Facultative nest switching by juvenile ospreys

Auk, The, Jan 2000 by Gilson, Lauren N, Marzluff, John M

When fledglings forego parental protection and provisioning to explore nests other than their own, they take a behavioral risk. In species whose young generally cannot leave the nest prior to developing flight, such movements among nests are unlikely to occur. Yet, this facultative behavior, which we refer to as "nest switching," has been noted in a variety of semialtricial species (e.g. Poole 1982, GonzAlez et al. 1986, Newton 1986, Bustamante and Hiraldo 1989, Donazar and Ceballos 1989, Kenward et al. 1993).

Two models may explain facultative nest switching in semialtricial species: (1) switching may arise purely from random predispersal movements of fledglings; or (2) switching may be a nonrandom, deterministic behavior that is influenced by conditions that fledglings encounter in their natal nest. The deterministic model predicts that some fledglings are more likely to switch than others, based on parental provisioning and individual food intake prior to fledging, both of which may be affected bv brood size. The random-movements model predicts no association between brood size or conditions within a nest and frequency of nest switching; rather, nest switching is influenced by opportunities available to a potential switcher (e.g. proximity and occupancy of nearby nests). The two models are not mutually exclusive, because spatial factors may influence the outcome of a behaviorally motivated switching attempt. However, the suite of predictions for each model should indicate whether the operant mechanism underlying nest switching is random or deterministic.

Facultative nest switching has been documented in several populations of Ospreys (Patidion haliaetus; Fernandez and Fernandez 1977, Judge 1981, Poole 1982). Through natural experiments and manipulations of Osprey broods in the field, we examined (1) the frequency of nest switching; (2) the effects of brood size on parental provisioning, nestling food intake, and nest switching; and (3) the influence of population density and dispersion on frequency of nest switching. Our objectives were to determine which fledglings switch nests and to assess factors that may explain why some fledglings switch while others do not.

Study area atid methods.-We observed 20 pairs of Ospreys that nested on artificial platforms in three areas of Cascade Reservoir, west-central Idaho, during 1993 and 1994. To monitor food intake and individual behavior, we dyed the head feathers (random assignment of colors within broods) of 32 nestlings as soon as the nestlings were visible above the nest rim (10 days or older) and banded all young at 23 to 35 days of age using standard USGS bands and anodized numerical bands. We equipped 28 of 30 fledglings (two nestlings died) with necklacemounted radio transmitters and fitted all fledglings in 1994 with both a transmitter and a color-coded patagial marker of rip-stop nylon (Gilson 1996).

To test the effects of brood size on food intake, parental provisioning, and ultimately on switching behavior, we augmented the number of three-young (3Y) broods in our sample by translocating nestlings among similarly aged broods (see Spitzer 1984). We observed three 3Y broods and seven one-young (1Y) broods in each year of study. We created two of the 3Y broods through translocation in 1993 and one in 1994. Translocated nestlings were chosen at random from a donor 2Y brood and placed into the nearest recipient 2Y brood (x = 1.07 /- SE of 0.41 km between nests) of similar age. We fostered young into recipient broods at 10 to 21 days of age to allow for a natural age/ size range among new brood mates (Forbes 1991). Details of observations and measurements are provided in Gilson (1996).

To determine whether switching was a random movement, we used analysis of variance (ANOVA) to compare internest distances, nearest-neighbor distances, and number of occupied nests and vacant platform nests within a 2-krn radius of nests that produced switchers with those of nests that did not produce switchers. We analyzed the use of perches by fledglings from 1Y versus 3Y broods using multivariate analysis of variance (MANOVA).

To assess whether switching behavior was determined by conditions at the nest prior to fledging (i.e. leaving the nest), we recorded parental provisioning rates and individual food intake (defined as bites per observation) of all juveniles in their nests. We assessed differences on two levels: 1Y versus 3Y broods, and among siblings (brood mates where manipulated) within 3Y broods. We assessed how brood size affected parental provisioning using a onetailed ANOVA, with the expectation that nestlings in larger broods receive lower per capita provisioning than those in single-chick broods. To describe how individual food intake varied among the two experimental brood sizes, we compared mean food intake, variance in intake, and average minimum daily intake during the two weeks prior to fledging using MANOVA and assessed statistical significance of the MANOVA results with a Bonferroni-adjusted a based on the number of dependent variables. We used a Wilcoxon matched-pairs signed-rank test to assess whether the food intake of switchers differed from that of nonswitchers within 3Y broods; all food-intake measures were log,, transformed.