander and James Gilroy. 2021. London: Helm.
“Birds have wings, and they use them.” [Often quoted in several variations in many bird books, but I have been unable to discover the origin.]
Way back on January 9, 1983, I was leading a BBC trip to South Quabbin, also known as Quabbin Park. There was not much around, which is typical for that time of the year, but we pulled into Enfield Lookout in the hopes of seeing an eagle. Back in those days many birders got their year or even life eagle from that lookout. If we were lucky, we were hoping to see turkeys across the water on the Prescott. Nobody even thought about ravens; they were not that widespread then. Unexpectedly, three geese flew in front of us, a bit of a way out, almost at eye level. They flew left and out of sight, then appeared again this time flying left to right, then away from us along the Prescott Peninsula, and eventually to the right and out of sight. These three geese were strikingly plumaged. Their entire inner forewing was white, they were gray underneath, but their heads had rusty tones. Their light eyes were outlined in black. The entire trip was dumbfounded because none of us had any idea what we had just seen. Did we witness a flock of vagrants? But what species and from where?
About an hour later, the trip took the short hike into Gate 52, and there we found the geese again perched across the water. Using our scopes, I identified them as Egyptian Geese. Do not ask me how I knew what they were; possibly I had seen pictures of them in some global waterfowl guide. Because there had been no previous records in the Quabbin area, and we knew of no accepted records for the state, we eventually assumed the Egyptian Geese were escapees from some exotic waterfowl collection. Therefore, they were not countable on our lists. Still, that momentary thrill of seeing what could have been genuine vagrants, possibly new to the state, remains vivid today. That is because birders crave vagrants. Vagrant species can determine where we go on a day trip. Vagrant species encourage us to wait for hours in bad weather for a bird to show up. Birders absolutely love vagrant species.
‘Vagrants,’ ‘accidentals,’ ‘rarities,’ ‘extralimitals,’ and ‘casuals’ are all synonyms for records of nominally ‘out of range’ individuals of a given bird species. Humans have long coveted records of these ‘lost individuals’, and there is a rich ornithological literature that describes various subcultures associated with their pursuit from 19th-century collectors to 21st-century twitchers (Mearns and Mearns,1998, Wallace 2004). The obsession surrounding vagrant birds has been historically derided by some ornithologists, who argued that records of vagrants are of little biological relevance, but we share the contention of others (e.g., Grinnell 1922, Rose & Polis 2000, Newton 2008) that vagrancy is a powerful biological phenomena [sic], whose study is fundamental to understanding the diversity of life on earth. (p. 7, Vagrancy in Birds)
Vagrancy In Birds is a unique book, a scholarly volume of ornithological research that birders will also find fascinating and enjoyable. Alexander Lees is a senior lecturer in Biodiversity at Manchester Metropolitan University and an associate of the Cornell Lab of Ornithology. James Gilroy is a lecturer in ecology at the University of East Anglia who studies the long-distance movements of animals. Together they have written a truly global assessment of how and why birds stray out of their normal ranges. It is a complex story for which there is still a lot to be learned.
Vagrancy does not occur only in birds, and there are records of butterflies, moths, dragonflies, bats, grasshoppers, sea turtles, seals, and cetaceans all straying out of their usual ranges. But in birds, because they can fly, we find the most frequent, widespread, and extreme examples of wandering far beyond where they usually migrate.
The first 70 pages of Vagrancy in Birds is a summation of what has been learned about how birds navigate during migration. According to the authors, birds have four basic compasses, ways in which they orient themselves on those long yearly migratory trips. They can use the sun, patterns of polarized light, reading the stars, and the earth’s magnetic fields to help find their way. No one species uses all these cues. Some birds also use visual cues of landmarks to develop a mental map of their routes. To aid in the timing of migration, birds also have an internal avian clock.
One possible cue that migrants may use to judge when to cease migration is time itself. Most animals possess an internal clock, and in birds the clock sense is known to be particularly advanced, with the capacity to keep track of time at high precision across daily and annual cycles. (p. 16)
Obviously, any breakdown in the ability of a bird to read these cues could lead to that bird wandering off its usual route. There can be compass errors like reverse migration (for example, going north when the bird should be heading south), or something the authors name as mirror-image misorientation (heading northwest, when they should be heading northeast). Magnetic anomalies in the earth’s field may also cause birds to stray. All of these subjects are described in detail in the text.
You would expect juvenile birds to wander off course most often because they have not yet learned the correct route to fly. But there are many records of adult vagrants.
An important question in the context of vagrancy, is if adult birds typically possess such a finely-tuned mental map of their world, why do some of them still end up as vagrants? One possibility is that some birds may reach adulthood without managing to accurately internalize a working map during their developmental years. (p. 19)
Vagrant birds of social species like waterfowl, cranes, or shorebirds may link up with migrating flocks of other species and continue their migration with them. Possible examples of this phenomenon here in New England occur when we see a species such as Pink-footed Goose in a large flock of Canada Geese or a Rufous-necked Stint among hundreds of Semipalmated Sandpipers.
