Volume 45, Number 2, December 1998:
Feral Pigeons

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ABOUT THIS ISSUE
- about KSN
- about the author

IN THIS ISSUE
- introduction
- origin of feral pigeons
- basic plumages
- mate choice and plumages
- advantages of different plumages
- advantages of choosing different mates
- breeding seasons
- reproducative data
- brood reduction
- living in colonies
- commuter pigeons
- relationships with people
- reference

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Updated: March 9, 2005
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Feral Pigeons
by Richard F. Johnston

BROOD REDUCTION

Since pigeons spend around fifty days caring for eggs and squabs, each young bird represents a major investment of time and energy. What should pigeons do if food shortages occur when they are breeding? Should they continue to feed both young, and risk both being underfed? Or should they concentrate on feeding just one of them? The answer is that feeding one is better biology. However, pigeons do not do this consciously and it is not a matter of simple choice, as the following discussion will show.

The two eggs of pigeons are laid about 40 hours apart. The first egg is partly incubated before the second appears, and it hatches about 24 hours earlier than the second. First eggs tend to be male, second eggs female. Thus, just after hatching, a pair of squabs usually consists of a day-old male and a recently-hatched female. Males are larger than females at hatching, and they have been fed for a whole day prior to the female's having hatched, so the size difference is pronounced.

Second hatchlings therefore need to compete with an older, more capable sibling for food and parental care. But second hatchlings have a faster rate of growth, and if food is abundant they can make up much of the size difference at around day 15. If food is scarce, the first squab is always fed more than the second, and the latter often dies. The surviving squab is fledged at a greater than average body weight, at the usual age of 30 days.

Pigeon reproduction thus includes this automatic brood reduction mechanism. It enables parents to rear at least one squab if food scarcity makes it difficult to rear two. Indeed, such brood reduction enables parents to raise one squab when trying to raise two would lead to death for both squabs. This is a remarkable adaptation for efficient reproduction.

The first hatchling does not have to be a male for this automatic mechanism to be brought into play. Brood reduction in pigeons would function even if the first egg were female. It would not function quite as efficiently because the size disparities of the sexes would, at least sometimes, inactivate the mechanism. Having the first egg male is a kind of insurance that the brood reduction strategy will work, and that probabilities of loss of both squabs will be reduced.

In birds the chance that the first egg will be male is ordinarily 50 percent, which makes great sense since the way in which eggs are made results in half of them being male and half female. But in pigeons, the first egg is male 70 percent of the time. No positively effective way for dictating sex of eggs is known, so that a certain mystery attends how pigeons manage such control 70 percent of the time. It is a mystery that should attract attention of students of biology.