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.
| Year | Jan. | Feb. | Mar. | April | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
| 1959 | 6.49 | 12.16 | 11.11 | 7.64 | 12.11 | 4.75 | 8.33 | 12.10 | 13.81 | |||
| 1960 | 9.24 | 8.17 | 3.76 | 10.65 | 8.84 | 11.00 | 6.31 | 11.25 | 8.56 | 5.49 | 8.39 | 11.81 |
| 1961 | 6.68 | 8.06 | 4.35 | 4.74 | 7.55 | 7.25 | 5.93 | 2.40 | 7.47 | 5.58 | 4.38 | 10.73 |
| 1962 | 3.82 | 6.76 | 13.72 | 9.68 | 6.82 | 7.49 | 6.59 | 5.82 | 7.81 | 9.47 | 19.80 | 9.28 |
| 1963 | 21.27 | 8.18 | 7.64 | 0.57 | 5.83 | 4.62 | 0.64 | 12.49 | 5.24 | 8.75 | 7.43 | 11.05 |
| 1964 | 4.17 | 7.92 | 4.40 | 11.20 | 11.82 | 8.04 | 2.42 | 7.52 | 5.69 | 13.15 | 8.82 | 9.88 |
| Average | 9.03 | 7.81 | 6.77 | 7.22 | 8.83 | 8.25 | 4.92 | 8.59 | 6.58 | 8.46 | 10.15 | 11.09 |
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.