I think we can learn more from studying the oddities in life than by simply examining the average and perhaps boring cases. The secretary bird (Sagittarius serpentarius) is truly one of those oddities, the only species in a whole taxonomic family of birds compared to a group like the Pigeons which has 348 species or the Pheasants with 181 species, or the 352 Hummingbird species. Having said that monotypic families like this are not uncommon amongst the birds, for example the Ostrich, Osprey and Sunbittern have just one species in each family. Initially, taxonomists assumed that the Secretary bird with its long striding legs, soft footfall and a hurried but delicate demeanor must surely be closely related to the Cranes. However, subsequent morphological and genetic analysis places this species smack in amongst the diurnal raptors. The Secretary bird family sits between the Osprey family and the diurnal raptor family that includes the 250 species of Hawk, Eagle, and Kite.
The cause of this confusion, no doubt arose because the Secretary bird exhibits parallel evolution. It has evolved to live in the African savannah lands where it feeds on Snakes, Lizards, large insects, and small mammals. To do this it needs to be long-legged, tall, and fast to see the prey and chase it down. A taller bird is a heavier bird so even though the Secretary bird is very light for its size, it still has difficulty and must run hard to get sufficient lift for flight. As such it is energetically economical just to chase the prey down rather than fly at it. This is a very similar feeding behavior to that observed amongst the Cranes and Bustards, hence they look somewhat similar, and I guess you could easily mistake the Secretary bird as a Crane, unless you stop and examine the skull and bill, then you would know it was indeed a raptor.
The most popular derivation of the Secretary bird’s name is that the feathers sticking out behind the bird’s head reminded 19th-century Europeans of the quill pens that secretaries then tucked behind their ears. More recently the etymologists have been arguing about it and a new suggestion is that the name derives from the Arabic ‘saqr-et-tair’, which means ‘hunter bird’. This fits of course and reflects the Latin name of Sagittarius, that in astrology also means the hunter. I suspect the former explanation will dominate although at the end of the day, the bird is still rather unique.
Long legs and venomous snakes
One clear difference between the Secretary birds and the Cranes is that the Cranes use their bill to catch prey while the Secretary bird, like so many other raptors, uses its feet. Indeed, watching a Secretary bird chase down and use well-aimed kicks and stamps to stun or kill a snake is amazing to watch. They exhibit remarkable force and accuracy as they basically lift their leg up to the body and punch the snake in the head at about 200 Newtons, which is basically 5 times their own body weight. As a comparison, a professional boxer will punch at about 3.5 times their body weight, so the secretary bird gives a big punch. The birds have legs almost twice as long as any bird of similar mass so we can assume the really long legs are an adaptation for killing prey. Interestingly, the Secretary bird shares many bone characteristics with the extinct ‘terror birds’ which also relied on the use of a strong kick to kill their prey. These amazing birds used to inhabit South America where some species were more than 10 feet (3m) tall and would kill mammals and reptiles.
Catching and eating venomous snakes is a dangerous game so many snake-eating vertebrates, like Mongoose, have developed special means of protecting themselves against the snake’s venom. The α-neurotoxins that many venomous snakes have act to bind tightly to a receptor that is found on skeletal muscles of vertebrates, and in so doing stops the neurotransmitters from activating the muscles. In effect this leads to paralysis so the snake can just make a rapid strike and then wait as the prey dies, ensuring it doesn’t get bitten or kicked before consuming a fresh meal. When you spend your life feeding on venomous snakes then selection should favor those individuals that have evolved venom resistance mechanisms. Indeed, most vertebrates accept that if they eat venomous snakes they are going to get bitten and have evolved the means by which the toxins are blocked from binding to the receptor and so avoid the paralyzing effects. The exception is the Secretary bird who have not evolved any obvious receptor blocking resistance to venom. This could mean one of two things, either the birds do not have the genetic ability for this to evolve or alternatively they simply do not face this problem because they rarely get bitten by snakes. Since they kick box the snake and have thick scales on their legs, it would appear the snakes may rarely if ever get their fangs into the soft tissue of the Secretary bird and so they avoid exposure to the toxin.
Threatened species
As a unique bird we should worry about the preservation of the Secretary bird. In 2016 it was designated by the IUCN as Vulnerable and recently, in 2020, listed as Threatened. The main reason was because the few indicators we have all show the population is decreasing throughout much of its range at an alarming rate. The birds are distributed over a wide part of sub Saharan Africa (excluding the rain forests) where there are now thought to be between 70,000 and 100,000 individuals. Indeed, it is the most threatened raptor species in western Africa, although in some areas it seems to be doing well in protected areas and interestingly amongst some types of crops where presumably their food is abundant. The decline has been very evident in Tanzania and South Africa, where it was once abundant and has exhibited a massive decline over the past 25 years. Indeed, part of the issue with the Secretary bird is that we know so very little about it, there are no systematic monitoring schemes, no recovery plans, no awareness programs and it is not included in any international conservation legislation.
The main threats the Secretary bird faces are mostly associated with human encroachment and habitat loss indicating both disturbance that may affect nesting trees and loss of places to capture food. In some parts of its distribution the species is not persecuted because of its good ability to kill snakes but in other areas the young are taken from the nest when the nest is accessible. They are mostly solitary and sedentary, although frequently pairs will forage together, and they will become nomadic and follow rainfall or fires and capture animals that are flushed from the flames.
I suspect it is the immature birds that suffer the most. While the adults tend to stay with a nest site and initially the young remain close, they subsequently wander up to 150km from their nesting site in search of unoccupied suitable territories before retuning close to the parents nest. This extensive wandering exposes them to accidents with fences and wires and brings them into contact with humans. Having this bird slowly slip into oblivion would be shameful, it is one of those charismatic and special species of Africa along with the Elephant, Giraffe, and Cheetah. We need to make people more aware about the stet and issues of the wonderful Secretary bird.
