We can agree to disagree, but the scientific literature is verbose, eloquent and certain on the subject. Inbreeding without culling reduces average fitness. Some species are, however, more strongly affected than others.
Wikipedia says "Inbreeding is the reproduction from the mating of two genetically related parents, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is called inbreeding depression. Deleterious alleles causing inbreeding depression can subsequently be removed through culling, which is also known as genetic purging.
Livestock breeders often practice controlled breeding to eliminate undesirable characteristics within a population, which is also coupled with culling of what is considered unfit offspring, especially when trying to establish a new and desirable trait in the stock. . . . Inbreeding may result in a far higher phenotypic expression of deleterious recessive genes within a population than would normally be expected. As a result, first-generation inbred individuals are more likely to show physical and health defects, including:
Reduced fertility both in litter size and sperm viability
Increased genetic disorders
Fluctuating facial asymmetry
Lower birth rate
Higher infant mortality
Slower growth rate
Smaller adult size
Loss of immune system function"
Note that individuals are not simply "inbred" "or non-inbred" (I'm sure you know this, coopslave, but some others might not). Inbreeding is measured through the inbreeding coefficient, which ranges from 0 to 1. The higher the value, the more inbred. Full siblings with unrelated parents (even if highly inbred themselves) have an inbreeding coefficient of 0.25. Half-siblings, 0.125. First cousins, 0.0625.
I agree that, in theory, those birds could have died from some disease my flock had grown immune to. However, I've brought in many new birds from trusted sources over the years, and have never had that happen again. As I said, I've heard many similar stories.
Last edited by ipf on Fri Jul 08, 2011 10:40 am; edited 1 time in total