RESUMO
Whether parents are able to adapt food gathering to rising offspring demands, or if they are controlled largely by extrinsic factors, is important for understanding key limits on fitness. Over seven breeding seasons, we studied the provisioning behavior of chinstrap penguins, Pygoscelis antarctica, at Seal Island, Antarctica, during parents' transition to leave broods of one or two chicks unguarded. By measuring the frequency, duration, and diel timing of foraging trips and the quantity of prey brought to chicks, we examined the extent to which variation in parents' feeding behavior could be attributed to provisioning costs which increase with chick growth. Estimates of the energy content of food loads were combined with foraging patterns and brood requirements to model parents' seasonal provisioning budget. The frequency of foraging trips increased from the guard to post-guard phase and was higher, and the seasonal effect larger, in parents with two chicks. The duration of overnight trips (~16 h) increased with seasonally increasing night length; diurnal trip duration (~8 h) showed no seasonal pattern. Birds exhibited a seasonal shift to diurnal foraging, a trend that was generally weaker in parents of smaller broods. Food loads increased with chick mass only in parents of one chick; parents of two chicks had larger but more constant food loads. Based on per trip calculations, parents foraging overnight could not have delivered to two-chick broods enough food to meet their demands unless chicks were small. Diurnal foragers (regardless of brood size) and overnight foragers with one chick could meet brood demands at chicks' peak mass. The combined daily effort of parents indicated that mated pairs on average had ample resources to meet chick demands through most of rearing. A brief period when demands could not be met was predicted in two-chick broods just before chicks were left unguarded and again as they neared fledging. Our findings suggest that penguins both increased provisioning frequency and favored foraging under higher light intensity in conjunction with increasing chick demands, tactics which required parents to leave chicks unattended. The ability to maintain intrinsic control over provisioning has bearing on how penguins may be limited by extrinsic constraints. Prey surveys conducted annually near colonies show abundant resources 10-20 km offshore with no consistent seasonal shifts in abundance. These findings support a prominent role for intrinsic factors in the foraging decisions of chinstrap penguins.
RESUMO
I conducted direct visual observations of aerial insect-eating birds concurrently with remote radar observations of aerial plankton before, during, and after the passage of an intense thunderstorm gust front in east-central Florida. Clear skies and convective conditions predominated in the area prior to local passage of the gust front. Shortly after passage, weather conditions shifted to a stratiform regime, with continuous low cloud cover and intermittent drizzly rain. Insectivorous birds appeared in the area briefly at the time of gust front passage, and became common following the onset of the drizzly rain. These birds remained in the area after the cessation of precipitation, feeding actively and very low, until mid-afternoon the following day, when the transition from stratiform back to convective conditions finally occurred. Almost immediately after this transition, the altitude of bird flight began increasing rapidly, and the birds quickly dispersed. I interpreted these observations together with radar data as indicating that (a) large quantities of aerial plankton were entrained by the gust front, "leaked" into the storm outflow, and were subsequently "scrubbed" out of the atmospheric boundary layer by precipitation; (b) after the rain ended, the insect species involved were prevented from reascending by stratiform sky conditions that inhibited thermal convection, and (c) the insects rapidly reascended at the first possible opportunity (i.e., almost immediately following the transition from stratiform back to convective conditions). This detailed case study showed that an individual convergence event in the atmospheric boundary layer caused a spatial redistribution of the aerial plankton which subjected the insects involved to locally intensified predation by avian consumers. However, despite the dramatic short-term effects, the apparently rapid reascent of the aerial plankton suggests that this particular scrubbing event probably had little impact on the local population dynamics of the insect species affected. Data on track directions of a large sample of summer gust fronts in east-central Florida suggest that the potential for net directional displacements of insect populations over seasonal time scales via the cumulative effects of individual convergence events and subsequent scrubbing is probably low.
