Breeding Habits of Boobies
Boobies live in colonies and, whereas most species nest on the ground or on rock ledges, some make rough nests of sticks in trees or bushes. They incubate their eggs by placing their feet over them and resting their entire body weight on the lower part of their legs. Two eggs are usually laid, a few days apart, and the first chick hatched is the one to survive. The second egg is thought to be a precaution against accident to the first and about 20 per cent of boobies are raised from second eggs.
Breeding takes place at different times depending on climate, sea conditions and species. At Ascension Island in the middle of the Atlantic Ocean where boobies ha -been studied in detail, the white booby breeds all the year round but the peak of egglaying is June and July with only a few clutches started during November, December and January. On the other hand, the egglaying of the brown booby is more spaced out and there are peaks of laying in April and December.
All boobies nest in colonies with the birds usually crowded together but nesting vary. White boobies nest on the flat tops of cliffs while the brown boobies choose more inaccessible places along the ledges on the cliff faces. Red-footed boobies and Abbott's boobies nest in trees or bushes and are remarkably agile in this arboreal life.
Pairs are slow to form and there is a prolonged courtship, but once formed the pair stay together for life. Like the albatrosses the boobies have an elaborate courtship dance. The male cocks his tail up and goose-steps, lifting his feet as high as possible and puffing out his breast. Sometimes he merely marks time, paddling in one place, then he struts up and down and eventually turns towards the female and, raising his wings, he emits a whistle. This is repeated several times, then the strutting continues. At first the female takes little notice of this performance but later she becomes more demonstrative, leaning forward to touch the male's bill or neck.
The nest is built of guano (the dried droppings of the birds themselves), seaweed, feathers or fish bones which the male collects and deposits in front of the female as she builds up a cup-shaped nest. Two eggs are laid but rarely is more than one chick reared. Eggs are sometimes kicked out of the nest as the incubating bird flies off. However, any eggs that a sitting booby can reach with its bill it will roll back into the nest, with the result that boobies are sometimes found sitting on six or seven eggs that have been retrieved after being knocked out of their neighbors' nests.
Boobies do not have a brood patch, the area of bare skin on the breast where the eggs are kept warm. Instead the webs of their feet develop a rich network of blood vessels which supply heat to the eggs balanced on their feet.
Both parents incubate the eggs, taking turns of a few hours each. When not incubating or feeding the boobies congregate with immature birds in groups called clubs outside the breeding colony.
Eggs hatch after 6 or 7 weeks and the chicks emerge naked and helpless. After a fortnight they are covered in a white down and in 5 months they have grown a full covering of feathers and are ready to take to the air.
This latter situation, together with recurrent food shortage, usually faces the three pan-tropical boobies so that at best they rear only single young (even though the Masked and Brown boobies lay two eggs) and many young die of starvation in the nest. Conditions are probably similar for Abbott's booby which has a particularly long breeding-cycle so that, if successful, it can nest only every other year.
The Blue-footed booby, on the other hand, often breeds in somewhat better circumstances and may rear two (occasionally three) young, while the Peruvian booby has easily the best feeding conditions of all and rears two or three (occasionally four). The nestling periods of boobies are mostly longer than in gannets, lasting up to 20 weeks or more in the Abbott's booby.
Unlike young gannets, the juvenile booby has no surplus fat but returns to the birth-site for several weeks at least after fledging to be fed by its parents.