SEASONALITY OF BREEDING

The breeding season in North Borneo.—Birds in the Quoin Hill area of eastern Borneo seem to breed most commonly in June, July, and August. Table 2 lists 34 of the more common species at Quoin Hill for which evidence on breeding was available. The actual evidence was provided by females with active brood patches or active ovaries, males with enlarged testes, birds in juvenal plumage, or birds actively in annual molt. From such data dates of presumed breeding were extrapolated. In Table 2, the solid black lines indicate dates for which both male and female were in breeding condition. The dotted lines indicate enlarged testes but no evidence of breeding in females. In the bottom line of Table 2, the figures indicate the percentage of the population breeding in any one month. For instance, 2.9 per cent of the birds were breeding in March, but 73 per cent were breeding in June. Rainfall records from the Cocoa Research Station from April, 1959, to December, 1964, were available to me. These data, along with the average for each month, are given in Table 3. There appears to be little correlation between rainfall and breeding season at Quoin Hill. A true dry season in the Quoin Hill area does not occur, but monthly rainfall has varied from 0.57 inches to 21.27 inches in a single year.

TABLE 2.—Seasonality of common breeding birds at Quoin Hill. Solid lines indicate times of occurrence of known breeding; dotted lines represent times of presumed breeding.

Species J F M A M J J A S O N D

Treron curvirostra

Cacomantis merulinus

Chalcites malayanus

Phaenicophaeus curvirostris

Collocalia fuciphaga

Chaetura leucopygialis

Harpactes diardi

Eurystomus orientalis

Calorhamphus fuliginosus

Megalaima chrysopogon

Megalaima mystacophanes

Sasia abnormis

Micropternus brachyurus

Dryocopus javensis

Pycnonotus cyaniventris

Pycnonotus atriceps

Pycnonotus brunneus

Pycnonotus erythrophthalmus

Zoothera interpres

Malacopteron magnirostre

Ptilocichla leucogrammica

Kenopia striata

Stachyris maculata

Orthotomus atrogularis

Orthotomus sericeus

Orthotomus sepium

Rhipidura perlata

Prionichilus xanthopygius

Anthreptes rhodolaema

Arachnothera flavigaster

Pityriasis gymnocephala

Oriolus xanthonotus

Platysmurus leucopterus

J F M A M J J A S O N D

The percentage of breeding in any one month is as follows: 0 0 2.9 8.8 38 73 58 50 35 17 11 8

Birds in the moss forest near Tenom appeared to be breeding in January, paralleling the trend found by Voous (1950a) for the lowlands of Borneo.

Other Bornean observations.—Voous (1950a) summarized data assembled by Coomans de Ruiter on the breeding of birds in the lowland of western Borneo near Pontianak. It appears that the breeding season in that part of Borneo, and indeed in all of western Borneo (Banks, 1950), starts in December and reaches a peak in March.

TABLE 3.—Monthly rainfall records, Cocoa Research Station, Quoin Hill.

YearJan.Feb.Mar.AprilMayJuneJulyAug.Sept.Oct.Nov.Dec.

19596.4912.1611.117.6412.114.758.3312.1013.81

19609.248.173.7610.658.8411.006.3111.258.565.498.3911.81

19616.688.064.354.747.557.255.932.407.475.584.3810.73

19623.826.7613.729.686.827.496.595.827.819.4719.809.28

196321.278.187.640.575.834.620.6412.495.248.757.4311.05

19644.177.924.4011.2011.828.042.427.525.6913.158.829.88

Average9.037.816.777.228.838.254.928.596.588.4610.1511.09

Gibson-Hill (1952) has questioned Banks' (1950) interpretation of data from the egg collection of V. W. Ryves. Gibson-Hill has shown that the data collected by Ryves covered two widely separated localities, one at Kiau near Kota Belud and the other near Sandakan. The former locality is on the west coast of North Borneo and the latter on the east coast of North Borneo. Gibson-Hill points out, and rightly so, that the timing of the rainfall in different parts of Borneo must be taken into account because of the large regional variation. The nesting data from the Ryves egg collection are scant and when used alone possibly yield a distorted view of the actual breeding season. Ryves did no collecting in the Sandakan area between September and March, and in the Kiau area between May and January. Although the breeding data from North Borneo accumulated by both Ryves and myself are limited, and records of rainfall are scant, there appears to be a trend toward breeding after the heavy rains have fallen.

Seasonality of breeding in tropical birds.—Possibly Bornean birds breed mostly in the "driest" part of the year. If so, this is in contrast with the time of breeding of birds of other tropical areas. Moreau (1950) found that in the Congo there was no distinct breeding season for most groups of birds, but that in East Africa there was a double breeding season; the peaks coincided with the two rainy seasons. Lack (1950) found that the Geospizinae of the Galapagos breed only when it rains and that rainfall causes a flurry of nest building and singing. If the rains stop, then the courtship activities stop until the next rains. Miller (1963) found that in birds of a western Andean cloud forest the breeding season was spread over the year and that breeding could not be correlated with rainfall.

Obviously more study is required on breeding of birds in Borneo before the timing of the annual cycle can be ascertained.