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I studied developmental rates in a suite of temperate and tropical passerine bird species from the New World to test the hypothesis that tropical passerines exhibit slower patterns of growth and development than temperate birds. I also investigated how the expression of several developmental rates varied with life history traits including mass and tarsus growth rate as well as nestling period. I present data on the developmental marker of age of eye-opening, and for the first time, relate this characteristic to a suite of life history traits. Additionally, I investigated how differences in the method used to calculate mass growth rate affected correlations to a suite of life history variables. I found that tropical passerines gained mass 17% more slowly than did temperate birds using the conventional means of calculating growth rate; however, there was a 23% difference between temperate and tropical passerines when using a modified logistic growth equation that set a fixed value of the adult mass as the upper asymptotic value (A) used to calculate growth rate. Tropical passerines also exhibited a 16% slower rate of tarsus, or long bone, growth. Other traits, such as nestling period and age at eye-opening, did not differ between temperate and tropical passerines. I found that nestling period is an inappropriate surrogate for growth rate since there is little correlation between these characteristics. Thus, previous studies that have used nestling period instead of growth rate are likely biasing the results. I confirm previous findings on differences in growth and development of passerines between temperate and tropical regions; however, tropical birds appear to exhibit greater variation in developmental characteristics of life histories than do temperate birds, especially among the suboscine passerines. |
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