For species that migrate within single species flocks, the social component of navigation may be even more direct. Evidence suggests that some species defer navigation decisions to a subset of experienced (usually older) individuals, with younger birds following behind and perhaps learning a trail of visual beacons along the way (Flack, et.al., 2012). (p. 34)
A common outcome for lost juvenile cranes (or geese) is to join flocks of other species, sometimes entirely different genera, and migrate with them. These flocks can become ‘carrier species’ that often lead the stray individuals into long range vagrancy. Each year birders across the Holarctic avidly search through wintering flocks of Arctic-breeding geese and cranes in search of stray vagrants that have fallen foul of this mechanism. Some individuals will then continue to migrate back to the Arctic with their adopted species year after year, producing hybrid offspring in some cases (Ottenbughs, et. al., 2016). (p. 34-35)
You may think adverse weather is the greatest cause of vagrancy, but Lees and Gilroy write that this is not the case.
It is understandable that laypeople assume that vagrant birds are usually ‘blown off course’ by adverse weather, but even the most cursory analysis reveals that a significant proportion of long-range vagrancy events cannot be explained by exogenous factors like winds alone. Vagrancy events are near constant, global, and often entirely uncorrelated with weather patterns, indicating that there are invariably other factors at play. Indeed it is likely that most incidences of long-distance vagrancy are driven by factors that are endogenous to the birds themselves. Failures in the compass system are perhaps the most obvious and pervasive mechanism that would cause birds to stray from their normal ranges. Even humans equipped with satellite GPS can still find navigation difficult and frequently get lost! (p. 21)
Still, weather events such as hurricanes or other powerful atmospheric events do cause vagrancy. You can be fairly sure that a strong southerly storm will be accompanied by a list of sought-after species. Here in Massachusetts, birders closely watch the pathway of incoming hurricanes. Depending on whether the eye passes well inland or closer to the coast determines where birders will venture out, braving destructive winds and falling limbs. Hopes are always high of spotting a Leach’s Storm-Petrel at Quabbin or Wachusett reservoirs or a Bermuda Petrel along the shore. Though this does not always work out for the vagrant seeker, some of these atmospheric events have produced historic outfalls of vagrant species.
A particularly notable drift event occurred in the Bering Sea during the peak El Niño year of 1998, when the convergence of two storm systems in mid-May brought an unusually strong and consistent vector of winds stretching from the Aleutian Islands as far east as the sea of Okhotsk. On 17 May 1998 observers on Attu tallied almost unbelievable totals of vagrant landbirds: some 180 Eyebrowed Thrushes Turdus obscurus, 223 Olive-backed Pipits Anthus hodgsoni, 193 Rustic Buntings Emberiza rustica, and 366 Bramblings Fringilla montifringilla, with no fewer than 700 Wood Sandpipers Tringa glareola being found the next day (Hameed et al. 2009). (p. 44–45)
Some species may even wander extensively on purpose as a way to explore potential new feeding or nesting territories.
As noted above, vagrancy among migratory species may often be driven by adverse weather conditions or navigational failings, but a significant proportion of vagrancy events among migrants may also involve ‘deliberate’ exploratory wandering in search of new territories. In many migratory species, juveniles can make apparently random movements away from their natal sites during the first weeks after fledging, prior to commencing their proper directional movement. (p. 55)
This brings us to human-driven vagrancy. This possibility is the bête noir of all regional rarities committees. Was that extra-limital bird, that everyone was so excited about adding to their lists, ship assisted? In a recent issue of British Birds it was revealed that the October 2019 British record of a Paddyfield Pipit (Anthus rufulus) was not added to the British list. Paddyfield Pipits are from Asia and are not migratory, so there was a real possibility that the bird was human assisted, either as a caged bird or brought to the area by accident by a ship or plane. You can imagine the outcries of the hardcore listers who had ticked the bird. Some of the examples cited by the authors of Vagrancy in Birds make for fascinating reading. There is also an additional concern that some of the birds that are human assisted may establish themselves in new areas and eventually transition from beloved vagrant to invasive pest.
The accidental transport of birds by boats and other vehicles arguably presents an even greater headache for birders trying to decide what vagrants count as ‘wild’ and therefore ‘tickable’. Shipping, in particular, plays a major role in influencing the movements of birds across oceans and one that is hotly debated amongst birders and ornithologists. Birds, and migratory landbirds in particular, frequently alight on vessels at sea, sometimes in truly spectacular numbers. Whilst ships are slow-moving in avian terms, birds that remain on board for long periods can inevitably find themselves moving inadvertently out of their normal ranges, and there are multitudes of examples of wild birds being carried vast distances aboard ships—for example Snowy Sheathbills Chionis albus transported from the Antarctic to the British Isles (Jay 1993) and Taiwan (Lin et al. 2018).
