Summary: Wintering Brown Shrikes frequent open lowland country, a habitat that is largely man-made in Malaya. Individuals are sedentary during the winter season, each occupying a restricted area and exhibiting territorial behavior.
The earliest observations each year in different parts of Southeast Asia indicate that the southward migratory journey is relatively rapid. In Malaya, migrants arrive from the first week of September to the third week of October. At a lowland netting station during 1964-68 a major part of the total catch was taken in the months of September and October. Only a small proportion of these early shrikes wintered in the netting area. No distant recoveries were reported, and the subsequent movements of birds that were not retrapped are unknown.
Shrikes netted in September-October comprised 2900 adults, 540/, full grown (i.e. immatures plus poorly-characterized adults), and 179, juveniles. The mean wing-length was significantly longer among adults than among both other classes, which did not differ significantly. During the winter, all ages showed a progressive decline in wing-length until the fight feathers were renewed in a premigratory moult falling in February-early April. Moult recorded in four Brown Shrikes taken in October-November is inte rpreted as the completion of a post-nuptial moult, commenced on the breeding grounds before autumn migration. In April, after the premigratory moult, confirmed adults constituted 70%, of the total trapped and apparent immatures 30%.
The mean weight of September birds was lower than any other month except November. The low weight in November is partly correlated with the shorter mean wing-length of the sample; it may also reflect the seasonally unfavourable weather of this month. The mean weight in February was high, although all birds were moulting; the weather in this month is typically hot and dry. Highest weights were recorded in April, indicating the premigratory deposition of fat. Weights of birds trapped more than once at different intervals showed a small initial weight loss (2 g), followed by a recovery within four days and no long-term adverse effects. A comparison of September weights in Taiwan and Malaya provides a tentative basis for the calculation of fat reserves utilized on the migratory fight.
The proportion of returns after one year was 11%, and after two years 1% only. Most returning birds were present in the netting area during the latter part of the winter of initial ringing; it is suggested tentatively that imprinting of the wintering grounds may occur during this period.
Ecologically in Malaya the Brown Shrike occupies a new habitat only gradually being filled by the resident Rufous-backed Shrike. There is no evidence of interaction between the two species.