Corresponding author: Reuven Yosef (

Academic editor: Grace Servat

Biometric analysis helps in sex differentiation, understanding development and for studies of avian biology such as foraging ecology, evolutionary ecology, and survivorship. We suggest that biometry can also be a reliable, practical and inexpensive tool to determine the age of nestlings in the field by non-invasive methods. As an example we studied the biometry of wing, culmen, talon, tarsus and body mass of nestling southern Indian Spotted Owlets (

Adult and juvenile birds of the same species are of similar size but are differentiated on the basis of plumage, fault bars, tail shape, castellated feathers, bill shape, cere color, and other parameters (eg.,

Our ability to monitor and understand biometric parameters is important from ecological and conservation perspectives. During the nestling period, these parameters allow one to evaluate parental feeding ability, and to monitor relative nutritional condition between siblings and between neighboring nests of different habitats. Parameters can also help in the rehabilitation of orphaned nestlings in determining whether they are fed appropriately so that growth rates are comparable to nestlings in the wild.

Retrograde calculation of hatching and egg-laying dates can be accurately determined from the age of the nestling or from time of fledging (Blotzheim and Bauer 1980;

In this paper we show how the use of biometry can be a reliable, practical and inexpensive tool to determine the age of nestlings in the field by non-invasive methods, and by taking the minimum required measurements through the use of a flow chart as described. A method to estimate the time of acquiring flight ability using biometric parameters is also investigated and described. We have taken southern Indian Spotted Owlets (

A total of 53 active nests of southern Indian Spotted Owlet

Out of the 53 active nests found, seven nests were intensively studied during the 2003 breeding season for the documentation of breeding biology and biometry. Biometry of eight nestlings from hatching till fledging, from 0 to 32 days, was done at weekly intervals, and the data was entered serially for each chick. A total of 136 measurements were taken, averaging 17 measurements per nestling. We included only those nests that were easily accessible where the exact dates of egg laying and hatching were recorded. Nestlings that died in the middle of the study were excluded. We ringed each nestling with a numbered aluminum ring placed on the tarsus to facilitate individual identification of each nestling. Sexes are alike in external appearance and thus we could not separate between the nestlings in this study based on their sexes (Hipkis et al. 2002).

We used a Vernier calipers (

To each of the biometric character we fitted the logistic model to understand its growth pattern and growth rate (

Character value = a/[1+b*exp(-c*

Where a, b and c are positive constants. Constant

We also studied the growth patterns by nullifying the effect of size using Principle Component Analysis (PCA) as described by ^{½}, where p is the number of characters, was 0.5. We calculated the angle theta between the eigenvector for each age class and isometric vector to understand the developmental pattern.

We performed Discriminant Factor Analysis (DFA) to understand whether different age groups form significantly different clusters and which factors can best discriminate between the clusters (

To predict the nestling age using biometric characters we constructed decision tree (regression tree) using exhaustive CHAID (Chi-squared Automatic Interaction Detection) algorithm. At each step, CHAID chooses the independent (predictor) variable that has the strongest interaction with the dependent variable. Categories of each predictor are merged if they are not significantly different with respect to the dependent variable (

Observations on the biometric parameters of nestling Spotted Owlets of different age groups are given in

Mean, standard deviation and coefficient of variation of biometric parameters for age in weeks of nestling Spotted Owlets (

Character | 1st Week(n =11) | 2nd Week(n =11) | 3rd Week(n = 6) | 4th Week(n = 6) | 5th Week(n = 6) | |||||
---|---|---|---|---|---|---|---|---|---|---|

xˉ (sd) | CV | xˉ (sd) | CV | xˉ (sd) | CV | xˉ (sd) | CV | xˉ (sd) | CV | |

Body Mass (g) | 23.36 (9.96) | 42.64 | 82.73 (10.08) | 12.19 | 117.17 (6.62) | 5.65 | 113.83 (12.50) | 10.98 | 126.83 (8.38) | 6.60 |

Wing Chord (mm) | 13.06 (3.49) | 26.69 | 33.45 (10.20) | 30.48 | 87.42 (5.14) | 5.88 | 103.17 (6.68) | 6.47 | 121.67 (6.53) | 5.37 |

Talon (mm) | 2.46 (0.81) | 32.89 | 6.43 (0.28) | 4.29 | 7.42 (0.25) | 3.35 | 7.48 (0.26) | 3.53 | 7.85 (0.10) | 1.34 |

Tarsus (mm) | 14.93 (3.16) | 21.18 | 30.17 (2.91) | 9.66 | 36.22 (1.22) | 3.36 | 38.40 (2.02) | 5.26 | 39.27 (1.76) | 4.49 |

Culmen (mm) | 6.74 (0.90) | 13.43 | 10.5 (0.40) | 3.86 | 11.33 (0.23) | 2.06 | 11.57 (3.36) | 3.36 | 12.40 (0.41) | 3.35 |

Biometry ofbody mass, wing chord length, talon length, tarsus length and culmen length plotted against the age of nestling Spotted Owlets (

After a small lag body mass displayed linear growth with a steep rise through 2.5 weeks. Eighty percent of adult growth was attained at the end of 2 weeks (

Eigenvalues of the first principle component for four ln-transformed biometric characters for eachage class along with percent variability explained by each principle axis and angle theta between eigenvector and isometric vector.

