analyses, no relationship between climatic factors and ecological
variables could be identified. This may be explained by the fact
that fox reproduction and lemming abundance are both influenced
by one another in a complex cycle (see Lemming).
The effects of this cycle are so strong that they may overshadow
other weaker influences.
Lapland Longspur and Climate
The potential relationships between climatic factors and Lapland
Longspur egg laying date, egg hatching date and clutch size (number
of eggs in the nest) were investigated. Our analyses revealed
that laying and hatching dates are strongly associated with climatic
conditions such as snow thickness on 1 June, snow-free area on
5 June, air temperature and number of days with average temperature
above 0°C. As for clutch size, it is influenced by air temperature
and number of days above 0°C.
Our results show that both laying and hatching dates are later
in years with later snowmelt (thicker snow pack on 1 June and
less snow-free area on 5 June). This relation may easily be explained
by the fact that Lapland longspurs, which are nesting on the ground,
cannot built their nests before nesting sites are snow-free. It
is also noteworthy that the same relationships are observed for
laying and hatching dates. This may be explained by the fact that
incubation has a set time length in birds, and for Lapland Longspurs,
hatching always occurs 12 to 13 days after the last egg has been
laid. Hatching dates are therefore following the same trends as
Laying and hatching dates are also associated with the number
of days above 0°C. Indeed, our analyses demonstrate that Lapland
Longspurs lay and hatch earlier in years with more days above
0°C. Similarly, laying and hatching dates are related to annual
and summer air temperature, with earlier dates in warmer years.
For the clutch size, our results show that females lay more eggs
when summer temperatures are higher. Similarly, clutch sizes are
larger when, in a year, there is a higher number of days above