Who wants to live forever?

Aging and the limits to bird lifespan

The natural limits to lifespan

The concept of species-specific life spans is widely accepted. This is the maximum lifespan of a species as determined by its genetic make-up, which in turn is shaped by evolution driven by environmental constraints. Maximum lifespan is intricately interwoven with life history strategies and, in mammals, there is a well-accepted continuum with short-lived species of small body size, early maturation with short gestation periods and giving birth at short intervals to numerous offspring (e.g., most rodents) and long-lived species (e.g., elephants, humans) with opposite life history parameters. Interestingly birds in general, however, live on average 1.5 times longer than similar-sized mammals despite their much higher (2–3 times) metabolic rates. Thus, life span cannot evolve independently from other life history traits.

Lifespan, longevity and life-expectancy

Lifespan and longevity both refer to the maximum length of an individual’s life, and should not be confused with life expectancy which is generally defined as the average length of time that an individual of any species may live.

Senescence

Little Sparrowhawk Accipter minullus, a predator which is one of the variables affecting life-expectancy in smaller birds.

Do birds eventually get so old that they are less likely to breed and survive than younger individuals? This is the definition of senescence, a decline with age in these key life history characteristics. Birds, however don’t age like we do and senescence is not easily detected in wild birds, because so few individuals live long enough to show signs of old age. In birds aging is difficult to see. In fact, once most birds develop their adult plumage, they essentially become impossible to age – no grey feathers develop around the temples to give us clues and a Bald Ibis is bald from the start. Instead, most wild birds will die from predation or other non‐age‐related causes such as food availability and environmental conditions. However, numerous field studies have now lasted long enough to provide good evidence for age‐related declines in birds if they make it through the gauntlet of predators and other natural hazards.

Source: www.outthere-bygeorge.com

What do we know about lifespans and life expectancy of our local birds?

South African bird species have lifespans that range from extremely short, just over a year, to seabirds who are known to have the longest lifespans of all birds. Species at the high end of the lifespan spectrum are characterised by slow metabolism and development, delayed reproduction, low reproductive investment, and long-term pair bonds, with the opposite characteristics at the short end. Precise information on the lifespans of different species is not easy to come by. It is usually impossible to follow large groups of individuals from hatching to death, so in addition to collecting data directly by banding and recapturing individuals, many indirect methods of estimating age are used. In practice, aging birds from banding is more complicated than it seems. Only a small percentage of banded birds are ever observed again, and if they were adults when they were first banded, their starting age is unknown.

Robin-chats

Terry Oatley’s work on this group of birds changed previous perceptions regarding lifespans and showed that they are surprisingly long-lived for small terrestrial, insectivorous birds. His ringing of birds in the Dargle area provided evidence of a Chorister Robin-chat Cossypha dichroa living to the amazing age of at least 24 years.

Chorister Robin-chat Cossypha dichroa – currently holding the longevity record amongst the robin-chats – showing the diagnostic hangman cap.

Of the 14 species of robin-chat which occur in South Africa probably the best known to upper Highway homeowners are the two common resident species, Red-capped Robin-chat (previously Natal Robin) Cossypha natalensis, and Cape Robin-chat Cossypha caffra. These two species have been determined, through ringing studies, to have lifespans of at least 12 and 17 years respectively. The Red-capped Robin-chats in particular are much loved garden birds with their beautiful russet and blue-grey plumage and distinctive cree-craw contact call. Their diet relies on a healthy garden, free of pesticides and herbicides to support their ground-foraging diets of invertebrates (insects, crabs) and fruit.

Red-capped Robin-chat Cossypha natalensis a garden favourite with its beautiful plumage and distinctive contact call.

Barn Swallows

Barn Swallows Hirundo rustica are the most abundant and widely distributed swallow in the world. Their return to KwaZulu-Natal is always greeted with joy as they herald that summer is on its way. These high energy little birds have a life-expectancy of around 3 years but some wily and successful individuals have been recorded to live 16 years, enough time for these amazing travellers to have covered roughly half the distance to the moon during their annual migration. The major causes of mortality affecting the life-expectancy of these birds are weather combinations of cold and rainy conditions which can occur late spring and early summer in the northern hemisphere breeding grounds. This affects food availability for adults and nestlings. Record cold weather conditions in 1974 in Europe killed at least one million migrating barn swallows with many of them juveniles. This can also be the case in their southern hemisphere overwintering grounds and starvation is the ultimate cause of death due to the non-availability of insect prey items.