Age | Characters | Variability (%) | theta | |||
---|---|---|---|---|---|---|

wing | culmen | talon | tarsus | |||

Week 1 | 0.5873 | 0.2099 | 0.7197 | 0.3051 | 76.14423 | 0.4252 |

Week 2 | 0.9869 | 0.0124 | 0.0549 | 0.1515 | 91.54507 | 0.9242 |

Week 3 | 0.8562 | 0.0239 | -0.4256 | 0.292 | 73.92306 | 1.1886 |

Week 4 | 0.6962 | 0.2667 | 0.338 | -0.5744 | 87.19698 | 1.1987 |

Week 5 | 0.9299 | 0.3455 | -0.0074 | -0.1261 | 53.56812 | 0.963 |

Out of five biometric parameters, wing chord length showed the longest lag phase. It started growing rapidly only after two weeks and attained maximum length after five weeks (

Identification key for ageing nestling Spotted Owlets (

The claw of the middle toe, talon, had the maximum growth rate among the five characters. It showed linear growth up to first two weeks and then it slowed down and attained mature size in the third week (

Tarsus showed a steep linear growth up to 2.5 weeks, and adult size was attained at 4.5 weeks (

Culmen had a growth rate equal to the wing chord length however it did not show any lag period and grew rapidly from hatching to the second week (

In the post-fledging and adult Spotted Owlets showed following biometric characters (n = 8): wing chord 150 mm (145–154), body mass 240 gm (235–245), talon 7.85 mm (7.8–7.9), tarsus 37 mm (33–40), culmen 14.5 mm (14–15)(Ali and Ripley 1969; SP, unpubl. data). One ringed fledgling was recaptured at 6.5 weeks following hatching, 2 weeks after fledging. Its biometrics were: wing chord 126 mm (84% of adult), body mass 125 gm (52.1%), talon 7.9 mm (100%), tarsus 40 mm (108%), and culmen 17 mm (117%). A sibling fledgling of this cohort was also recaptured but only body mass was measured.

We also studied the developmental patterns in the size adjusted characters and compared it with the isometric developmental pattern (

Growth rate of different parameters varied significantly with respect to age. This allowed us to derive useful biometric parameters to predict age during the nestling period (

Discriminant Factor Analysis of the age classes based on five biometric characters of nestling Spotted Owlets (

To predict the age of a nestling from the minimum biometric characters, we constructed a regression tree using CHAID algorithm. The regression tree could separate the nestling of different ages using three characters - wing chord length, culmen length and tarsus length (

Decision tree based on exhaustive CHAID algorithm. Bar diagrams show the number of individuals in each age class 1 to 5 weeks of nestling Spotted Owlets (

If wing chord length (mm) is in the range [9.5, 66.15] then Age = 1 week in 50% of cases.

If wing chord length (mm) is in the range [66.15, 127] then Age = 3 weeks in 33.3% of cases.

If culmen length (mm) is in the range [5.6, 9.1] and wing chord length (mm) is in the range [9.5, 66.15] then Age = 1 week in 100% of cases.

If culmen length (mm) is in the range [9.1, 11.3] and wing chord length (mm) is in the range [9.5, 66.15] then Age = 2 weeks in 100% of cases.

If wing chord length (mm) is in the range [66.15, 115] then Age = 3 weeks in 46.2% of cases.

If wing chord length (mm) is in the range [115, 127] then Age = 5 weeks in 100% of cases.

If tarsus length (mm) is in the range [35, 38.5] and wing chord length (mm) is in the range [66.15, 115] then Age = 3 in 75% of cases.

If tarsus length (mm) is in the range in [38.5, 40.7] and wing chord length (mm) is in the range [66.15, 115] then Age = 4 weeks in 80% of cases.

If wing chord length (mm) is in the range [66.15, 102.5] and tarsus length (mm) is in the range [35, 38.5] then Age = 3 weeks in 100% of cases.

If wing chord length (mm) is in the range [102.5, 115] and tarsus is in the range [35, 38.5] then Age = 4 weeks in 100% of cases.

