December. 1999 
15 
Schwartz, H J . S. Shaabani and D Walther, (eds), 
(1991) Range Management Handbook of Kenya. 
Vol II. I: Marsabii District. Kenya 
Wass. P. . ( 1995) Kenya s Indigenous Forests: Status. 
Management and Conservation. IUCN Forest 
Conservation Programme. IUCN Publications. 
Cambridge 
EARTHWATCH 
IMPACT OF TROPICAL FOREST 
FRAGMENTATION UPON BIRD 
POPULATIONS IN THE USAMBARA 
MOUNTAINS, TANZANIA 
AUGUST 23 TO SEPTEMBER 7 1999 
Tropical forest fragmentation and destruction is an issue 
or global concern While there is considerable concern 
about the loss of tropical forest, there are very few 
long-term studies examining the impact of tropical forest 
fragmentation upon vertebrate communities In his 
study. Dr. William Newmark has been monitoring the 
impact of forest fragmentation upon understory bird 
populations in the East and West Usambara Mountains 
of Tanzania for twelve consecutive years The 
Earthwatch fellowships have allowed several Kenyan 
researchers to take pari in this project 
THE STUDY 
The Study Area 
The Usambara Mountains contain the highest known 
diversity and endemism of plant and animal species in 
East Africa. They are part c»t a larger range nt 
mountains in eastern Kenya and Tanzania termed ihc 
Eastern Art Mountains, which have been recently 
declared one of the fourteen most endangered tropical 
forest ecosystems or hotspots world wide 
Located m North-eastern Tanzania about 50 km 
from the coast, the East Usambaias arc made up of 
intermediate forest fragments scattered in the vicinity 
of Amam These fragments range in altitude from 300- 
1500 m As a result of their physical proximity to the 
Indian Ocean and their elevation, these forests ate very 
effective in capturing precipitation and receive fairly 
high rainfall all year round The sludy site at East 
Usambara was located at an altitude of about 901) m 
Nine forest fragments have been selected lor this study 
Three of these arc "mainland" or control sites with 
fairly undisturbed intact forest, and the other six are 
smaller fragments at different levels of disturbance 
Objectives and Methods 
The main objective of the study was to investigate the 
effects of forest fragmentation and disturbance on 
understory birds at nine selected sues in East Usambara 
We did this by: 
• Sampling understory bird species using mist nets at 
three control sites and six fragments differing in 
disturbance 
• Documenting population variability of understory 
bird species between and within the sites 
• Measuring vegetation structure at randomly selected 
points in the nine study sites by estimating 
percentage herbaceous ground cover, density oi 
trees greater than 30 ern in girth, density of woody 
vegetation less than 30 cm in circumference, 
under story vegetation vertical cover less than 2.5 cm 
in height and measuring Macsopsis (a colonising 
tree species) less than 10 cm and greater than 10 cm 
in girth 
• Estimating food availability by collecting and sorting 
leaf-litter invertebrates 
• Scaling ihc effect of fragmentation on microclimate 
by taking simultaneous measurements of 
temperature, relative humidity and luminescence at 
three sues differing in terms of forest disturbance 
Understory Bird Sampling 
Understory birds were sampled using mist nets The 
nets were deployed from dawn lo dusk in .i linear lashmn 
through the forest so as to completely bisect the fores! 
fragments This made il possible to show bird 
distribution and composition from ihc forest edge to 
the interior 
At each study site, nets were put up for an average 
of three days each During our slay . wc mist netted at 
the three control sites and in two slightly disturbed 
fragments. The remaining lour sites were to be sampled 
after wc departed. All the nine forest fragments being 
studied are located on lea estates created between 60- 
100 years ago when the native Ibrcst was felled 
Each net is 13 m in length and is suspended between 
two numbered poles. When a bird was trapped in the 
nets, the pole numbers were noted. Tins helped m 
estimate at what distance from Ihc Ibresi edge the bird 
was caughi Fellows assisted in selling up ihc nets on 
arrival, and helped carry ihc birds lo and from the sludy 
Mies lor processing The Principal Investigator allowed 
a few fellows lo handle the birds, bul carried oul all 
che biometric measurements himself. He explained 
about the different types of rings used, and other ringing 
equipment 
Newly captured birds were ringed, and weight, tail 
and bill measurements taken. Recaptures had the ring 
number recorded and were weighed. Where possible, 
age and sex were noted 
Preliminary results showed there was a decline in 
ihc number of Thrushes ringed ai the condo! sites. 
Vegetation Structure 
There have been suggestions that forest interior species 
are adversely affected by changes in the structure of 
understory vegetation Wc sampled vegetation structure 
