LeGrosetal. • URBANIZATION AND HOST-PARASITE INTERACTIONS 
793 
Habitat 
BG. 2. Proportion of parasitized Northern Mocking- 
hird nests in each habitat. WP = wildlife preserve, PAST = 
pa>ture, RES = residential, and PL = parking lot. Asterisks 
show habitats with significantly different parasite preva¬ 
lence. WPand RES were significantly different at the 10% 
fcvcl but not 5%. 
W, clutch size, land-use category, study site, 
and P ,ant type did not have a significant effect on 
parasitism status of the nest. The percentage of 
building, tree, and open area all had a positive 
cdecl 011 the probability of a nest to be parasitized 
l I ' able 3 * X 2 = 6.67. df = 1, P = 0.010; x 2 = 
df» 1, P = 0.005, and x 2 = 7.60, df = 1, P 
~ respectively). There was no effect of any 
y ariab!c on parasite intensity per nestling. 
DISCUSSION 
Ttle prevalence of P. porteri differed among 
abitats with fewer parasites in nests in the 
wildlife preserve and parking lots than in pastures 
and residential areas (Fig. 2). This result was not 
consistent with the parasite-release hypothesis 
which indicates parasitism would decrease with 
urbanization. The percentages of buildings, trees, 
and open areas positively affected the probability 
of being parasitized. The relationship among 
these three parameters and urbanization, however, 
is not straightforward and their interactions could 
explain the observed habitat effects. The percent¬ 
age of ground covered by buildings increases with 
urbanization, but the percentage of ground 
covered by trees tends to decrease with urbani¬ 
zation. The percentage of open areas does not 
seem to be correlated with urbanization as it is 
lowest in residential areas and highest in pastures 
(Table 3). Little is known about the life cycle of 
P. porteri and it is difficult to explain the pattern 
of higher prevalence of parasitism at moderate 
levels of urbanization. It could be tied to 
abundance of food resources for adult flies, 
presence of predators of adult flies including 
their parasites, distance between nests, or other 
mechanisms. We detected at least one parasite of 
P. porteri (a tachinid fly found at the wildlife 
preserve), but we know nothing of the abundance 
of this parasite or even the identity of potential 
predators of adult flies. 
The abundance of the host could also influence 
the prevalence of parasitism by providing more or 
fewer nests in which the parasites can lay eggs. 
High densities of hosts could result in a dilution 
effect (Ostefeld and Keesing 2000), i.e„ a lower 
proportion of mockingbird nests being parasitized. 
Fewer hosts would result in a higher proportion of 
nests being parasitized. Our results, however, 
showed little relationship between host density 
and parasite prevalence as the highest parasite 
prevalence was in habitats with both high 
r TA f LE 3. Percentage of ground covered by buildings, open areas (sum of pavement and gnua* trees 
. ^determined surface, calculated for each Northern Mockingbird study site from satellite images 
•'W-2008. ORD = Ordway-Swisher Biological Station. BRU = Beef Research Urut, SF = Santa Fe River Ranch Beef 
DUCK = Duckoond. CAPRI = Capri. OM = Oaks Mall, and BP - Butler Plaza. 
Rural 
Urban 
Hjbiui 
Preserve 
Pastures 
Residential 
areas 
Parking lots 
Si# 
ORD 
BRU 
SF 
DUCK 
CAPRI 
OM 
BP 
Building s 
0.00 
0.14 
0.37 
7.06 
28.49 
25.91 
22.58 
areas 
frees 
23.53 
70.17 
71.79 
13.10 
38.65 
52.93 
60.40 
72.21 
28.16 
26.24 
79.60 
32.78 
19.52 
16.07 
Other 
4.26 
1.53 
1.60 
0.24 
0.08 
1.64 
0.95 