A juvenile Barn Swallow Hirundo rustica

Sunbirds

Although there are 81 sunbird species in Africa longevity records largely come from work done on only 18 species in southern Africa. Although generally small birds with high metabolic rates this is a group with longer lifespans than would be expected. The largest sunbird (by length and weight), the Malachite Sunbird Nectarinia famosa, which although not common does occur in the upper Highway area. These emerald jewels can live for at least 10 years.

An unmistakeable male Malachite Sunbird Nectarinia famosa in breeding plumage

The smaller and less visually obvious, coastal Grey Sunbird, Cyanomitra veloxii which inhabits forests and gardens in KwaZulu-Natal is often located by its call. This noisy sunbird has a large down-curved bill which it uses to probe its favourite flowers such as those of the Tree Fuschia Halleria lucida.

A young Grey Sunbird Cyanomitra veloxii showing a hint of a red epaulet at the shoulder.

The abundant nectar-rich flowers of Halleria lucida being visited by an Olive Sunbird Cyanomitra olivacea.

Cape White-eye

Cape White-eye Zosterops virens

These are ubiquitous and attractive birds which flock in the canopy of a wide variety of KZN habitats. A species that gleans insects from the mid to top layers of trees and eat aphids in large quantities particularly an important food source to these birds in winter. They are gregarious and flock in small parties throughout the year and can even make up large flocks of 100+ birds during the breeding season. They have high mortality rates with 35% of adults lost annually. Being small they have many predators including larger bird species, snakes and even spiders who can ensnare them in their webs. This means that although their possible lifespan is around 11 years their average life expectancy is closer to 7-8 years.

Wisdom

Indian Yellow-nosed albatross soaring within sight of the city of Durban

Many large seabirds are especially long-lived. Albatross species in general will survive at least 50 years. One of the most famous of these is Wisdom a wild female Laysan Albatross. She is the oldest confirmed wild bird as well as the oldest banded bird in the world. She was originally ringed in 1956 with the red ankle band Z333 and she is known to have successfully hatched a chick this past February, 2021 making her at least 70 years old. She has survived natural disasters and has outlived the person who banded her, the US Fish and Wildlife Service biologist Chandler Robbins, who died in 2017 at age 98.

Clues for identifying the longer (and shorter) lived birds around you

For those watching birds in our gardens or adjacent open spaces, we can begin to predict the birds that are likely to be longer and shorter lived. Longer lifespan is often associated with features of a bird’s biology and natural history. Here are five characteristics that can help to make an educated guess about which species are likely to be longer-lived:

Body size: generally larger species tend to live longer than smaller species.

Number of young: birds with longer lifespans often have fewer young, while those with shorter lifespans tend to have more.

Years to adulthood: shorter-lived species usually reach adulthood more quickly than longer-lived species.

Terrestrial ground dwelling species: birds that live and nest on the ground have often adapted for shorter lifespans than those that live higher up, such as in the shelter of the tree canopy.

Island life: birds that live and nest on islands are often longer-lived than their mainland counterparts.

So on the basis of the bullet points above of this which do you think lives longer: a Natal Spurfowl Pternistis natalensis or a Ring-necked Dove, Streptopelia capicola?

Natal Spurfowl

Ring-necked Dove

To help you make this choice here is information on these two species. Natal Spurfowl, are larger than Ring-necked doves, (410g versus 153g), have substantially more chicks (up to 5-10 eggs versus up 1-2 eggs), reach adulthood more quickly (one year versus three years), and live on the ground.

If you chose Ring-necked Dove, you were right. The longevity record for these doves is 37 years, while the oldest recorded Natal Spurfowl is 7 years old.

In addition to these biological and ecological features, there is another factor that often predicts how long a bird species is known to live and that relates to how many studies have been carried out. In general, birds that have been better studied are more likely to have records of long-lived individuals. We actually have no idea about the life history traits of many of even our common garden birds. For example we have no idea of the two common garden species in the Upper Highway area, Goldentailed Woodpecker, Campethera abingoni and Cardinal Woodpecker, Dendropicos fuscescens with no details of age to sexual maturity nor longevity.

Two common garden species, the Golden tailed Woodpecker (left) with diagnostic black speckled forecrown and spotted back and a Cardinal Woodpecker (right) with its brown forecrown and barred (not spotted) back.

It’s a tough life!

As the song says ‘the world has only one sweet moment set aside for us’ and that moment is strongly influenced by other factors rather than the genetics that determine lifespan. The highest mortality, from the moment a bird embarks upon its adult journey and leaves its nest, is generally found in inexperienced young birds. Once they are adults, after they have successfully reared young, the probability of death each year remains more or less constant. In other words, few birds die of ‘old’ age – they just run the same gamut of risks year in and year out until their ‘one sweet moment’ comes to an end.