Technical Report #197.
William Vogt with translation and notes by David Cameron Duffy. July
2018. A report on the guano-producing birds of Peru (“Informe sobre
Aves Guaneras”). 198 Pages
(Modified from the original) Vogt studied the Guanay Cormorant (Phalacrocorax
bougainvillii), Peruvian Booby (Sula variegata), and Peruvian
Pelican (Pelecanus thagus) for almost three years (1938 - 1940).
The nesting range of these species extends from 04035'S to 380S,
covering an area of considerable geographical diversity. The climate is
briefly described. The report contains details of the oceanography and
marine biology of a part of the nesting range of the birds, and a
rejection of the theory that a warm southward-flowing surface current
causes major changes on climate and breeding season for guano birds. The
histories of past abnormal years and their effects on the birds are
The average density of Guanay Cormorant nests is 313.9 ± 3.76
per 100 m2. The ecological efficiency of the islands and their
microclimates are linked. The Guanay Cormorant is limited to nesting on
the windy parts of the island, which are the coolest, as there is an
inverse relation between wind and temperature.
A detailed description of the social behavior of the Guanay
Cormorant, including its feeding and nesting, is given. Nesting is
concentrated in spring and summer. The breeding season keeps the adults
on the island for four months every year. The average Guanay Cormorant
clutch size is 3.13 ± 0.101 eggs and incubation lasts 27 days. Nesting
in large colonies protects nests because fewer birds are at the edges in
larger colonies. Various causes of mortality are described. The only
significant predator of the Guanay Cormorant may be the Andean Condor (Vultur
Food is probably the most important limiting factor for the
birds. There is a
correlation between the abundance of food for birds and the abundance of
diatoms. Each Guanay Cormorant eats approximately 215 g/day of food and
no more than 316 g/day.
Annually, each Guanay Cormorant eats 78.4 to 115.4 kg/year of the
Peruvian Anchoveta (Engraulis ringens) and produces approximately 15.8
kg of guano per year. The quantity of fish consumed by birds in the year
prior to the guano harvest of 1940 was between 711,903 and 917,150
Based on extensive data, this is much lower than previous
estimates. Each ton of guano is the result of 4.95 - 7.3 tons of fish
consumed, but the guano that the company removes from the islands is
only a small proportion of the guano that the birds deposit at sea,
where it may act as an important fertilizer for plankton.
Preliminary studies suggest that anchoveta are migratory. I
have rejected, because of a lack of supporting data, the theory that
anchoveta are still present during food shortages, but at depths too
great for them to be taken by birds. A preliminary study of 1,427
anchoveta indicated a marked reduction in the year-classes hatched in
the spring and summer of 1939 and 1940, when the birds died of hunger.
Anchoveta spawned in 1938 were the most abundant. The Peruvian Anchoveta
appears similar in biology to the California Sardine (Sardinops sagax
caerulea). The commercial anchoveta fishery should be carefully
monitored as it represents one of the most serious potential threats to
the guano industry.
Various interactions of the birds and humans are discussed. I
review the history of the guano birds, based on available data, from the
pre-Columbian period to the present. The ecologies of the Peruvian Booby
and Peruvian Pelican are discussed. Each species seems to occupy a
distinct niche so that, within broad limits, the three do not compete
with one another. Interactions or the synecology of plants and animals
connected with the guano birds appear to be so complex that they require
more thorough study. Various management methods are suggested that might
allow an increase in the numbers of birds and the proportion of guano
harvested. Various management methods are suggested that might allow an
increase in the numbers of birds and the proportion of guano harvested.