Thomas E. Martin Lab:

Geographic Variation in Life History Traits
The University of Montana

Geographic variation in avian life history traits and parental care behaviors

Life history traits, such as clutch size, renesting rates, developmental rates, age of first reproduction, and adult survival, along with parental care behaviors (incubation, brooding, feeding young), vary extensively in geographic space.

A major question being examined is: why do these traits vary geographically? Bird studies, historically, have focused on the importance of food, following the pioneering work of David Lack. However, life history theory and studies in systems other than birds suggest that age-specific mortality are important sources of selection. Study sites were chosen around the world to allow examination of the potential influence of age-specific mortality and other environmental factors on life history variation of birds.

A key comparison is between South America and North America (Arizona), allowing examination of latitudinal variation in life history among closely-related pairs of species, as well as among endemics. Three years of study were first conducted in northwestern Argentina, and now ongoing (3 years) work is conducted in the northern Andes of Venezuela to examine tropical species. This work is conducted in collaboration with Carlos Bosque. The Venezuela work is a mid-elevation (1600-2000 m) site because nest predation is reduced compared to lowland tropics and similar to that observed in Arizona. This controls nest predation as a determinant of latitudinal differences in life histories. Some key differences between sites are smaller clutch sizes, longer development periods, different parental care behavior, larger eggs and higher adult survival in Venezuela.

The South Africa site, on the other hand, is the same latitude in the south (34 S) as Arizona is to the north (34 N) but nest predation in South Africa is outrageously high compared with any of the other sites. One interesting difference between South Africa and other sites is that hatching asynchrony is higher there than in any other site. Other life history traits (i.e., clutch size, developmental periods, etc) varied from typical tropical expression to typical north temperate expression among species all in one site. This work is conducted in collaboration with the Percy Fitzpatrick Institute of African Ornithology.

Work in New Zealand and Tasmania is a collaboration with Dr. James Briskie, who is the lead investigator. This work provides several important avenues of investigation. First, native species in New Zealand evolved largely in the absence of predators. Second, species have been introduced from Europe that evolved in the presence of predators. This allows a comparison of species that evolved in the presence and absence of nest predation but living in a common environment (removing environmental effects). Comparisons between New Zealand and Tasmania control for phylogeny for species that evolved in the absence (NZ) versus presence (Tasmania) of predators.

Life history work in birds has focused on clutch size, but life history strategies include a complex interaction of many traits with each other and the environment (see figure). The hypothesized relationships represent key avenues of investigation.

I have shifted my focus to traits that have received less attention. As one example, developmental periods are substantially longer in many southern locations, particularly the tropics, even controlling for differences in nest predation (see graph). Longer periods in the nest automatically increase the probability of nest predation, given that it is a time-dependent event. Consequently, it is unclear why periods would ever be lengthened, or what mechanisms underlie these differences. New work on incubation periods is focused on looking at parental care behavior (e.g., incubation behavior) and the consequences for egg temperatures during development.

Ambient temperature and its influence on egg temperature and egg viability is hypothesized to underlie early onset of incubation and asynchronous hatching. These relationships also are under investigation.