Higher densities of the threatened little bustard Tetrax tetrax occur in larger grassland fields: management implications

1. 7th European Ornithologists' Union
Conference - Zurich
Higher densities of the threatened little
g e de s t es o t e t eate ed tt e
bustard Tetrax tetrax occur in larger grassland
fields: management implications
g p
JOÃO PAULO SILVA1,2,3, JORGE MESTRE PALMEIRIM2 & FRANCISCO MOREIRA3
1-N t
Nature C
Conservation I tit t (ICNB)
ti Institute
2 - Centre for Environmental Biology, Faculty of Sciences - University of Lisbon
3 - Centre for Applied Ecology "Prof. Baeta Neves", Institute of Agronomy – Technical University
of Lisbon

2. • The little bustard is medium sized grassland bird mostly
g y
adapted to extensive agricultural environments and pastures
• Presents nowadays a highly fragmented distribution
• The most viable population is found in the Iberian Peninsula
with more than half of the world’s population

3. • It is a leking bird, where breeding males display in an
aggregated manner, with f
t d ith females attending primarily f
l tt di i il for
the purpose of mating
• Breeding population estimates are mainly based on adult
male densities because females are inconspicuous
• Little bustard breeding male density has been found to
depend on small agricultural fields, presumably due to
increased habitat diversity
• However, exceptionally high densities are found in large
fields in Portugal, suggesting that the influence of field
size varies geographically and is not yet fully
understood.

4. Objectives
• With this work we sought to explore to what extent field
size and vegetations structure influence little bustard
male densities during the breeding season
• Develop management guidelines from which agro- agro
environmental schemes can be delineated

5. Study area

6. Study area
• Diversified landscape with agricultural areas and pastures
occurring interspersed with oak Quercus rotundifolia
woodlands
• Over 70% of the study area is covered with farmland fields
with over 20 ha

7. Methods
• Previous works showed (e.g. Silva 2005) a clear preference
for fallow lands and pastures (Grasslands)
• With this work we chose to study grassland fields since they
represent the manageable unit
• Grassland field selection:
– Over 20 ha (large enough to fit a sampling plot)
– Further from paved roads and inhabited houses
• Fi ld work
Field k
– During the spring of 2007 and 2008
– 3 hours after sunrise and before sunset
– Male counts were done within sampling plots
consisting of circular areas with a radius of 250 m
m,
thus occupying an area of c.20 ha

8. Methods
• 2007
– All available grassland fields were studied – 54 fields
– 88 sampling plots
• 2008
– 29 random fields
– 43 sampling p
p g plots

9. Methods
• V i bl
Variables
Variables (units) Description Source Mean (range)
Mean number of males per
Male density (males/100 ha) 100 ha pooled from all
ha, Fieldwork 6.2 (0.0 37.4)
6 2 (0 0 – 37 4)
sampling plots
Mean vegetation height of
Vegetation height (cm) the field, pooled from all Fieldwork 39.3 (10.1 – 95.8)
sa p g p ots
sampling plots
Mean vegetation cover of
Vegetation cover (%) the field, pooled from all Fieldwork 69.6 (40.0 – 94.0)
sampling plots
Standard deviation of the
Vegetation height heterogeneity vegetation height, pooled Derived from variable 2 12.8 (3.6 – 45.2)
from all sampling plots
Standard deviation of the
Vegetation cover heterogeneity vegetation cover, pooled Derived from variable 3 14.0 (6.2 – 22.4)
from all sampling plots
Number of grazing animals
Number of grazers (grazers/ha) Fieldwork 0.21 (0.0 – 3.9)
in the field
Size of the agricultural
Field area size (ha) GIS and fieldwork 57.3 (23.1 – 171.7)
field
Whether the field is
Type of land use pasture (0) or fallow land Fieldwork 0.41 (0 – 1)
(1)
Number of different
Number of neighbouring land uses landuses in the GIS and fieldwork 2.9 (1 – 6)
neighbouring fields
Number of neighbouring Number of neighbouring GIS and fieldwork
1.3 (0 – 4)
grasslands with LB grasslands with LB

10. Methods
• Statistical analysis
– 1st Logistic regression to determine the environmental
variables influencing the presence of breeding males
• repeated fields were selected randomly to avoid
p y
prevalence problems
• Forward stepwise selection was used to build the model
• The model’s to the data was assessed using ROC of
AUC
• Model was validated with a Jackknife procedure
– Generalised Linear Model (GLM) only considering presence
data was used to model male density
• Akaike’s Information Criteria (AIC) and Backward
stepwise selection were used to build the model
• The final model presented the lowest AIC

11. Results
• Male presence was found in 47 of 83 samples, with an overall
count of 183 breeding males
• Densities reached up to 37.4 males/100 ha
• Logistic Regression
– 2 variables entered the model: field size and vegetation
i bl t d th d l fi ld i d t ti
height
– AUC 82 4%
82.4%
– Validation AUC 79.9%
• GLM
– The only variable explaining male density is field size
– R2 = 0.46

12. Results
• Logistic Regression Variables Coefficient ‐2 log LR p‐value
Field size 0.035 11.830 <0.001
g
Vegetation
‐0.058
0 058 20.011 <0.001
20 011 <0 001
height
Constant 0.503 ‐‐‐ ‐‐‐
- Probability for the presence of breeding males in grasslands
of different sizes assuming different vegetation heights
1.0
0.9
0.8
lity
0.7
Probabil
0.6
0.5 20 cm
0.4
40 cm
P
0.3
0.2 60 cm
0.1
0.0
00
20 30 40 50 60 70 80 90 100 110 120 130 140 150
Field size (ha)

13. Results
• Plot of male density versus field size in fields where little bustards
were present. The fitted equation is y 0.001 (
p q y= (field size)2 + 6.697
)

14. Discussion
• Little bustards prefer larger grassland fields with adequate
vegetation structure
– Larger fields seem to congregate independent leks, since the
autocorrelation factor did not enter the model
– P f
Preference for a particular vegetation structure is well known
f ti l t ti t t i ll k
– Larger habitat patches are also likely to present less disturbance
and smaller rates of predation, due to reduced edge effects
p , g
– There are several aspects relating to their exploded lek mating
system that might explain higher densities in larger fields:
• most lek theories admit an increase of reproductive success
in larger leks
• aggregations with minimum distances between territorial
males are more probable in continuous areas of high-quality
habitat, rather than in suitable but discontinuous and spatially
scattered habitats
tt d h bit t

15. Discussion
• Conservation and management implications
• Management implications of our study are clear since fields
clear,
represent the agricultural management unit:
o (1) conservation efforts should be channelled to farms
( )
containing the largest fields
o (2) careful crop rotation planning and adequate livestock
grazing are needed t create suitable h bit t f th b di
i d d to t it bl habitat for the breeding
season
o (3) management at the landscape, rather than the farm level
landscape level,
is needed to ensure the most continuous grassland habitat
patches possible through careful and synchronized planning
of field rotation within farms and with neighbouring farms
ff f f