Seasonal Decline in Nestling Growth: Support for the Parental-Quality Hypothesis in Cassin's Auklets

Growth rates of nestlings often decline over the breeding season (e.g. Birkhead and Nettleship 1982, Gaston et al. 1983, Morbey and Ydenberg 1997, Lepage et al. 1999). Although several mechanistic hypotheses have the potential to explain this phenomenon (see Nilsson 1999), we investigated the two that are considered most often. Under the date hypothesis, nestling growth depends indirectly on current environmental conditions through direct effects on parental provisioning. A seasonal decline in nestling growth is expected if feeding conditions deteriorate over the season. Under the parental-quality hypothesis, young or inexperienced parents, or those with lower provisioning ability, initiate breeding later, leading to a seasonal decline in nestling growth. We determined which hypothesis could best explain the observed seasonal decline in nestling growth rates of Cassin's Auklets (Ptychoramphus aleuticus) by comparing growth of nestlings whose hatching dates were normal versus experimentally delayed. Breeding phenology and nestling growth of Cassin's Auklets were monitored in 1994 on Triangle Island (50°52′N, 129°05′W; Morbey and Ydenberg 1997). Cassin's Auklets lay one egg and incubate it for approximately 38 days, with parents switching incubation duties approximately every 24 h (Astheimer 1991). Prior to egg laying, we excavated 82 burrows to create access holes and began daily monitoring. If twigs that we placed in burrow entrances were knocked down the following day, we inferred that the burrow had been visited the previous night. Active burrows were checked every third day for eggs. Upon discovery of a newly laid egg, we alternately assigned the burrow to one of two experimental groups. In the delayed group (n = 27), the egg was replaced with a hard-boiled chicken egg for five days. In the interim, the auklet egg was left in a carton buried in the sand to keep it safe and cool. Enforcing egg neglect at the beginning of the incubation period was done to minimize any negative effects of interrupting embryonic growth. In the control group (n = 25), eggs were handled as in the delayed group but were not removed for more than a few seconds. Hatching dates of these experimental burrows were expected to span the natural range.

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