However, we also present a simple flow-chart style of the biometrics with the minimum-maximum measurements (

Correlation between wing chord size and body mass was examined in order to understand when the fledglings are capable of flight. Examination of biometric data of mass gain and wing chord growth revealed that in the early nestling period the growth rate of wing size was less than that of mass, but was equal at 4.5 weeks.

Based on the above, we devised a formula to examine this correlation and determine the optimal wing chord length to body mass ratio to predict when the nestling would be capable of initiating its first attempt at flight.

The average value of flight formula of fifth week nestling was 0.96 while nestlings of younger age showed lesser values (^{st} to 4^{th} week showed average flight formula values 0.59, 0.41, 0.75 and 0.91 respectively. Adults had a test value of 0.97 for males (n = 7) and 0.97 for females (n = 6). However, two fledglings that were flying well at 6.5 weeks showed a test value of 0.99.

Flight formula for different age groups of nestling Spotted Owlets (

In this study we have used only externally measured biometric parameters in order to understand nestling growth rate and to present an idea of how we can use these data to help future studies that may need to evaluate age of hatchlings in Spotted Owlets. However, we consider our limited sources to also be a limiting factor in the depth to which this study could be done at present. Our inability to sex individuals at the nest prevents us from knowing the differential growth rates of males compared to females.

In our study, all five biometric parameters studied in Spotted Owlets showed logistic growth during the nestling period from hatching till 4.5 weeks when they fledged. The growth rate of all the parameters examined in this study is differential, this heterochrony is used to estimate the age of the nestlings. Three parameters - talon, tarsus and culmen - achieve 100 % growth at 4.5 weeks, while body mass gain is 89% and wing chord growth is 80% of adults at 4.5 weeks.

We attribute this differential growth to the required capabilities that enable fledging. Upon fledging each of the individuals has to fend for itself – from predators and for food. This requires fully developed talons, culmen and tarsus, and a significant increase in body mass. In many cases we have observed Spotted Owlets nestlings - similar to many other owl species - walk out of the nest hole and perch on branches of the nest tree prior to gaining the ability of flight. We assume that the reason for this is to escape increased risk of predation at the nest, where the stench and odor from accumulated pellets, fecal matter and other debris is likely to attract predators. We base this assumption on our having documented predation of eggs and nestlings from the nest hole by Small Indian Civet (

Owing to the fact that the mean values of wing chord, tarsus and body mass differed significantly between the age groups, based on the flow chart, a comparison of the field measurement with our data will allow the researcher to estimate age with a high level of accuracy. We suggest that only three characters (tarsus length, culmen length and wing chord length) are sufficient for determining the age of the nestling.

The estimation of nestling age from this flow chart is useful for conservation and rescue work. The biometric parameters obtained can be compared with the values of various parameters plotted against age in normal wild nestlings given in this paper. From this normal base line trend, one may evaluate the growth and assess the nutritional status of nestlings reared in an orphanage or by foster parents. If a discrepancy is apparent, appropriate corrective measures can be taken by adjustments in feeding.

The nestlings attained 91.9% of adult body mass at 4.5 weeks but experienced a drop in mass at 3.5 weeks, one week prior to fledging. This trend is also recorded in other birds of prey towards the end of nestling stage and can be explained as a response to achieve appropriate wing loading in order to make the first flight easier (

An analysis of the wing chord length and the body mass using our flight formula (eq. 2) suggests that as the nestling approaches maturity and is capable of first attempt of flight the flight formula value approached unity. In the simplest words our analysis shows that as the wing chord length (mm) approaches body mass (g) the bird becomes capable of flight. This finding is important for rescued fledglings that have suffered injuries or fractures, and for which, appropriate time of release needs to be calculated. The optimal body mass to wing chord ratio helps decide the amount of feeding and time required between rescue and release. We have observed that hand-reared nestlings and rehabilitated adult owls may exhibit good wing flapping, but do not immediately take flight when released. Hence, with the application of our formula one can predict the capability of flight and thus prevent predation after release.

In summary, biometry of wing, body mass, talon, tarsus and culmen of nestling Spotted Owlets is an easy, reliable and inexpensive method to determine nestling age, to assess growth rate and nutritional status, and to predict ability to initiate first flight. This method is described here for the first time for Spotted Owlets, and we postulate that such charts can be devised for other avian species as well.

We thank Anand Pandit and Ashish Bavdekar provided the facilities for this analysis at K. E. M. Hospital and Research Center, Pune. Field work was assisted by Banda Pednekar, Pramod Pawashe, Prashant Deshpande, Kumar Pawar and Unmesh Barbhai. Susan Craig improved an earlier draft of the paper. The study was assisted by the Vatsala Joshi Grant donated by Govind Mudholkar, Rochester, New York, to ELA Foundation, Pune. We are grateful to Indian Forest Department for allowing the study.