Inbreeding is the mating of closely related individuals, such as siblings or parent and offspring. This can lead to a phenomenon called inbreeding depression, where the offspring have reduced biological fitness. In birds, inbreeding depression has been shown to result in smaller egg size, lower hatching success, higher embryo mortality, slower growth, decreased survival, weaker immune function, and reduced fertility. However, the effects of inbreeding can vary substantially depending on the species and specific individuals involved.
Why do birds sometimes mate with siblings?
There are several reasons why sibling mating occurs in birds:
- Limited dispersal – Siblings remain in close proximity after fledging, increasing the chances of mating.
- Small population size – When populations are very small, birds have few options for non-related mates.
- Isolation – Birds on islands or in fragmented habitats may be geographically isolated from unrelated mates.
- Lack of mate choice – Some species have little or no mate selection, leading to more random mating.
- Inbreeding avoidance failure – Mechanisms to avoid inbreeding sometimes break down, allowing sibling pairs to form.
In general, birds tend to avoid mating with close relatives, but limitations on dispersal and population size can override this in some cases. Captive breeding programs are especially prone to inbreeding when the founding population is small.
What are the genetic risks of sibling mating in birds?
When closely related birds mate, it increases homozygosity – the chances of both parents passing down identical gene copies to offspring. This can have several genetic consequences:
- Expression of deleterious recessive alleles – Harmful mutations that are normally masked by a dominant allele can become expressed.
- Loss of heterozygote advantage – Beneficial genetic diversity from heterozygous loci is lost.
- Accumulation of new mutations – New recessive mutations are more likely to become homozygous.
These effects reduce the overall “genetic fitness” of inbred offspring. High homozygosity makes embryos more vulnerable to inheriting two copies of any deleterious alleles carried by the parents.
How does inbreeding affect egg size and hatchability?
Multiple studies on wild and captive bird populations have shown that increased inbreeding is associated with reductions in egg size, hatching success, and nestling survival. Some suspected mechanisms behind this include:
- Reduced maternal provisioning – Inbreeding depression impairs maternal health, resulting in fewer resources allocated to eggs.
- Developmental instability – Increased homozygosity disrupts optimal gene expression during development.
- Embryonic mortality – Lethal recessive alleles or loss of heterozygote advantage increases embryo death.
In an Ohio song sparrow population, inbred eggs were 3-5% smaller and had 6% lower hatching success. Similar effects have been found in captive populations of various bird species. However, a minority of studies have failed to detect impacts of inbreeding on egg and hatchling fitness.
Examples of reduced hatchability
- Domestic chickens – 13% lower hatch of eggs from full sibling matings vs unrelated.
- Lesser kestrels – 32% reduction in hatching success between outbred vs extremely inbred chicks.
- Song sparrows – 6% lower hatchability among most inbred offspring.
What developmental issues can inbreeding cause?
Inbred birds that do successfully hatch often show issues with growth, condition, and survival as nestlings or fledglings. Likely mechanisms include developmental instability, impaired immunity, and reduced competitive ability:
- Developmental instability – Disruption of optimal gene expression and developmental pathways.
- Weakened immunity – Greater susceptibility to pathogens and parasites.
- Reduced competition – Less vigorous begging, decreased motor skills, smaller size.
Studies on various species have found inbred chicks have slower growth rates, smaller body size, weaker immune responses, and higher rates of abnormalities and deformities. However, these effects are not always seen, suggesting impacts may depend on the specific genes involved.
Examples of inbreeding effects on nestlings
- Japanese quails – 8% smaller body mass among inbred chicks.
- Great tits – Reduced T cell immune response in inbred nestlings.
- Darwin’s finches – 3X higher fledgling mortality for inbred offspring.
Does inbreeding reduce reproductive success?
Inbreeding depression can carry over to reduce reproductive success and fertility in birds that reach adulthood. Some hypothesize this is due to developmental instability of the reproductive system. Impacts may include:
- Reduced semen quality in males.
- Developmental issues with reproductive organs.
- Impaired mate attraction cues like plumage and courtship behavior.
- Lower fertility rates for both sexes.
- Greater dysfunction of hormone regulation.
In song sparrows, inbred males have been found to be less successful at holding high quality territories. Female zebra finches produce 40% fewer offspring when paired with sibling males vs unrelated males. Such reductions in fitness can cause population declines.
Reproductive effects of inbreeding
- 44% reduction in egg hatchability in inbred pairings of Japanese quail.
- 24-40% fewer offspring produced by inbred zebra finches and prairie chickens.
- Lower testosterone levels measured in inbred male birds.
Does inbreeding always have negative effects?
While inbreeding depression is common, some exceptions have been observed in wild and captive bird populations. Mild inbreeding doesn’t always impart significant fitness costs. Several factors influence this:
- Overdominance vs. partial dominance – Impacts depend on type of gene action involved.
- Population history – Purging of genetic load can occur over multiple generations.
- Environmental conditions – Good conditions may mask inbreeding effects.
- Trait specificity – Not all traits are equally affected.
In one population of song sparrows, offspring had highest survival when the parents were moderately inbred, but survival declined when parental relatedness was too high or low. Inbreeding effects also tended to be stronger in years with poorer environmental conditions.
How does inbreeding impact species conservation?
Inbreeding is a major concern in conservation of small, fragmented, or declining populations. When populations lose genetic diversity, inbreeding depression can lead to:
- Reduced reproductive success
- Impaired adaptation to changes
- Increased extinction risk
Many threatened species have very small populations, making inbreeding unavoidable. Conservation programs aim to maintain genetic diversity through strategies like:
- Establishing interconnected populations
- Introducing unrelated individuals
- Captive breeding with genetic management
However, inbreeding effects can be unpredictable, and introducing outbred individuals risks outbreeding depression. Each conservation situation requires careful genetic monitoring and planning.
| Species | Inbreeding Effects Observed |
|---|---|
| White-tailed sea eagle | Lower hatching rate and fledging success |
| Lesser kestrel | Smaller egg size and reduced fertility |
| Kakapo | Poorer sperm quality and motility |
| Spanish imperial eagle | Lower offspring survival and defective feathers |
Conclusions
In summary, inbreeding between sibling birds can have a variety of negative consequences due to increased genetic homozygosity and loss of diversity. These include reduced embryo viability, poorer growth and condition of chicks, weakened immune function, lower reproductive success, and overall reductions in fitness. However, the magnitude of inbreeding depression varies across species and situations based on the genetic details involved. Inbreeding remains a significant concern in conservation, as it increases extinction risk, especially for small populations. Careful genetic management is necessary to maintain diversity and fitness for threatened species vulnerable to inbreeding.
