Ecological factors influencing the reproductive ecology, territoriality and foraging behaviour of fiscal shrikes.

Abstract

This thesis describes the behavioural ecology of South African fiscal shrikes through detailed field observations and experimental manipulations, and discusses these in the context of reproductive ecology, life history strategies, territoriality, and foraging theory. A population of fiscal shrikes was observed during the period October 1995-0ctober 1997 in a 336 Ha area of grazed bushveld near Pietermaritzburg, South Africa. Fiscal shrike reproduction coincided with the onset of the rainy season. Nests were placed in the centre or north-west regions of tree canopies and contained three or four eggs. Incubation and brooding were performed by females, and shading behaviour was observed in hot weather. Hatching and fledging success averaged 48%. High, exposed nests were less successful than other nests as they were prone to damage during inclement weather. There was a trend in life history strategies between equatorial and South African fiscal shrikes (an increase in clutch size, incubation and brooding durations, and territory size and a decrease in breeding season length), and between northern hemisphere Laniinae (increased intensity in the northern hemisphere), that could be explained by variability in precipitation and temperature, and clutch survival. Fiscal shrikes maintained individual or pair-held territories throughout the year using visual and vocal signalling. Territories averaged 2.2 Ha in area and almost all contained man-made structures. Territory size was inversely proportional to tall tree density, as trees were used for foraging. Fiscal shrikes resided in areas with few natural perch sites by utilising fence lines and overhead cables. When perch density was manipulated fiscal shrikes decreased and increased territory size accordingly. Fiscal shrikes were sit-and-wait foragers and primarily used perch-to-ground forays. The diet of breeding birds consisted of large invertebrates. Small items were consumed and larger items were cached or fed to offspring or females. Fiscal shrikes were central place foragers. Prey size increased in patches over 35 m from the nest. Perch height affected search area, which in turn affected foraging methods, search duration and subsequent movements. Perches may be the limiting resource for other sit-and-wait foragers and the manipulation of perch density could act as a conservation strategy for declining populations.