Assessing the Development of Shorebird Eggs Using the Flotation Method: Species-Specific and Generalized Regression Models

Abstract

We modeled the relationship between egg flotation and age of a developing embryo for 24 species of shorebirds. For 21 species, we used regression analyses to estimate hatching date by modeling egg angle and float height, measured as continuous variables, against embryo age. For eggs early in incubation, we used linear regression analyses to predict hatching date from logit-transformed egg angles only. For late incubation, we used multiple regression analyses to predict hatching date from both egg angles and float heights. In 30 of 36 cases, these equations estimated hatching date to within four days of the true hatching date for each species. After controlling for incubation duration and egg size, flotation patterns did not differ between shorebirds grouped by mass (≥100 g or <100 g) or taxonomy (Scolopacidae versus Charadriidae). Flotation progressed more rapidly in species in which both adults incubate the clutch versus species in which only one adult incubates the clutch, although this did not affect prediction accuracy. We also pooled all continuous data and created a generalized regression equation that can be applied to all shorebird species. For the remaining three species, we estimated hatching date using five float categories. Estimates of hatching date using categorical data were, overall, less accurate than those generated using continuous data (by 3%–5% of a given incubation period). Our equations were less accurate than results reported in similar studies; data collected by multiple observers and at multiple sites, as well as low sample sizes for some species, likely increased measurement error. To minimize flotation method prediction error, we recommend sampling in early incubation, collecting both egg angle and float height data in late incubation, and developing site- and species-specific regression models where possible.

Why is timing of bird migration advancing when individuals are not?

“…The laying date of each nest is estimated by conventional egg-floating techniques [22] and, in many cases, confirmed by recording hatching date. To compare advances in arrival among species with differing times of arrival and nest-laying, the first known date of nest initiation for each of these species (between 34 and 161 nests per species) was extracted from these data and used to calculate the time interval (days) between arrival and laying for each species.…”

Biparental incubation patterns in a high-Arctic breeding shorebird: how do pairs divide their duties?

“…The start of incubation and hatching was estimated by laying date for clutches found during laying and by measuring the height and inclination of the eggs floated in water for clutches found complete. This floating technique is based on the fact that eggs lose weight as the embryo develops: freshly laid eggs sink to the bottom of a water column and lay horizontal; as eggs develop, they rise with their blunt end and eventually float on the water surface (Liebezeit et al 2007; median estimation of all floated eggs within the nest was used). Each nest was visited at the estimated hatching date to capture both parents to estimate their condition; to measure, bleed, and ring the chicks; and to determine the fate of the nest.…”

Arctic-nesting birds find physiological relief in the face of trophic constraints

“…Hatch dates were based on confirmed sightings of newly hatched chicks in the nest. When hatch dates were not confirmed by chicks in the nests, hatch dates were estimated based on either 1) known laying dates (adding 22 days incubation + # eggs laid), 2) egg flotation30, or 3) signs of hatch in the nest (starred or pipped eggs).…”