The debate usually starts when such a bird makes landfall in a new area—should we count it as ‘wild’ and a ‘natural vagrant’, even though it has not made the journey under its own steam? This is a tricky question—and one that troubles legislators as well as birders, as ‘wild’ status typically determines whether individuals are protected under regional wildlife laws. Usually, organisms transported outside their native ranges by boats are considered non-native in their destination regions and may thus be targeted for culling to prevent them establishing invasive populations (Gilroy et al. 2017). Some of the most adept ship-born hitchhikers in the avian world are potentially significant pests. (p. 59)
Cases of escapes from collections or ship assists have caused numerous headaches for birders here in Massachusetts. In January 1991 a flock of six Barnacle Geese was seen flying about the Osterville area on Cape Cod. Up until that point all state records of Barnacle Geese were of single birds, and all were considered to be escapes from waterfowl collectors. But six birds couldn’t escape, right? That this was a flock was first considered to add credence that these were genuine and therefore countable birds. Many birders spent many hours trying to track these birds down only to learn later that the flock had indeed escaped en masse from a collection. (See Birds of Massachusetts p. 95.)
Some accounts of human-assisted vagrants have unique consequences. In 1982, a Eurasian Jackdaw was found at Tom Nevers Neck, Nantucket. Just previous to its appearance, I had vowed to Sheila C. that if I ticked a lifer in Massachusetts before the end of the year, we would get married. It was December, so I thought, “What were the chances?” To make a long story short, we traveled to Nantucket, I ticked the jackdaw, and we got married. It was later discovered that several jackdaws had appeared in eastern North America, all being clearly ship assisted. I crossed jackdaw off my state list, but we stayed married and are still married to this day.
If you think just because a bird is found aboard ship it is automatically not countable in all cases, the authors of Vagrancy in Birds offer this:
When considering whether ship-assisted birds should be classed as ‘wild’, there is a huge grey area between the clear-cut examples of ‘unnatural’ port-to-port transport for largely sedentary species (such as House Crow) and the potentially much shorter (but no less ‘unnatural’) transport of other migratory birds that alight on vessels for a few days during their long overwater flights. At what point do we draw the line between unnatural vagrancy caused by ship assistance and natural transoceanic vagrancy? (p. 59)
Think about the times when you have been aboard an overnight pelagic and a migrant warbler has landed on the deck. This bird may remain on board the ship for some time and may even accept water or food—such as peanut butter—from the passengers. Eventually the bird leaves and takes off across the water. Yes, the bird was ship assisted. It may not have survived but for that pause aboard ship. So, is that bird countable?
The bulk of Vagrancy in Birds are the “Family Accounts” (p.71–329) in which the authors describe patterns of vagrancy for every bird family. Many of these accounts are quite short, amounting to a single paragraph. You can imagine how little there is to write about accounts of vagrancy in Struthionidae (Ostriches) or Tinamidae (Tinamous) (both on p.71). You may think some species are rather sedentary and there is little chance they could wander, but there are surprises. Under Phasianidae (p. 82–83) there is an amazing account of mass vagrancy of ptarmigans.
In North America, where it is known as Willow Ptarmigan, Willow Grouse routinely make significant migrations in flocks numbering up to 2,000 individuals, particularly in Manitoba, Saskatchewan, and Alberta (Hannon et al. 1998), with more than 1,000 observed on intertidal flats at Kotzebue sound, Alaska, in September 1988 (Lehman, 1989). This species is apparently found with some regularity as a migrant offshore, with Zimmerman et al. (2005) describing an encounter with a group of more than 100 in August 2003 in Kuskokwim Bay, Alaska, many of which landed on the ship, and were even observed landing—and taking off from—the sea surface! (p. 83)
The much longer accounts of vagrancy in families like Anatidae (waterfowl p.73–81), Procellariidae (Shearwaters and Petrels p.163–68), Trochilidae (hummingbirds p.103–7), Accipitridae (Kites, Hawks, and Eagles p.186–91), and many others have a lot of information that will be of interest to New England birders.
A final important chapter discusses how global climate change may affect patterns of vagrancy in some species.
Vagrancy in Birds is an important contribution to the vast literature on bird movements and migration. It is well illustrated with numerous color photographs from around the world. Vagrancy is a complicated subject, particularly when considered on a global scale, yet the authors present the essential information, references, and examples to make these odd cases of vagrancy something that should be of real interest to both ornithologists and birders. In the birding world, sightings of vagrants will continue to challenge and frustrate us as we try to tease out how any one vagrant got here, always asking: “countable or not?” Vagrancy in Birds will be a reference cited by rarity committees in the future. But this is a book that should be enjoyed by all birders. For those of you who thrilled at the Steller’s Sea-Eagle on the South Shore or enjoyed a Rufous Hummingbird at a feeder in December, Vagrancy in Birds is a volume that will give you some understanding about how these unexpected birds wandered here.
“Migration is the greatest adventure in the life of a bird, the greatest risk it must take.” Roger Tory Peterson
- Pitches, Adrian. 2022. Paddyfield Pipit NOT added to the British List. News and Comment section, British Birds 115:126.
- Veit, Richard R. and Wayne R. Petersen. 1993. Birds of Massachusetts. Lincoln, Massachusetts: Massachusetts Audubon Society.
To listen to my interview with Alexander Lees, please go to: https://www.wicn.org/podcast/alexander-lees/