Avian ecologists have long been interested in understanding the mechanisms that drive differences in life history traits, such as clutch size, between temperate and tropical birds.

Until now, however, advancing our understanding of these mechanisms has proceeded slowly, largely because we lack information on the demography and breeding ecology of tropical species.

Moreover, comparisons between tropical to temperate trends in life history traits typically fail to control for phylogenetic influences. In other words, we don't know whether similarities among birds living in either habitat are due to the birds being related or due to their sharing similar habitats.

Yellow Warbler, Ideal Study Species

Javier is removing a mangrove warbler from a mist net so that it can be measured, and given a unique identifier (colored ankle bracelets) so its future movements can be followed.

The yellow warbler (Dendroica petechia) is an ideal species in which to tease this apart. Among the several yellow warbler subspecies are forms that are tropical residents and other forms that are long-distance migrants that breed in the temperate zone and winter in the tropics.

For my Ph. D. dissertation, I studied the breeding biology of the mangrove warbler (Dendroica petechia bryanti), a tropical subspecies of yellow warbler from southern Mexico.

My main objective was to describe the life history traits of the mangrove warbler and examine how they differ from those of its temperate relative, the yellow warbler.

Second, I examined how factors such as rainfall, food abundance, nest depredation, and habitat influence reproduction and patterns of habitat occupancy in the mangrove warbler.

I conducted my research over three consecutive years (2001-2003) at Celestn Biosphere Reserve in the Yucatan Peninsula.

Over the 3 years, we monitored the fate of 285 active nests—the largest number of nests ever found for a tropical population of the yellow warbler.

Results

Results so far indicate that life history traits vary from temperate to tropical regions, providing a convincing case for the role that the tropical environment plays in driving the evolution of life history traits.

Life history traits of the mangrove warbler correspond to descriptions typical of tropical species.

Below is a table comparing life history traits of mangrove and yellow warblers:

Life History Traits	Mangrove Warbler	Yellow Warbler
Territoriality	Year-round	Seasonal
Breeding season length	3.5 months	2.5 months
Bigamous male percent	10	5 or less
Average clutch size	3 eggs	4.5 eggs
Average incubation time	13 days	11 days
Average brooding time	11 days	8.5 days
Depredation percent	65	30
Nesting success percent	26	55
Nesting attempts	2	?
Females double brooding percent	5	1 or less
Cowbird parasitism percent	8	40
Parental care percent	44	57
Adult survivorship	65	50

The tropical mangrove warblers benefit by having a longer season in which to breed, adults live longer, and cowbird parasitism is lower. The costs appear to be greater chance that the nest will be depredated. So, mangrove warblers respond by laying fewer eggs but spending more time incubating and brooding the young. If the nest fails, as often happens, the birds will often try nesting again.

For temperate yellow warblers a shorter breeding season means they tend to "put all their eggs in one basket", they have larger clutches, and quickly incubate and brood the youngsters. If the nest fails, they often don't have time to nest again.

With respect to the timing of breeding, the results indicate that in contrast to the traditional belief that rainfall is the main factor stimulating breeding in tropical birds, photoperiod is the main factor explaining when birds begin nesting while food availability may be used as a short-term cue to adjust the timing of breeding.

Bird pictures courtesy of Javier Salgado-Ortiz

This article summarizes the information in this publication:

Salgado-Ortiz, Javier, Marra, Peter P., Sillett, Terence Scott and Robertson, Raleigh J. 2008. Breeding Ecology of the Mangrove Warbler (Dendroica petechia Bryanti) and Comparative Life History of the Yellow Warbler Subspecies Complex. The Auk, 125(2): 402-410.

View abstract

It is widely accepted that tropical birds differ from temperate species in life-history traits and social behaviors, yet baseline ecological data are lacking for most tropical species and comparative studies often fail to control for phylogenetic influences. Within the Americas, the Yellow Warbler (Dendroica petechia) is ideal for such comparisons because its subspecies encompass a range of life-history strategies; the subspecies include long-distance migrants, temperate-tropical migrants, and tropical residents. We collected baseline data on the breeding ecology of Mangrove Warblers (D. p. bryanti) from southern Mexico (2001-2003) and compared their life-history traits with those of temperate and other tropical subspecies using existing data. Mangrove Warblers actively defended territories year-round during both nonbreeding and breeding seasons. The timing of breeding varied by year, and clutch size averaged 2.9 + 0.5 [SD] eggs, with both a median and a mode of 3 eggs. Annual estimates of nesting success ranged from 18% to 33%, and nest depredation was the primary cause of nest failure. Annual survival was significantly higher for males (0.65) than for females (0.52) and, given our data, did not vary by age or year. On the basis of a comparative analysis of life-history data from published studies on the Yellow Warbler subspecies complex, we found that most life-history traits differed between tropical and temperate latitudes. Specifically, compared with temperate Yellow Warblers, Mangrove Warblers exhibited longer breeding seasons, smaller clutch sizes, longer incubation and nestling periods, lower nesting success, higher rates of nest depredation, and higher annual adult survival rates.

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