Many people wonder about the survival rates of baby birds. How many actually make it to adulthood? The answer depends on many factors.
Nestling mortality rates
Nestling mortality can be quite high for many species of birds. One study examined nestling survival rates for 34 species of open-cup nesting birds. They found an average nestling mortality rate of 28% prior to fledging. Rates ranged from 12% to 58% depending on the species. Similar nestling mortality rates have been found across numerous other studies.
There are many causes of nestling mortality including:
- Starvation
- Exposure to extreme weather
- Predation
- Parasitism
- Disease
- Accidents such as nest collapse or sibling aggression
The most vulnerable time is often the first few days after hatching. Mortality rates tend to stabilize or even decline somewhat after this initial period.
Fledgling mortality rates
The story does not end once birds leave the nest. Young birds that have fledged still face high mortality rates. One study found an average post-fledging mortality rate of 49% across 33 species studied. Rates ranged from 27% to 67%.
Major causes of fledgling mortality include:
- Starvation
- Predators
- Collisions with windows or vehicles
- Human activities like agriculture
- Severe weather events
Mortality is highest immediately after leaving the nest. Fledglings are inexperienced, often poor flyers, and at risk until they develop stronger flying abilities and some survival skills.
First year mortality rates
Mortality remains high beyond the fledgling stage. One study estimated that on average, 63% of birds die within their first year. There is a great deal of variation by species and other factors.
Some major factors impacting first year mortality include:
- Starvation, especially during migration or winter when food is scarce
- Predation
- Disease
- Severe weather such as droughts, cold spells, or storms
- Habitat loss from human activities
The pressures of migration, finding food and shelter, establishing a territory, avoiding predators, and surviving weather extremes all contribute to high mortality rates for young birds in their first year after hatching.
Lifespan and mortality overview
Here is an overview of average lifespan and mortality rates for songbirds:
Stage | Average Mortality Rate |
---|---|
Nestling | 25-30% |
Fledgling | 35-55% |
First Year | 50-70% |
Adult Annual | 30-60% |
Average Lifespan | 2-5 years |
As these numbers show, mortality is extremely high in a bird’s first year of life. If they can make it through that initial year, their chances improve significantly and average adult lifespans are generally 2-5 years depending on species.
Mortality rates by species
There is considerable variation in mortality rates across different bird species. Here are some examples:
Tree swallow
- Nestling mortality: 15%
- Fledgling mortality: 55%
- First year mortality: 65%
- Adult annual mortality: 47%
- Average lifespan: 2.7 years
Savannah sparrow
- Nestling mortality: 28%
- Fledgling mortality: 63%
- First year mortality: 72%
- Adult annual mortality: 59%
- Average lifespan: 2 years
Western bluebird
- Nestling mortality: 19%
- Fledgling mortality: 32%
- First year mortality: 49%
- Adult annual mortality: 38%
- Average lifespan: 5 years
As you can see, mortality rates are significantly lower for Western bluebirds compared to tree swallows and Savannah sparrows. This demonstrates how lifespans and survival rates can vary among species.
Factors impacting mortality rates
Many factors influence mortality rates for nestlings, fledglings, and adult birds. Here are some of the major factors:
Weather
Severe weather events like storms, flooding, heat waves or cold spells can all lead to direct mortality or impact food availability. Birds are very vulnerable to weather extremes.
Predators
Common nest predators include snakes, squirrels, raccoons, crows, jays, owls, hawks, and domestic cats. Fledglings and juveniles are also heavily predated by a wide array of predators.
Starvation
Lack of food is a huge threat. Nestlings rely completely on their parents for food delivery. Fledglings must learn to forage on their own. Migration and winter can be times when starvation risk is high.
Habitat quality
Loss of habitat from human activities reduces available nesting spots and food resources. Fragmented or degraded habitats increase mortality.
Parasitism
Nest parasites like brown-headed cowbirds can reduce nestling survival. Parasitic flies, mites, and blood parasites can also impact adult birds.
Disease
Bacterial, viral and fungal infections are common and may impact fledglings and adults more than nestlings. Salmonellosis, avian influenza, trichomoniasis, aspergillosis and avian pox are examples.
Nest characteristics
The nest location, structure, concealment from predators, and shelter from weather impacts nestling survival rates.
Parental care
Experienced parents tend to have higher nesting success. Quality of parental care influences growth and survival of young. Male songbirds often help feed nestlings.
Population density
Higher densities of birds may increase disease transmission risks and competition for food or nesting sites.
Human impacts on mortality
Some direct and indirect ways humans increase mortality risks for wild birds include:
- Habitat loss from development, agriculture, logging, etc.
- Buildings and windows collisions
- Vehicle collisions
- Power line electrocutions
- Light pollution interfering with migration
- Pesticides reducing insect food sources
- Outdoor cats killing billions of birds each year
Habitat loss may be the biggest human-caused problem facing many bird populations. Providing quality habitat is key to helping bird populations thrive.
Conclusions
In summary:
- Mortality is extremely high in the first year of life for most songbirds.
- Nestling mortality ranges 15-30% on average.
- Fledgling mortality ranges 35-65% on average.
- First year mortality ranges 50-70% on average.
- If birds survive their first year, average lifespan is 2-5 years for most species.
- Many factors impact mortality rates including weather, predators, disease, competition, and human activities.
- Mortality rates vary significantly between species.
- Habitat quality is a key factor influencing populations and mortality rates.
Providing quality habitat is essential to help bird populations thrive. While mortality is naturally high in young birds, humans can take actions to give them the best chance at survival.